SQL Server, DBA interview questions
Questions are categorized under the following sections, for your convenience:
- Database design (8 questions)
- SQL Server architecture (12 questions)
- Database administration (13 questions)
- Database programming (10 questions)
- Database design
Q1. What is normalization? Explain different levels of normalization?
ü Check out the article Q100139 from Microsoft knowledge base and of course, there’s much more information available in the net. It’ll be a good idea to get a hold of any RDBMS fundamentals text book, especially the one by C. J. Date. Most of the times, it will be okay if you can explain till third normal form.
Q2. What is denormalization and when would you go for it?
ü As the name indicates, denormalization is the reverse process of normalization. It’s the controlled introduction of redundancy in to the database design. It helps improve the query performance as the number of joins could be reduced.
Q3. How do you implement one-to-one, one-to-many and many-to-many relationships while designing tables?
ü One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign key relationships. One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key relationships. Many-to-Many relationships are implemented using a junction table with the keys from both the tables forming the composite primary key of the junction table. It will be a good idea to read up a database designing fundamentals text book.
Q4. What’s the difference between a primary key and a unique key?
ü Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key doesn’t allow NULLs, but unique key allows one NULL only.
Q5. What are user defined datatypes and when you should go for them?
ü User defined datatypes let you extend the base SQL Server datatypes by providing a descriptive name, and format to the database. Take for example, in your database, there is a column called Flight_Num which appears in many tables. In all these tables it should be varchar(8). In this case you could create a user defined datatype called Flight_num_type of varchar(8) and use it across all your tables. See sp_addtype, sp_droptype in books online.
Q6. What is bit datatype and what’s the information that can be stored inside a bit column?
ü Bit datatype is used to store boolean information like 1 or 0 (true or false). Untill SQL Server 6.5 bit datatype could hold either a 1 or 0 and there was no support for NULL. But from SQL Server 7.0 onwards, bit datatype can represent a third state, which is NULL.
Q7. Define candidate key, alternate key, composite key.
ü A candidate key is one that can identify each row of a table uniquely. Generally a candidate key becomes the primary key of the table. If the table has more than one candidate key, one of them will become the primary key, and the rest are called alternate keys. A key formed by combining at least two or more columns is called composite key.
Q8. What are defaults? Is there a column to which a default can’t be bound?
ü A default is a value that will be used by a column, if no value is supplied to that column while inserting data. IDENTITY columns and timestamp columns can’t have defaults bound to them. See CREATE DEFAULT in books online.
Q9. What is a transaction and what are ACID properties?
ü A transaction is a logical unit of work in which, all the steps must be performed or none. ACID stands for Atomicity, Consistency, Isolation, Durability. These are the properties of a transaction. For more information and explanation of these properties, see SQL Server books online or any RDBMS fundamentals text book. Explain different isolation levels An isolation level determines the degree of isolation of data between concurrent transactions. The default SQL Server isolation level is Read Committed. Here are the other isolation levels (in the ascending order of isolation): Read Uncommitted, Read Committed, Repeatable Read, Serializable. See SQL Server books online for an explanation of the isolation levels. Be sure to read about SET TRANSACTION ISOLATION LEVEL, which lets you customize the isolation level at the connection level. Read Committed - A transaction operating at the Read Committed level cannot see changes made by other transactions until those transactions are committed. At this level of isolation, dirty reads are not possible but nonrepeatable reads and phantoms are possible. Read Uncommitted - A transaction operating at the Read Uncommitted level can see uncommitted changes made by other transactions. At this level of isolation, dirty reads, nonrepeatable reads, and phantoms are all possible. Repeatable Read - A transaction operating at the Repeatable Read level is guaranteed not to see any changes made by other transactions in values it has already read. At this level of isolation, dirty reads and nonrepeatable reads are not possible but phantoms are possible. Serializable - A transaction operating at the Serializable level guarantees that all concurrent transactions interact only in ways that produce the same effect as if each transaction were entirely executed one after the other. At this isolation level, dirty reads, nonrepeatable reads, and phantoms are not possible.
Q10. CREATE INDEX myIndex ON myTable(myColumn)What type of Index will get created after executing the above statement?
