SQL Server - Common Interview Questions and Answers

  1. What is difference between DELETE and TRUNCATE commands?
    Delete command removes the rows from a table based on the condition that we provide with a WHERE clause. Truncate will actually remove all the rows from a table and there will be no data in the table after we run the truncate command.
    1. TRUNCATE:
      1. TRUNCATE is faster and uses fewer system and transaction log resources than DELETE.
      2. TRUNCATE 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.
      3. TRUNCATE removes all rows from a table, but the table structure, its columns, constraints, indexes and so on, remains. The counter used by an identity for new rows is reset to the seed for the column.
      4. You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY constraint. Because TRUNCATE TABLE is not logged, it cannot activate a trigger.
      5. TRUNCATE cannot be rolled back.
      6. TRUNCATE is DDL Command.
      7. TRUNCATE Resets identity of the table
    2. DELETE:
      1. DELETE removes rows one at a time and records an entry in the transaction log for each deleted row.
      2. 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.
      3. DELETE Can be used with or without a WHERE clause
      4. DELETE Activates Triggers.
      5. DELETE can be rolled back.
      6. DELETE is DML Command.
      7. DELETE does not reset identity of the table.

    Note: DELETE and TRUNCATE both can be rolled back when surrounded by TRANSACTION if the current session is not closed. If TRUNCATE is written in Query Editor surrounded by TRANSACTION and if session is closed, it can not be rolled back but DELETE can be rolled back.

  2. When is the use of UPDATE_STATISTICS command?
    This command is basically used when a large processing of data has occurred. If a large amount of deletions any modification or Bulk Copy into the tables has occurred, it has to update the indexes to take these changes into account. UPDATE_STATISTICS updates the indexes on these tables accordingly.
  3. What is the difference between a HAVING CLAUSE and a WHERE CLAUSE?
    They specify a search condition for a group or an aggregate. But the difference is that HAVING can be used only with the SELECT statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVING behaves like a WHERE clause. Having Clause is basically used only with the GROUP BY function in a query whereas WHERE Clause is applied to each row before they are part of the GROUP BY function in a query.
  4. What are the properties and different Types of Sub-Queries?
    1. Properties of Sub-Query
      1. A sub-query must be enclosed in the parenthesis.
      2. A sub-query must be put in the right hand of the comparison operator, and
      3. A sub-query cannot contain an ORDER-BY clause.
      4. A query can contain more than one sub-query.
    2. Types of Sub-Query
      1. Single-row sub-query, where the sub-query returns only one row.
      2. Multiple-row sub-query, where the sub-query returns multiple rows,. and
      3. Multiple column sub-query, where the sub-query returns multiple columns
  5. What is SQL Profiler?

    SQL Profiler is a graphical tool that allows system administrators to monitor events in an instance of Microsoft SQL Server. You can capture and save data about each event to a file or SQL Server table to analyze later. For example, you can monitor a production environment to see which stored procedures are hampering performances by executing too slowly.

    Use SQL Profiler to monitor only the events in which you are interested. If traces are becoming too large, you can filter them based on the information you want, so that only a subset of the event data is collected. Monitoring too many events adds overhead to the server and the monitoring process and can cause the trace file or trace table to grow very large, especially when the monitoring process takes place over a long period of time.

  6. What are the authentication modes in SQL Server? How can it be changed?
    Windows mode and Mixed Mode - SQL and Windows. To change authentication mode in SQL Server click Start, Programs, Microsoft SQL Server and click SQL Enterprise Manager to run SQL Enterprise Manager from the Microsoft SQL Server program group. Select the server then from the Tools menu select SQL Server Configuration Properties, and choose the Security page.