SQL SERVER – Interview Questions and Answers – Frequently Asked Questions – Day 7 of 31

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What are Different Types of Locks?

• Shared Locks: Used for operations that do not change or update data (read-only operations), such as a SELECT statement.
• Update Locks: Used on resources that can be updated. It prevents a common form of deadlock that occurs when multiple sessions are reading, locking, and potentially updating resources later.
• Exclusive Locks: Used for data-modification operations, such as INSERT, UPDATE, or DELETE. It ensures that multiple updates cannot be made to the same resource at the same time.
• Intent Locks: Used to establish a lock hierarchy. The types of intent locks are as follows: intent shared (IS), intent exclusive (IX), and shared with intent exclusive (SIX).
• Schema Locks: Used when an operation dependent on the schema of a table is executing. The types of schema locks are schema modification (Sch-M) and schema stability (Sch-S).
• Bulk Update Locks: Used when bulk-copying data into a table and the TABLOCK hint is specified.

What are Pessimistic Lock and Optimistic Lock?

Optimistic Locking is a strategy where you read a record, take note of a version number and check that the version hasn’t changed before you write the record back. If the record is dirty (i.e. different version to yours), then you abort the transaction and the user can re-start it.

Pessimistic Locking is when you lock the record for your exclusive use until you have finished with it. It has much better integrity than optimistic locking but requires you to be careful with your application design to avoid Deadlocks.

When is the use of UPDATE_STATISTICS command?

This command is basically used when a large amount of data is processed. If a large amount of deletions, modifications 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.

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. (Read more here)

What is Connection Pooling and why it is Used?

To minimize the cost of opening and closing connections, ADO.NET uses an optimization technique called connection pooling.

The pooler maintains ownership of the physical connection. It manages connections by keeping alive a set of active connections for each given connection configuration. Whenever a user calls Open on a connection, the pooler looks for an available connection in the pool. If a pooled connection is available, it returns it to the caller instead of opening a new connection. When the application calls Close on the connection, the pooler returns it to the pooled set of active connections instead of closing it. Once the connection is returned to the pool, it is ready to be reused on the next Open call.

What are the Properties and Different Types of Sub-Queries?

Properties of a Sub-Query

• A sub-query must be enclosed in the parenthesis.
• A sub-query must be put on the right hand of the comparison operator, and
• A sub-query cannot contain an ORDER BY clause, however sub-query can use ORDER BY when used with TOP clause. Read Comment by David Bridge
  
• A query can contain more than one sub-query.

Types of Sub-query

• Single-row sub-query, where the sub-query returns only one row.
• Multiple-row sub-query, where the sub-query returns multiple rows, and
• Multiple column sub-query, where the sub-query returns multiple columns

What is an 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 very 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.

What are the Authentication Modes in SQL Server? How can it be Changed?

There are two authentication modes in SQL Server.

• Windows Mode
• Mixed Mode – SQL and Windows

To change authentication mode in SQL Server, go to 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.

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Reference: Pinal Dave (https://blog.sqlauthority.com)