ü Non-clustered index. Important thing to note: By default a clustered index gets created on the primary key, unless specified otherwise.
Q11. What’s the maximum size of a row?
ü 8060 bytes. Don’t be surprised with questions like ‘what is the maximum number of columns per table’. 1024 columns per table. Check out SQL Server books online for the page titled: "Maximum Capacity Specifications". Explain Active/Active and Active/Passive cluster configurations Hopefully you have experience setting up cluster servers. But if you don’t, at least be familiar with the way clustering works and the two clusterning configurations Active/Active and Active/Passive. SQL Server books online has enough information on this topic and there is a good white paper available on Microsoft site. Explain the architecture of SQL Server This is a very important question and you better be able to answer it if consider yourself a DBA. SQL Server books online is the best place to read about SQL Server architecture. Read up the chapter dedicated to SQL Server Architecture.
Q12. What is lock escalation?
ü Lock escalation is the process of converting a lot of low level locks (like row locks, page locks) into higher level locks (like table locks). Every lock is a memory structure too many locks would mean, more memory being occupied by locks. To prevent this from happening, SQL Server escalates the many fine-grain locks to fewer coarse-grain locks. Lock escalation threshold was definable in SQL Server 6.5, but from SQL Server 7.0 onwards it’s dynamically managed by SQL Server.
Q13. What’s the difference between DELETE TABLE and TRUNCATE TABLE commands?
ü DELETE TABLE is a logged operation, so the deletion of each row gets logged in the transaction log, which makes it slow. TRUNCATE TABLE also deletes all the rows in a table, but it won’t log the deletion of each row, instead it logs the deallocation of the data pages of the table, which makes it faster. Of course, TRUNCATE TABLE can be rolled back. TRUNCATE TABLE is functionally identical to DELETE statement with no WHERE clause: both remove all rows in the table. But TRUNCATE TABLE is faster and uses fewer system and transaction log resources than DELETE. The DELETE statement removes rows one at a time and records an entry in the transaction log for each deleted row. TRUNCATE TABLE removes the data by deallocating the data pages used to store the table’s data, and only the page deallocations are recorded in the transaction log. TRUNCATE TABLE removes all rows from a table, but the table structure and its columns, constraints, indexes and so on remain. The counter used by an identity for new rows is reset to the seed for the column. If you want to retain the identity counter, use DELETE instead. If you want to remove table definition and its data, use the DROP TABLE statement. You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY constraint; instead, use DELETE statement without a WHERE clause. Because TRUNCATE TABLE is not logged, it cannot activate a trigger. TRUNCATE TABLE may not be used on tables participating in an indexed view
Q14. Explain the storage models of OLAP
ü Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more infomation.
Q15. What are the new features introduced in SQL Server 2000 (or the latest release of SQL Server at the time of your interview)? What changed between the previous version of SQL Server and the current version?
ü This question is generally asked to see how current is your knowledge. Generally there is a section in the beginning of the books online titled "What’s New", which has all such information. Of course, reading just that is not enough, you should have tried those things to better answer the questions. Also check out the section titled "Backward Compatibility" in books online which talks about the changes that have taken place in the new version.
Q16. What are constraints? Explain different types of constraints.
ü Constraints enable the RDBMS enforce the integrity of the database automatically, without needing you to create triggers, rule or defaults. Types of constraints: NOT NULL, CHECK, UNIQUE, PRIMARY KEY, FOREIGN KEY. For an explanation of these constraints see books online for the pages titled: "Constraints" and "CREATE TABLE", "ALTER TABLE"
Q17. What is an index? What are the types of indexes? How many clustered indexes can be created on a table? I create a separate index on each column of a table. What are the advantages and disadvantages of this approach?
ü Indexes in SQL Server are similar to the indexes in books. They help SQL Server retrieve the data quicker. Indexes are of two types. Clustered indexes and non-clustered indexes. When you create a clustered index on a table, all the rows in the table are stored in the order of the clustered index key. So, there can be only one clustered index per table. Non-clustered indexes have their own storage separate from the table data storage. Non-clustered indexes are stored as B-tree structures (so do clustered indexes), with the leaf level nodes having the index key and it’s row locater. The row located could be the RID or the Clustered index key, depending up on the absence or presence of clustered index on the table. If you create an index on each column of a table, it improves the query performance, as the query optimizer can choose from all the existing indexes to come up with an efficient execution plan. At the same t ime, data modification operations (such as INSERT, UPDATE, DELETE) will become slow, as every time data changes in the table, all the indexes need to be updated. Another disadvantage is that, indexes need disk space, the more indexes you have, more disk space is used.
