SQL Database Concepts Interview Questions And Answers

HAVING CLAUSE

- HAVING CLAUSE is used only with the SELECT statement.
- It is generally used in a GROUP BY clause in a query.
- If GROUP BY is not used, HAVING works like a WHERE clause.

WHERE Clause

- It is applied to each row before they become a part of the GROUP BY function in a query.

- The optimizer available in SQL Server optimizes the code to be effectively executed.
- A query execution plan shows how this optimizer would run the query.
- Query execution plan can be viewed by :
- Using the Show Execution Plan option available in Query Analyzer,
- Displaying Estimated Execution Plan on the query dropdown menu,
- Use the SET SHOWPLAN_TEXT ON command before running a query and capturing the execution plan event in a SQL Server Profiler trace.

- Using transactions we can group all SQL commands into a single unit.
- The transaction begins with some task and finishes only when all tasks within it are over.
- The transaction gets over successfully only when all commands in it are successfully over. Even if one command fails, the whole transaction fails.
- The BEGIN TRANSACTION, ROLLBACK TRANSACTION, and COMMIT TRANSACTION statements are used to work with transactions.
- A group of tasks starts with the begin statement.
- In case of any problem, the rollback command is executed to abort the transaction.
- If all the tasks run successfully, all commands are executed through commit statement.

Recursion is method of problem solving where the solution is arrived at by repetitively applying the logic and solution to the subsets of the problem.

Transact-SQL supports recursion. So, yes it is possible for a stored procedure to call itself.

Stored procedures and managed code references can be nested up to 32 levels.

a.) An execution plan can be called as a road map that graphically or textually shows the data retrieval methods which have been chosen by the SQL
Server query optimizer, for a stored procedure or ad- hoc query.

b.) It is used because it is a very useful tool for a developer to understand the performance characteristics of a query or stored procedure.

c.) There exists an option called "Show Execution Plan" in Query Analyzer. If this option is turned on, it will display query execution plan in separate window when the query is run again.

- When the processing of large data is done, this command is used.
- Whenever large number of deletions, modification or copy takes place into the tables, the indexes need to be updated to take care of these changes. UPDATE_STATISTICS performs this job.

- A view is a virtual table that consists of fields from one or more real tables.
- It is usually used to join multiple tables and get the data.
- The WITH CHECK OPTION for a view prevents any modification to the data that does not confirm to the WHERE clause of the view definition.
- This allows the data belonging to the view to be updated through the view.

- It is a Windows service which handles the tasks scheduled within the SQL Server environment. These tasks are also called as job and are stored with in SQL server. The jobs may run through a trigger, a predefined schedule or on demand.
- This service is very useful in determining why a particular job did not run as intended.

- By default, clustered index on the column are created by the primary key whereas nonclustered index are created by unique key.
- Primary key doesn't allow NULLs, but unique key allows one NULL.

- They help in reducing the network traffic and latency which in turn boosts application performance.
- They help in promoting code reuse.
- They provide better security to data.
- It is possible to encapsulate the logic using stored procedures. This allows to change stored procedure code without affecting clients.
- It is possible to reuse stored procedure execution plans, which are cached in SQL Server's memory. This reduces server overhead.

a.) One-to-One relationship - can be implemented as a single table and rarely as two tables with primary and foreign key relationships.

b.) One-to-Many relationships - by splitting the data into two tables with primary key and foreign key relationships.

c.) Many-to-Many - by using a junction table with the keys from both the tables forming the composite primary key of the junction table.

- Truncate can not be rolled back while Delete can be.
- Truncate keeps the lock on table while Delete keeps the lock on each row.
- Truncate resets the counter of the Identity column while Delete doesn't do so.
- Trigger is not fired in Truncate while it happens in Delete.

Relational tables have six properties:
1. Values are atomic.
2. Column values are of the same kind.
3. Each row is unique.
4. The sequence of columns is insignificant.
5. The sequence of rows is insignificant.
6. Each column must have a unique name.

Collation is a type of sort order. There are mainly three types of sort orders, namely:
i.) Dictionary case sensitive
ii.)Dictionary - case insensitive
iii.)Binary.

- It is a set of T-SQL statements combined together to perform a single task formed by combining many small tasks.
- When you actually run a Stored procedure, a set of statements is run.

