The primary job of a constraint is to enforce a rule in the database. Together, the constraints in a database maintain the integrity of the database. SQL Constraints. NOT NULL - Ensures that a column cannot have a NULL value. UNIQUE - Ensures that all values in a column are different. PRIMARY KEY - A combination of a NOT NULL and UNIQUE. FOREIGN KEY - Uniquely identifies a row/record in another table. CHECK - Ensures that all values in a column satisfies a specific. This SQL Server tutorial explains how to use the check constraints in SQL Server (Transact-SQL) with syntax and examples. A check constraint in SQL Server.


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Constraint names must follow the rules for identifiersexcept that the constraints in sql server cannot start with a number sign. Columns that are of the ntext, text, varchar maxnvarchar maxvarbinary maxxml, or image data types cannot be specified as columns for an index.

The default is ASC. User-specified fillfactor values can be from 1 through If a value is not specified, the default is 0.


Specifies the storage location of the index created for the constraint. Below is create constraints in sql server query in which column TotalMarks is set as Default to and MarksObtained column check the data to be more than Whereas the second insert command gets failed as MarksObtained are less than As total constraints in sql server column is specified as default we need not put the data while using insert command.

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 constraints in sql server key constraints are used to enforce entity integrity. A different syntax allows you to create a table constraint.

Unlike a column constraint, a table constraint is able to enforce a rule across multiple columns.


We will see an example of using multiple columns when we build a special CHECK constraint later in the article. Notice there is now a comma after the ProductName column definition.

SQL Server Constraints with Example :

Most significant primary key is 'Hamburger'. MsgLevel 16, State 1, Line 1 Could not create constraint. The statement has been terminated.

Check Constraints Check constraints contain an expression the database will evaluate when constraints in sql server modify or insert a row. If the expression evaluates to false, the database will not save the row.

SQL Server Constraints with Example

You can use check constraints in sql server to implement business rules, and tighten down the allowed values and formats allowed for a particular column. We can use the same three techniques we learned earlier to create a check constraint using SQL.

The first technique places the constraint after the column definition, as shown below. Note the constraint name is optional for unique and check constraints. The following sample creates the constraint as a table constraint, separate from the column definitions.

The logical expression would be the following: This allows for multiple conditions to be checked in one location. The difference is in how they determine which values are valid:

  • Constraints in SQL Server |
  • Unique Constraints and Check Constraints | Microsoft Docs
  • SQL Constraints
  • Unique Constraints and Check Constraints
  • ALTER TABLE table_constraint (Transact-SQL)
  • CHECK Constraints
  • Constraints in SQL Server

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