Q18. What is RAID and what are different types of RAID configurations?
ü RAID stands for Redundant Array of Inexpensive Disks, used to provide fault tolerance to database servers. There are six RAID levels 0 through 5 offering different levels of performance, fault tolerance. MSDN has some information about RAID levels and for detailed information, check out the RAID advisory board’s homepage
Q19. What are the steps you will take to improve performance of a poor performing query?
ü This is a very open ended question and there could be a lot of reasons behind the poor performance of a query. But some general issues that you could talk about would be: No indexes, table scans, missing or out of date statistics, blocking, excess recompilations of stored procedures, procedures and triggers without SET NOCOUNT ON, poorly written query with unnecessarily complicated joins, too much normalization, excess usage of cursors and temporary tables. Some of the tools/ways that help you troubleshooting performance problems are: SET SHOWPLAN_ALL ON, SET SHOWPLAN_TEXT ON, SET STATISTICS IO ON, SQL Server Profiler, Windows NT /2000 Performance monitor, Graphical execution plan in Query Analyzer. Download the white paper on performance tuning SQL Server from Microsoft web site. Don’t forget to check out sql-server-performance.com
Q20. What are the steps you will take, if you are tasked with securing an SQL Server?
ü Again this is another open ended question. Here are some things you could talk about: Preferring NT authentication, using server, databse and application roles to control access to the data, securing the physical database files using NTFS permissions, using an unguessable SA password, restricting physical access to the SQL Server, renaming the Administrator account on the SQL Server computer, disabling the Guest account, enabling auditing, using multiprotocol encryption, setting up SSL, setting up firewalls, isolating SQL Server from the web server etc. Read the white paper on SQL Server security from Microsoft website. Also check out My SQL Server security best practices
Q21. What is a deadlock and what is a live lock? How will you go about resolving deadlocks?
ü Deadlock is a situation when two processes, each having a lock on one piece of data, attempt to acquire a lock on the other’s piece. Each process would wait indefinitely for the other to release the lock, unless one of the user processes is terminated. SQL Server detects deadlocks and terminates one user’s process. A livelock is one, where a request for an exclusive lock is repeatedly denied because a series of overlapping shared locks keeps interfering. SQL Server detects the situation after four denials and refuses further shared locks. A livelock also occurs when read transactions monopolize a table or page, forcing a write transaction to wait indefinitely. Check out SET DEADLOCK_PRIORITY and "Minimizing Deadlocks" in SQL Server books online. Also check out the article Q169960 from Microsoft knowledge base.
Q22. What is blocking and how would you troubleshoot it?
ü Blocking happens when one connection from an application holds a lock and a second connection requires a conflicting lock type. This forces the second connection to wait, blocked on the first. Read up the following topics in SQL Server books online: Understanding and avoiding blocking, Coding efficient transactions. Explain CREATE DATABASE syntax Many of us are used to creating databases from the Enterprise Manager or by just issuing the command: CREATE DATABAE MyDB.
Q23. But what if you have to create a database with two filegroups, one on drive C and the other on drive D with log on drive E with an initial size of 600 MB and with a growth factor of 15%?
ü That’s why being a DBA you should be familiar with the CREATE DATABASE syntax. Check out SQL Server books online for more information.
Q24. How to restart SQL Server in single user mode? How to start SQL Server in minimal configuration mode?
ü SQL Server can be started from command line, using the SQLSERVR.EXE. This EXE has some very important parameters with which a DBA should be familiar with. -m is used for starting SQL Server in single user mode and -f is used to start the SQL Server in minimal configuration mode. Check out SQL Server books online for more parameters and their explanations.
Q25. As a part of your job, what are the DBCC commands that you commonly use for database maintenance?
