SQL Server- Basic Questions

What are different types of Collation Sensitivity?

• Case sensitivity - A and a, B and b, etc.
• Accent sensitivity
• Kana Sensitivity - When Japanese kana characters Hiragana and Katakana are treated differently, it is called Kana sensitive.
• Width sensitivity - A single-byte character (half-width) and the same character represented as a double-byte character (full-width) are treated differently than it is width sensitive.

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.

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.

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

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.

What are the authentication modes in SQL Server? How can it be changed?

• Windows mode
• SQL Authentication
• 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.

What is Log Shipping?

• Log shipping is the process of automating the backup of database and transaction log files on a production SQL server, and then restoring them onto a standby server. 
• Enterprise Editions only supports log shipping. In log shipping the transactional log file from one server is automatically updated into the backup database on the other server. 
• If one server fails, the other server will have the same db and can be used this as the Disaster Recovery plan. The key feature of log shipping is that it will automatically backup transaction logs throughout the day and automatically restore them on the standby server at defined interval.

What does it mean to have QUOTED_IDENTIFIER ON? What are the implications of having it OFF?

• When SET QUOTED_IDENTIFIER is ON, identifiers can be delimited by double quotation marks, and literals must be delimited by single quotation marks.
• When SET QUOTED_IDENTIFIER is OFF, identifiers cannot be quoted and must follow all Transact-SQL rules for identifiers.

What is the difference between a Local and a Global temporary table?

1.    A local temporary table exists only for the duration of a connection or, if defined inside a compound statement, for the duration of the compound statement.

2.    A global temporary table remains in the database permanently, but the rows exist only within a given connection. When connection is closed, the data in the global temporary table disappears. However, the table definition remains with the database for access when database is opened next time.

 What is an execution plan? When would you use it? How would you view the execution plan?

• An execution plan is basically a road map that graphically or textually shows the data retrieval methods chosen by the SQL Server query optimizer for a stored procedure or ad- hoc query.
• Its a very useful tool for a developer to understand the performance characteristics of a query or stored procedure since the plan is the one that SQL Server will place in its cache and use to execute the stored procedure or query. 
• From within Query Analyzer is an option called "Show Execution Plan" (located on the Query drop-down menu). If this option is turned on it will display query execution plan in separate window when query is ran again.

What is BCP? When does it used?

• BulkCopy is a tool used to copy huge amount of data from tables and views. 
• BCP does not copy the structures same as source to destination. 
• BULK INSERT command helps to import a data file into a database table or view in a user-specified format.

What is a Scheduled Jobs or What is a Scheduled Tasks?

• Scheduled tasks let user automate processes that run on regular or predictable cycles. 
• User can schedule administrative tasks, such as cube processing, to run during times of slow business activity.
•  User can also determine the order in which tasks run by creating job steps within a SQL Server Agent job. E.g. back up database, Update Stats of Tables.
•  Job steps give user control over flow of execution. If one job fails, user can configure SQL Server Agent to continue to run the remaining tasks or to stop execution.