Tuesday 5 April 2011

SQL Server 2008 Interivew Questions 2

  1. Difference between char and nvarchar / char and varchar data-type?
    char[(n)] - Fixed-length non-Unicode character data with length of n bytes. n must be a value from 1 through 8,000. Storage size is n bytes. The SQL-92 synonym for char is character.
    nvarchar(n) - Variable-length Unicode character data of n characters. n must be a value from 1 through 4,000. Storage size, in bytes, is two times the number of characters entered. The data entered can be 0 characters in length. The SQL-92 synonyms for nvarchar are national char varying and national character varying.
    varchar[(n)] - Variable-length non-Unicode character data with length of n bytes. n must be a value from 1 through 8,000. Storage size is the actual length in bytes of the data entered, not n bytes. The data entered can be 0 characters in length. The SQL-92 synonyms for varchar are char varying or character varying.
  2. GUID datasize?
  3. How GUID becoming unique across machines?
    To ensure uniqueness across machines, the ID of the network card is used (among others) to compute the number.
  4. What is the difference between text and image data type?
    Text and image. Use text for character data if you need to store more than 255 characters in SQL Server 6.5, or more than 8000 in SQL Server 7.0. Use image for binary large objects (BLOBs) such as digital images. With text and image data types, the data is not stored in the row, so the limit of the page size does not apply.All that is stored in the row is a pointer to the database pages that contain the data.Individual text, ntext, and image values can be a maximum of 2-GB, which is too long to store in a single data row.

  5. What are joins?
    Sometimes we have to select data from two or more tables to make our result complete. We have to perform a join.
  6. How many types of Joins?
    Joins can be categorized as:
    • Inner joins (the typical join operation, which uses some comparison operator like = or <>). These include equi-joins and natural joins.
      Inner joins use a comparison operator to match rows from two tables based on the values in common columns from each table. For example, retrieving all rows where the student identification number is the same in both the students and courses tables.
    • Outer joins. Outer joins can be a left, a right, or full outer join.
      Outer joins are specified with one of the following sets of keywords when they are specified in the FROM clause:
      • LEFT JOIN or LEFT OUTER JOIN -The result set of a left outer join includes all the rows from the left table specified in the LEFT OUTER clause, not just the ones in which the joined columns match. When a row in the left table has no matching rows in the right table, the associated result set row contains null values for all select list columns coming from the right table.
      • RIGHT JOIN or RIGHT OUTER JOIN - A right outer join is the reverse of a left outer join. All rows from the right table are returned. Null values are returned for the left table any time a right table row has no matching row in the left table.
      • FULL JOIN or FULL OUTER JOIN - A full outer join returns all rows in both the left and right tables. Any time a row has no match in the other table, the select list columns from the other table contain null values. When there is a match between the tables, the entire result set row contains data values from the base tables.
    • Cross joins - Cross joins return all rows from the left table, each row from the left table is combined with all rows from the right table. Cross joins are also called Cartesian products. (A Cartesian join will get you a Cartesian product. A Cartesian join is when you join every row of one table to every row of another table. You can also get one by joining every row of a table to every row of itself.)
  1. What is self join?
    A table can be joined to itself in a self-join.
  2. What are the differences between UNION and JOINS?
    A join selects columns from 2 or more tables. A union selects rows.
  3. Can I improve performance by using the ANSI-style joins instead of the old-style joins?
    Code Example 1:
    select o.name, i.name
    from sysobjects o, sysindexes i
    where o.id = i.id
    Code Example 2:
    select o.name, i.name
    from sysobjects o inner join sysindexes i
    on o.id = i.id
    You will not get any performance gain by switching to the ANSI-style JOIN syntax.
    Using the ANSI-JOIN syntax gives you an important advantage: Because the join logic is cleanly separated from the filtering criteria, you can understand the query logic more quickly.
    The SQL Server old-style JOIN executes the filtering conditions before executing the joins, whereas the ANSI-style JOIN reverses this procedure (join logic precedes filtering).
    Perhaps the most compelling argument for switching to the ANSI-style JOIN is that Microsoft has explicitly stated that SQL Server will not support the old-style OUTER JOIN syntax indefinitely. Another important consideration is that the ANSI-style JOIN supports query constructions that the old-style JOIN syntax does not support.
  4. What is derived table?
    Derived tables are SELECT statements in the FROM clause referred to by an alias or a user-specified name. The result set of the SELECT in the FROM clause forms a table used by the outer SELECT statement. For example, this SELECT uses a derived table to find if any store carries all book titles in the pubs database:
    SELECT ST.stor_id, ST.stor_name
    FROM stores AS ST,
         (SELECT stor_id, COUNT(DISTINCT title_id) AS title_count
          FROM sales
          GROUP BY stor_id
         ) AS SA
    WHERE ST.stor_id = SA.stor_id
    AND SA.title_count = (SELECT COUNT(*) FROM titles)