ü DBCC CHECKDB, DBCC CHECKTABLE, DBCC CHECKCATALOG, DBCC CHECKALLOC, DBCC SHOWCONTIG, DBCC SHRINKDATABASE, DBCC SHRINKFILE etc. But there are a whole load of DBCC commands which are very useful for DBAs. Check out SQL Server books online for more information.
Q26. What are statistics, under what circumstances they go out of date, how do you update them?
ü Statistics determine the selectivity of the indexes. If an indexed column has unique values then the selectivity of that index is more, as opposed to an index with non-unique values. Query optimizer uses these indexes in determining whether to choose an index or not while executing a query. Some situations under which you should update statistics: 1) If there is significant change in the key values in the index 2) If a large amount of data in an indexed column has been added, changed, or removed (that is, if the distribution of key values has changed), or the table has been truncated using the TRUNCATE TABLE statement and then repopulated 3) Database is upgraded from a previous version. Look up SQL Server books online for the following commands: UPDATE STATISTICS, STATS_DATE, DBCC SHOW_STATISTICS, CREATE STATISTICS, DROP STATISTICS, sp_autostats, sp_createstats, sp_updatestats
Q27. What are the different ways of moving data/databases between servers and databases in SQL Server?
ü There are lots of options available, you have to choose your option depending upon your requirements. Some of the options you have are: BACKUP/RESTORE, dettaching and attaching databases, replication, DTS, BCP, logshipping, INSERT…SELECT, SELECT…INTO, creating INSERT scripts to generate data.
Q28. Explain different types of BACKUPs avaialabe in SQL Server? Given a particular scenario, how would you go about choosing a backup plan?
ü Types of backups you can create in SQL Sever 7.0+ are Full database backup, differential database backup, transaction log backup, filegroup backup. Check out the BACKUP and RESTORE commands in SQL Server books online. Be prepared to write the commands in your interview. Books online also has information on detailed backup/restore architecture and when one should go for a particular kind of backup.
Q29. What is database replication? What are the different types of replication you can set up in SQL Server?
ü Replication is the process of copying/moving data between databases on the same or different servers. SQL Server supports the following types of replication scenarios: · Snapshot replication · Transactional replication (with immediate updating subscribers, with queued updating subscribers) · Merge replication See SQL Server books online for indepth coverage on replication. Be prepared to explain how different replication agents function, what are the main system tables used in replication etc.
Q30. How to determine the service pack currently installed on SQL Server?
ü The global variable @@Version stores the build number of the sqlservr.exe, which is used to determine the service pack installed. To know more about this process visit SQL Server service packs and versions.
Q31. What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?
ü Cursors allow row-by-row processing of the resultsets. Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. See books online for more information. Disadvantages of cursors: Each time you fetch a row from the cursor, it results in a network roundtrip, where as a normal SELECT query makes only one roundtrip, however large the resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Further, there are restrictions on the SELECT statements that can be used with some types of cursors. Most of the times, set based operations can be used instead of cursors. Here is an example: If you have to give a flat hike to your employees using the following criteria: Salary between 30000 and 40000 — 5000 hike Salary between 40000 and 55000 — 7000 hike Salary between 55000 and 65000 — 9000 hike. In this situation many developers tend to use a cursor, determine each employee’s salary and update his salary according to the above formula. But the same can be achieved by multiple update statements or can be combined in a single UPDATE statement as shown below:
ü UPDATE tbl_emp SET salary = CASE WHEN salary BETWEEN 30000 AND 40000 THEN salary + 5000 WHEN salary BETWEEN 40000 AND 55000 THEN salary + 7000 WHEN salary BETWEEN 55000 AND 65000 THEN salary + 10000 END
ü Another situation in which developers tend to use cursors: You need to call a stored procedure when a column in a particular row meets certain condition. You don’t have to use cursors for this. This can be achieved using WHILE loop, as long as there is a unique key to identify each row. For examples of using WHILE loop for row by row processing, check out the ‘My code library’ section of my site or search for WHILE. Write down the general syntax for a SELECT statements covering all the options. Here’s the basic syntax: (Also checkout SELECT in books online for advanced syntax).
