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What are constraints in SQL Server?

Posted By :Syed Shakeer Hussain     Posted Date :April 23, 2009    Points :10   Category :Sql Server 
Constraints enforce the data integrity to the database and protect columns of the table from unwanted values. NOT NULL, CHECK, UNIQUE, PRIMARY KEY, FOREIGN KEY are the types of contraints define in SQL Server.

You can also find related Interview Question to What are constraints in SQL Server?  below: 

Explain Constraints in SQL SERVER?

Data integrity is an important feature in SQL Server. When used properly, it ensures that data is accurate, correct, and valid. It also acts as a trap for otherwise undetectable bugs within applications.


A PRIMARY KEY constraint is a unique identifier for a row within a database table. Every table should
have a primary key constraint to uniquely identify each row and only one primary key constraint can be created for each table. The primary key constraints are used to enforce entity integrity.

UNIQUE constraint:

A UNIQUE constraint enforces the uniqueness of the values in a set of columns, so no duplicate values are entered. The unique key constraints are used to enforce entity integrity as the primary key

FOREIGN KEY constraint:

A FOREIGN KEY constraint prevents any actions that would destroy links between tables with the corresponding data values. A foreign key in one table points to a primary key in another table. Foreign
keys prevent actions that would leave rows with foreign key values when there are no primary keys with that value. The foreign key constraints are used to enforce referential integrity.

CHECK constraint:

A CHECK constraint is used to limit the values that can be placed in a column. The check constraints are used to enforce domain integrity. A NOT NULL constraint enforces that the column will not accept null values. The not null constraints are used to enforce domain integrity, as the check constraints. (More...)

Define Constraints in SQL SERVER

Constraints Provide the Mechanism provide integrity in the data base to create a table to check some conditions.

There are Three type of constraints in sql server

1) Domain Integtity Constraint(not null,check)
2) Entity Integrity Constrinnt( Primary key ,Unique Key)
3) Referential Integrity Constraint(Parent table,Child Table Relationship)..

hope this will Hellp..
Let me know any Queries

Sriram.R (More...)

Database Concurrency in Sql Server.

When two or more user try to update same type of data in a table then Database Concurrency helps in this situation.

There are 2 types of Database Concurrency

Pessimistic: When one user try to change the data with pessimistic concurrency then a lock is placed on the data in a table so that another user cannot change data. when one user finishes then other user can able to change.

Optimistic: When two user works on the same data in a table and one change that data first then second user cannot change that same data becasue the which he have using is allready changed so he cannot do the change becasue change apply to another data that is changed by first user. (More...)

Is it true that COM objects no longer need to be registered on the server?

Yes and No. Legacy COM objects still need to be registered on the server before they can be used. COM developed using the new .NET Framework will not need to be registered. Developers will be able to auto-register these objects just by placing them in the 'bin' folder of the application. (More...)

What are server controls?

ASP.NET server controls are components that run on the server and encapsulate user-interface and other related functionality. They are used in ASP.NET pages and in ASP.NET code-behind classes. (More...)

Which authentication mode lets you use both SQL Server logins and Windows logins?

Mixed Mode authentication mode lets you use both SQL Server logins and Windows logins? (More...)

What are the types of indexes available with SQL Server?

There are basically two types of indexes that we use with the SQL Server. Clustered and the Non-Clustered. (More...)

What is the use of CASCADE CONSTRAINTS?

When this clause is used with the DROP command, a parent table can be dropped even when a child table exists. (More...)

What is the difference between Server. Transfer and Response. Redirect?

The Transfer method allows you to transfer from inside one ASP page to another ASP page. All of the state information that has been created for the first (calling) ASP page will be transferred to the second (called) ASP page. This transferred information includes all objects and variables that have been given a value in an Application or Session scope, and all items in the Request collections. For example, the second ASP page will have the same SessionID as the first ASP page.

When the second (called) ASP page completes its tasks, you do not return to the first (calling) ASP page. All these happen on the server side browser is not aware of this.
The redirect message issue HTTP 304 to the browser and causes browser to got the specified page. Hence there is round trip between client and server. Unlike transfer, redirect doesn't pass context information to the called page.

Shashi Ray (More...)

How many types of tables are present in SQL SERVER?

There are 2 types of temporary tables, local and global in sql server.

Local temporary tables are created using a single pound (#) sign and are visible to a single connection and automatically dropped when that connection ends.

Global temporary tables are created using a double pound (##) sign and are visible across multiple connections and users and are automatically dropped when all SQL sessions stop referencing the global temporary table. (More...)

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