- ACID (Atomicity Consistency Isolation Durability) is a quality sought after in a reliable database. Here's the relevance of each quality:
- Atomicity is an all-or-none proposition.
- Consistency - it guarantees that your database is never left by a transaction in a half-finished state.
- Isolation - it keeps transactions separated from each other until they’re finished.
- Durability - it ensures that the database keeps a track of pending changes in a way that the server can recover from an abnormal termination.

These are the buffer pages that contain modifications which have not been written to disk.

- It is the process of copying and cleaning data from heterogeneous sources.
- It is an important part of development projects for data warehousing and business intelligence.

- 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.

- Global temporary tables (created with a double “##”) are visible to all sessions.
- Global temporary tables are dropped when the session that created it ends, and all other sessions have stopped referencing it.

There are 3 kinds of locks in SQL Server

i.) Shared locks - they are used for operations which do not allow any change or update of data. For e.g. SELECT.

ii.) Update locks - they are used when SQL Server wants to modify a page. The update page lock is then promoted to an exclusive page lock before actually making the changes.

iii.) Exclusive locks - they are used for the data modification operations. For e.g. UPDATE, INSERT, or DELETE.

Data regions are report items used to display data from a single dataset. You can perform grouping, sorting and various aggregate functions on data in data region. In SSRS 2005, there were 4 data regions:-
1. Table
2. Matrix
3. List
4. Chart
While in SSRS 2008, there are one additional data region namely Gauge.
Let’s explain each one of them:

1. Table - Table Data region has fixed tabular structure i.e. fixed number of columns. It is useful for displaying data grouped by row. You can have maximum of 1 report item per cell. The size of table depends on number of rows dataset fetches i.e., if number of rows returned by dataset is more; it can expand to multiple pages.

2. Matrix – A matrix data region display data in pivot table format, hence also popularly known as pivot table or crosstab report. It has minimum of one row group and one column group. The size of matrix data region depends on columns and rows fetched.

3. List - A list data region is free layout. It is useful for complex reporting resign. The list can be used to display multiple table and matrix. Each getting data from different dataset.

4. Chart – This data region is for displays the data graphically i.e., in form of chart. A various chart types are available in SSRS 2008 namely line, pie chart, columns etc.

5. Gauge - This can be used in a table or matrix to show the relative value of a field in a range of values in the data region. You can also add a gauge to the design surface to show a single relative value.

There are various ways in which you can enhance your report:

1. Display your data in graphic format using Chart Region.
2. Use sorting.
3. If couple of reports are related, you can make them interactive using connect them using bookmark link, hyper link or drill through report link.
4. Adding sub-report. Sub-report is a stand-alone report which can be link to another report based on its content using parameter.
5. Add custom fields. Custom fields provide with same functionality as alias columns provide in SQL server query. It is the timing of the operation that differs from the alias columns. The calculation is performed on dataset by report server.
6. Using expression.
7. Using custom code. SSRS allows including custom code written in VB.Net. 8. Add document map (navigational links to report item once report is rendered) to report.

The following are various aggregate functions available:-
1. SUM
2. AVG
3. COUNT
4. COUNTDISTINCT
5. MAX
6. MIN
7. STDEV
8. STDEVP
9. VAR
10. VARP

By default, SUM is the aggregate function used for numeric data type.

There are 3 ways in which you can integrate reports into your application:-
1. Navigating to URL i.e. https:\\servername\reportservername\reportname – This is simplest and most popular way. A separate login might be required since we are directly calling the report from report server. Address of report server gets expose to user.
2. Using IFrame, Browser control or Report Viewer Control – In this approach, we embed the URL of report server in our application, hence address of reportserver is not exposed. No separate window opens. A user does not come to know that he has moved to different server.
3. Programmatically sending a web request using SOAP to report server.

Custom fields can be defined as alias column of the report since the operation is performed on report server rather than on database server. The custom field is very useful for the data manipulation like adding some fields whose value can be calculated based on expression, text e.g. instead of CName fetched from database, I want the dataset to display Customer Name etc.

We can add custom fields as right click on dataset, select add in Dataset window. The New field dialog box will open, we can add name of custom field and also mention whether it is database field or calculated one. If it is calculated, then we can mention the computation in this window.