ü SELECT select_list [INTO new_table_] FROM table_source [WHERE search_condition] [GROUP BY group_by_expression] [HAVING search_condition] [ORDER BY order_expression [ASC | DESC] ]
Q32. What is a join and explain different types of joins.
ü Joins are used in queries to explain how different tables are related. Joins also let you select data from a table depending upon data from another table. Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS. For more information see pages from books online titled: "Join Fundamentals" and "Using Joins".
Q33. Can you have a nested transaction?
ü Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and @@TRANCOUNT
Q34. What is an extended stored procedure? Can you instantiate a COM object by using T-SQL?
ü An extended stored procedure is a function within a DLL (written in a programming language like C, C++ using Open Data Services (ODS) API) that can be called from T-SQL, just the way we call normal stored procedures using the EXEC statement. See books online to learn how to create extended stored procedures and how to add them to SQL Server. Yes, you can instantiate a COM (written in languages like VB, VC++) object from T-SQL by using sp_OACreate stored procedure. Also see books online for sp_OAMethod, sp_OAGetProperty, sp_OASetProperty, sp_OADestroy. For an example of creating a COM object in VB and calling it from T-SQL, see ‘My code library’ section of this site.
Q35. What is the system function to get the current user’s user id?
ü USER_ID(). Also check out other system functions like USER_NAME(), SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(), HOST_NAME().
Q36. What are triggers? How many triggers you can have on a table? How to invoke a trigger on demand?
ü Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table. In SQL Server 6.5 you could define only 3 triggers per table, one for INSERT, one for UPDATE and one for DELETE. From SQL Server 7.0 onwards, this restriction is gone, and you could create multiple triggers per each action. But in 7.0 there’s no way to control the order in which the triggers fire. In SQL Server 2000 you could specify which trigger fires first or fires last using sp_settriggerorder. Triggers can’t be invoked on demand. They get triggered only when an associated action (INSERT, UPDATE, DELETE) happens on the table on which they are defined. Triggers are generally used to implement business rules, auditing. Triggers can also be used to extend the referential integrity checks, but wherever possible, use constraints for this purpose, instead of triggers, as constraints are much faster. Till SQL Server 7.0, triggers fire only after the data modification operation happens. So in a way, they are called post triggers. But in SQL Server 2000 you could create pre triggers also. Search SQL Server 2000 books online for INSTEAD OF triggers. Also check out books online for ‘inserted table’, ‘deleted table’ and COLUMNS_UPDATED()
Q37. There is a trigger defined for INSERT operations on a table, in an OLTP system. The trigger is written to instantiate a COM object and pass the newly insterted rows to it for some custom processing. What do you think of this implementation? Can this be implemented better?
ü Instantiating COM objects is a time consuming process and since you are doing it from within a trigger, it slows down the data insertion process. Same is the case with sending emails from triggers. This scenario can be better implemented by logging all the necessary data into a separate table, and have a job which periodically checks this table and does the needful.
Q38. What is a self join? Explain it with an example.
ü Self join is just like any other join, except that two instances of the same table will be joined in the query. Here is an example: Employees table which contains rows for normal employees as well as managers. So, to find out the managers of all the employees, you need a self join.
ü CREATE TABLE emp ( empid int, mgrid int, empname char(10) )
ü INSERT emp SELECT 1,2,’Vyas’ INSERT emp SELECT 2,3,’Mohan’ INSERT emp SELECT 3,NULL,’Shobha’ INSERT emp SELECT 4,2,’Shridhar’ INSERT emp SELECT 5,2,’Sourabh’
ü SELECT t1.empname [Employee], t2.empname [Manager] FROM emp t1, emp t2 WHERE t1.mgrid = t2.empid Here’s an advanced query using a LEFT OUTER JOIN that even returns the employees without managers (super bosses)
ü SELECT t1.empname [Employee], COALESCE(t2.empname, ‘No manager’) [Manager] FROM emp t1 LEFT OUTER JOIN emp t2 ON t1.mgrid = t2.empid
Interview questions for Oracle database administrator
Q) Differentiate between TRUNCATE and DELETE.
The Delete command will log the data changes in the log file where as the truncate will simply remove the data without it. Hence Data removed by Delete command can be rolled back but not the data removed by TRUNCATE. Truncate is a DDL statement whereas DELETE is a DML statement.