  5. What is Stored procedure?
    A stored procedure is a set of Structured Query Language (SQL) statements that you assign a name to and store in a database in compiled form so that you can share it between a number of programs.
    • They allow modular programming.
    • They allow faster execution.
    • They can reduce network traffic.
    • They can be used as a security mechanism.
  6. What are the different types of Storage Procedure?
    1. Temporary Stored Procedures - SQL Server supports two types of temporary procedures: local and global. A local temporary procedure is visible only to the connection that created it. A global temporary procedure is available to all connections. Local temporary procedures are automatically dropped at the end of the current session. Global temporary procedures are dropped at the end of the last session using the procedure. Usually, this is when the session that created the procedure ends. Temporary procedures named with # and ## can be created by any user.
    2. System stored procedures are created and stored in the master database and have the sp_ prefix.(or xp_) System stored procedures can be executed from any database without having to qualify the stored procedure name fully using the database name master. (If any user-created stored procedure has the same name as a system stored procedure, the user-created stored procedure will never be executed.)
    3. Automatically Executing Stored Procedures - One or more stored procedures can execute automatically when SQL Server starts. The stored procedures must be created by the system administrator and executed under the sysadmin fixed server role as a background process. The procedure(s) cannot have any input parameters.
    4. User stored procedure
  1. How do I mark the stored procedure to automatic execution?
    You can use the sp_procoption system stored procedure to mark the stored procedure to automatic execution when the SQL Server will start. Only objects in the master database owned by dbo can have the startup setting changed and this option is restricted to objects that have no parameters.
    USE master
    EXEC sp_procoption 'indRebuild', 'startup', 'true')
  2. How can you optimize a stored procedure?
  3. How will know whether the SQL statements are executed?
    When used in a stored procedure, the RETURN statement can specify an integer value to return to the calling application, batch, or procedure. If no value is specified on RETURN, a stored procedure returns the value 0.  The stored procedures return a value of 0 when no errors were encountered. Any nonzero value indicates an error occurred.
  4. Why one should not prefix user stored procedures with sp_?
    It is strongly recommended that you do not create any stored procedures using sp_ as a prefix. SQL Server always looks for a stored procedure beginning with sp_ in this order:
    1. The stored procedure in the master database.
    2. The stored procedure based on any qualifiers provided (database name or owner).
    3. The stored procedure using dbo as the owner, if one is not specified.
Therefore, although the user-created stored procedure prefixed with sp_ may exist in the current database, the master database is always checked first, even if the stored procedure is qualified with the database name.
  1. What can cause a Stored procedure execution plan to become invalidated and/or fall out of cache?
    1. Server restart
    2. Plan is aged out due to low use
    3. DBCC FREEPROCCACHE (sometime desired to force it)
  2. When do one need to recompile stored procedure?
    if a new index is added from which the stored procedure might benefit, optimization does not automatically happen (until the next time the stored procedure is run after SQL Server is restarted).
  3. SQL Server provides three ways to recompile a stored procedure:
    • The sp_recompile system stored procedure forces a recompile of a stored procedure the next time it is run.
    • Creating a stored procedure that specifies the WITH RECOMPILE option in its definition indicates that SQL Server does not cache a plan for this stored procedure; the stored procedure is recompiled each time it is executed. Use the WITH RECOMPILE option when stored procedures take parameters whose values differ widely between executions of the stored procedure, resulting in different execution plans to be created each time. Use of this option is uncommon, and causes the stored procedure to execute more slowly because the stored procedure must be recompiled each time it is executed.
    • You can force the stored procedure to be recompiled by specifying the WITH RECOMPILE option when you execute the stored procedure. Use this option only if the parameter you are supplying is atypical or if the data has significantly changed since the stored procedure was created.
  1. How to find out which stored procedure is recompiling? How to stop stored procedures from recompiling?
  2. I have Two Stored Procedures SP1 and SP2 as given below. How the Transaction works, whether SP2 Transaction succeeds or fails?
    EXEC SP2

    commit tran

    Both will get roll backed.
        BEGIN TRAN
        commit tran

    Both will get roll backed.
  4. How will you handle Errors in Sql Stored Procedure?
    INSERT NonFatal VALUES (@Column2)
    IF @@ERROR <>0
      PRINT 'Error Occured'
  5. How will you raise an error in sql?
    RAISERROR - Returns a user-defined error message and sets a system flag to record that an error has occurred. Using RAISERROR, the client can either retrieve an entry from the sysmessages table or build a message dynamically with user-specified severity and state information. After the

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