Top SQL Database Concepts Interview Questions & Answers:

1. What is buffer cash and log Cache in sql server?

Buffer Cache: Buffer cache is a memory pool in which data pages are read. It performance of the buffer cache is indicated as follows: 95% indicates that pages that were found in the memory are 95% of time. Another 5% is needed for physical disk access. If the value falls below 90%, it is the indication of more physical memory requirement on the server.

Log Caches: Log cache is a memory pool used to read and write the log pages. A set of cache pages are available in each log cache. The synchronization is reduced between log and data buffers by managing log caches separately from the buffer cache.

2. Do you know what is a Trace frag? Where do we use it?

Temporary setting of specific server characteristics is done by trace tags. DBCC TRACEON is the command to set the trace flags. Once activated, trace flag will be in effect until the server is restarted. Trace frags are frequently used for diagnosing performance issues.

For example, the trace flag 3205 is used for disabling hard compression for tape drives, when an instance of SQL Server starts.

3. Explain difference between cross join and Full outer join?

Cross Join :
No join conditions are specified.
Results in pairs of rows.
Results in Cartesian product of two tables.

Full Outer Join:
A combination of both left and right outer joins.
Results in every row from both of the tables , at least once.
Assigns NULL for unmatched fields.

4. Do you know how to send email from database?

SQL Server has a feature for sending mail. Stored procedures can also be used for sending mail on demand. With SQL Server 2005, MAPI client is not needed for sending mails.

The following is the process for sending emails from database.

- Make sure that the SQL Server Mail account is configured correctly and enable Database Mail.

- Write a script to send an e-mail. The following is the script.

USE [YourDB]
EXEC msdb.dbo.sp_send_dbmail
@recipients = 'xyz@xyz.com; xyz@xyz.com;abc@edf.com'
@body = ' A warm wish for your future endeavor',
@subject = 'This mail was sent using Database Mail' ;
GO

5. Do you know how to make remote connection in database?

The following is the process to make a remote connection in database:

Use SQL Server Surface Area Configuration Tool for enabling the remote connection in database.
Click on Surface Area Configuration for Services and Connections.
Click on SQLEXPRESS/Database Engine/RemoteConnections
Select the radio button: Local and Remote Connections and select ‘Using TCP/IP only' under Local and Remote Connections.
Click on OK button / Apply button

6. Explain how to use Linked Server?

MS SQL Server supports the connection to different OLE DB on an ad hoc basis. This persistent connection is referred as Linked Server.

The following are the steps to use Linked Server for any OLE DB. I refer this to use an MS-Excel workbook.

Open SQL Server Management Studio in SQL Server 2005
Expand Server Objects in Object Explorer.
Right-click on Linked Servers. Click on New Linked Server.
Select General page in the left pane and
i. Type any name for the linked server in the first text box
ii. Select the Other Data Source option.
iii. Click on Microsoft Jet 4.0 OLE DB Provider from the Provider list.
iv. Type the Excel as the name of the OLE DB data source.
v. Type the full path and file name of the Excel file in Data Source box.
vi. Type the Excel version no. (7.0, 8.0 etc) in the Provider String. Use Excel 8.0 for Excel 2000, Excel 2002 or Excel 97.
vii. To create a linked server click on OK.

7. Do you know concepts and capabilities of SQL Server?

Microsoft SQL server is a relational database management system. It uses MS- SQL as the query language. SQL Server offers a high level of security, reliability and scalability depending on the business needs. The server offers a wide data storage, full text query search, buffer management, logging and transaction, fast data retrieval etc. it offers a variety of replication services to avoid loosing data. It offers SQL Server Reporting Services for data gathered from the database.

8. Tell me what is the order in which the SQL query is executed?

The following is the order of executing SQL query:

The query goes to the shared pool that has information like parse tree and execution plan for the corresponding statement.

Then validates the SQL statement and validates the source(table).

Acquire locks.

Checks all the privileges.

Execute the query.

Fetch the values for SELECT statement

Displays the fetched values.

To sum up, the sequence is:

SELECT .........
FROM ..........
WHERE ..........
GROUP BY ...........
HAVING .............

9. How to store pdf file in sql server?

Create a column as type ‘blob' in a table. Read the content of the file and save in ‘blob' type column in a table.

Or store them in a folder and establish the pointer to link them in the database.

10. Tell me what is the STUFF and how does it differ from the REPLACE function?

Both STUFF and REPLACE are used to replace characters in a string.

select replace('abcdef','ab','xx') results in xxcdef

select replace('defdefdef','def','abc') results in abcabcabc
We cannot replace a specific occurrence of “def” using REPLACE.

select stuff('defdefdef',4, 3,'abc') results in defabcdef

where 4 is the character to begin replace from and 3 is the number of characters to replace.