Q) What is the maximum buffer size that can be specified using the DBMS_OUTPUT.ENABLE function?
A) 1000000
Q) Can you use a commit statement within a database trigger?
A) Yes, if you are using autonomous transactions in the Database triggers.
Q)What is an UTL_FILE? What are different procedures and functions associated with it?
A)The UTL_FILE package lets your PL/SQL programs read and write operating system (OS) text files. It provides a restricted version of standard OS stream file input/output (I/O).
Subprogram -Description
FOPEN function-Opens a file for input or output with the default line size.
IS_OPEN function -Determines if a file handle refers to an open file.
FCLOSE procedure -Closes a file.
FCLOSE_ALL procedure -Closes all open file handles.
GET_LINE procedure -Reads a line of text from an open file.
PUT procedure-Writes a line to a file. This does not append a line terminator.
NEW_LINE procedure-Writes one or more OS-specific line terminators to a file.
PUT_LINE procedure -Writes a line to a file. This appends an OS-specific line terminator.
PUTF procedure -A PUT procedure with formatting.
FFLUSH procedure-Physically writes all pending output to a file.
FOPEN function -Opens a file with the maximum line size specified.
Q) Difference between database triggers and form triggers?
A) Database triggers are fired whenever any database action like INSERT, UPATE, DELETE, LOGON LOGOFF etc occurs. Form triggers on the other hand are fired in response to any event that takes place while working with the forms, say like navigating from one field to another or one block to another and so on.
Q) What is OCI. What are its uses?
A) OCI is Oracle Call Interface. When applications developers demand the most powerful interface to the Oracle Database Server, they call upon the Oracle Call Interface (OCI). OCI provides the most comprehensive access to all of the Oracle Database functionality. The newest performance, scalability, and security features appear first in the OCI API. If you write applications for the Oracle Database, you likely already depend on OCI. Some types of applications that depend upon OCI are:
· PL/SQL applications executing SQL
· C++ applications using OCCI
· Java applications using the OCI-based JDBC driver
· C applications using the ODBC driver
· VB applications using the OLEDB driver
· Pro*C applications
· Distributed SQL
Q) What are ORACLE PRECOMPILERS?
A) A precompiler is a tool that allows programmers to embed SQL statements in high-level source programs like C, C++, COBOL, etc. The precompiler accepts the source program as input, translates the embedded SQL statements into standard Oracle runtime library calls, and generates a modified source program that one can compile, link, and execute in the usual way. Examples are the Pro*C Precompiler for C, Pro*Cobol for Cobol, SQLJ for Java etc.
Q) What is syntax for dropping a procedure and a function? Are these operations possible?
A) Drop Procedure/Function ; yes, if they are standalone procedures or functions. If they are a part of a package then one have to remove it from the package definition and body and recompile the package.
Q) Can a function take OUT parameters. If not why?
A) yes, IN, OUT or IN OUT.
Q) Can the default values be assigned to actual parameters?
A) Yes. In such case you don’t need to specify any value and the actual parameter will take the default value provided in the function definition.
Q) What is difference between a formal and an actual parameter?
A) The formal parameters are the names that are declared in the parameter list of the header of a module. The actual parameters are the values or expressions placed in the parameter list of the actual call to the module.
Q) What are different modes of parameters used in functions and procedures?
A) There are three different modes of parameters: IN, OUT, and IN OUT.
IN - The IN parameter allows you to pass values in to the module, but will not pass anything out of the module and back to the calling PL/SQL block. In other words, for the purposes of the program, its IN parameters function like constants. Just like constants, the value of the formal IN parameter cannot be changed within the program. You cannot assign values to the IN parameter or in any other way modify its value.
IN is the default mode for parameters. IN parameters can be given default values in the program header.
OUT - An OUT parameter is the opposite of the IN parameter. Use the OUT parameter to pass a value back from the program to the calling PL/SQL block. An OUT parameter is like the return value for a function, but it appears in the parameter list and you can, of course, have as many OUT parameters as you like.
Inside the program, an OUT parameter acts like a variable that has not been initialised. In fact, the OUT parameter has no value at all until the program terminates successfully (without raising an exception, that is). During the execution of the program, any assignments to an OUT parameter are actually made to an internal copy of the OUT parameter. When the program terminates successfully and returns control to the calling block, the value in that local copy is then transferred to the actual OUT parameter. That value is then available in the calling PL/SQL block.
IN OUT - With an IN OUT parameter, you can pass values into the program and return a value back to the calling program (either the original, unchanged value or a new value set within the program). The IN OUT parameter shares two restrictions with the OUT parameter:
An IN OUT parameter cannot have a default value.
An IN OUT actual parameter or argument must be a variable. It cannot be a constant, literal, or expression, since these formats do not provide a receptacle in which PL/SQL can place the outgoing value.
Q) Difference between procedure and function.
A) A function always returns a value, while a procedure does not. When you call a function you must always assign its value to a variable.
Q) Can cursor variables be stored in PL/SQL tables. If yes how. If not why?
A) Yes. Create a cursor type - REF CURSOR and declare a cursor variable of that type.
DECLARE
/* Create the cursor type. */
TYPE company_curtype IS REF CURSOR RETURN company%ROWTYPE;
/* Declare a cursor variable of that type. */
company_curvar company_curtype;
/* Declare a record with same structure as cursor variable. */
company_rec company%ROWTYPE;
BEGIN
/* Open the cursor variable, associating with it a SQL statement. */
OPEN company_curvar FOR SELECT * FROM company;
/* Fetch from the cursor variable. */
FETCH company_curvar INTO company_rec;
/* Close the cursor object associated with variable. */
CLOSE company_curvar;
END;
Q) How do you pass cursor variables in PL/SQL?
A) Pass a cursor variable as an argument to a procedure or function. You can, in essence, share the results of a cursor by passing the reference to that result set.
Q) How do you open and close a cursor variable. Why it is required?
A) Using OPEN cursor_name and CLOSE cursor_name commands. The cursor must be opened before using it in order to fetch the result set of the query it is associated with. The cursor needs to be closed so as to release resources earlier than end of transaction, or to free up the cursor variable to be opened again.
Q) What should be the return type for a cursor variable. Can we use a scalar data type as return type?
A) The return type of a cursor variable can be %ROWTYPE or record_name%TYPE or a record type or a ref cursor type. A scalar data type like number or varchar can’t be used but a record type may evaluate to a scalar value.
Q) What is use of a cursor variable? How it is defined?
A) Cursor variable is used to mark a work area where Oracle stores a multi-row query output for processing. It is like a pointer in C or Pascal. Because it is a TYPE, it is defined as TYPE REF CURSOR RETURN ;
Q) What WHERE CURRENT OF clause does in a cursor?
A) The Where Current Of statement allows you to update or delete the record that was last fetched by the cursor.
Q) Difference between NO DATA FOUND and %NOTFOUND
A) NO DATA FOUND is an exception which is raised when either an implicit query returns no data, or you attempt to reference a row in the PL/SQL table which is not yet defined. SQL%NOTFOUND, is a BOOLEAN attribute indicating whether the recent SQL statement does not match to any row.
Q) What is a cursor for loop?
A) A cursor FOR loop is a loop that is associated with (actually defined by) an explicit cursor or a SELECT statement incorporated directly within the loop boundary. Use the cursor FOR loop whenever (and only if) you need to fetch and process each and every record from a cursor, which is a high percentage of the time with cursors.
Q) What are cursor attributes?
A) Cursor attributes are used to get the information about the current status of your cursor. Both explicit and implicit cursors have four attributes, as shown:
Name Description
%FOUND Returns TRUE if record was fetched successfully, FALSE otherwise.
%NOTFOUND Returns TRUE if record was not fetched successfully, FALSE otherwise.
%ROWCOUNT Returns number of records fetched from cursor at that point in time.
%ISOPEN Returns TRUE if cursor is open, FALSE otherwise.
Q) Difference between an implicit & an explicit cursor.
A) The implicit cursor is used by Oracle server to test and parse the SQL statements and the explicit cursors are declared by the programmers.
Q) What is a cursor?
A) A cursor is a mechanism by which you can assign a name to a “select statement” and manipulate the information within that SQL statement.
Q) What is the purpose of a cluster?
A) A cluster provides an optional method of storing table data. A cluster is comprised of a group of tables that share the same data blocks, which are grouped together because they share common columns and are often used together. For example, the EMP and DEPT table share the DEPTNO column. When you cluster the EMP and DEPT, Oracle physically stores all rows for each department from both the EMP and DEPT tables in the same data blocks. You should not use clusters for tables that are frequently accessed individually.
Q) How do you find the number of rows in a Table ?
A) select count(*) from table, or from NUM_ROWS column of user_tables if the table statistics has been collected.
Q) Display the number value in Words?
A)
Q) What is a pseudo column. Give some examples?
A) Information such as row numbers and row descriptions are automatically stored by Oracle and is directly accessible, ie. not through tables. This information is contained within pseudo columns. These pseudo columns can be retrieved in queries. These pseudo columns can be included in queries which select data from tables.
Available Pseudo Columns
· ROWNUM - row number. Order number in which a row value is retrieved.
· ROWID - physical row (memory or disk address) location, ie. unique row identification.
· SYSDATE - system or today’s date.
· UID - user identification number indicating the current user.
· USER - name of currently logged in user.
Q) How you will avoid your query from using indexes?
A) By changing the order of the columns that are used in the index, in the Where condition, or by concatenating the columns with some constant values.
Q) What is a OUTER JOIN?
A) An OUTER JOIN returns all rows that satisfy the join condition and also returns some or all of those rows from one table for which no rows from the other satisfy the join condition.
Q) Which is more faster - IN or EXISTS?
A) Well, the two are processed very differently.
Select * from T1 where x in ( select y from T2 )
is typically processed as:
select *
from t1, ( select distinct y from t2 ) t2
where t1.x = t2.y;
The sub query is evaluated, distinct’ed, indexed (or hashed or sorted) and then joined to the original table — typically. As opposed to
select * from t1 where exists ( select null from t2 where y = x )
That is processed more like:
for x in ( select * from t1 )
loop
if ( exists ( select null from t2 where y = x.x )
then
OUTPUT THE RECORD
end if
end loop
It always results in a full scan of T1 whereas the first query can make use of an index on T1(x). So, when is where exists appropriate and in appropriate? Lets say the result of the sub query ( select y from T2 ) is “huge” and takes a long time. But the table T1 is relatively small and executing ( select null from t2 where y = x.x ) is very fast (nice index on t2(y)). Then the exists will be faster as the time to full scan T1 and do the index probe into T2 could be less then the time to simply full scan T2 to build the sub query we need to distinct on.
Lets say the result of the sub query is small — then IN is typically more appropriate. If both the sub query and the outer table are huge — either might work as well as the other — depends on the indexes and other factors.
Q) When do you use WHERE clause and when do you use HAVING clause?
A) The WHERE condition lets you restrict the rows selected to those that satisfy one or more conditions. Use the HAVING clause to restrict the groups of returned rows to those groups for which the specified condition is TRUE.
Q) There is a % sign in one field of a column. What will be the query to find it?
A) SELECT column_name FROM table_name WHERE column_name LIKE ‘%\%%’ ESCAPE ‘\’;
Q) What is difference between SUBSTR and INSTR?
A) INSTR function search string for sub-string and returns an integer indicating the position of the character in string that is the first character of this occurrence. SUBSTR function return a portion of string, beginning at character position, substring_length characters long. SUBSTR calculates lengths using characters as defined by the input character set.
Q) Which data type is used for storing graphics and images?
A) Raw, Long Raw, and BLOB.
Q) What is difference between SQL and SQL*PLUS?
A) SQL is the query language to manipulate the data from the database. SQL*PLUS is the tool that lets to use SQL to fetch and display the data.
Q) What is difference between UNIQUE and PRIMARY KEY constraints?
A) An UNIQUE key can have NULL whereas PRIMARY key is always not NOT NULL. Both bears unique values.
Q) What is difference between Rename and Alias?
A) Rename is actually changing the name of an object whereas Alias is giving another name (additional name) to an existing object.
What are various joins used while writing SUBQUERIES?
IN, NOT IN, IN ANY, IN ALL, EXISTS, NOT EXISTS.
