NOT NULL Constraint

NULL is unknown or missing data in table.

When you create a new NOT NULL constraint on a column, SQL Server checks the column’s current contents for any NULL values. If the column currently contains NULL values, the constraint creation fails. Otherwise, SQL Server adds the NOT NULL constraint and any future INSERT or UPDATE commands that would cause the existence of a NULL value fail.

Example

CREATE TABLE Employee

(

EID INT PRIMARY KEY IDENTITY NOT NULL,

ENAME VARCHAR(50) NOT NULL,

SALARY DECIMAL(10,2) NOT NULL,

DEPT VARCHAR(50)

)

INSERT INTO Employee(ENAME,SALARY,DEPT)

VALUES('abc',20000,'HR')

The following SQL enforces the "EID" column and the "ENAME, SALARY " column to not accept NULL values:

INSERT INTO Employee(ENAME,DEPT)

VALUES('pqr','HR')

Msg 515, Level 16, State 2, Line 1
Cannot insert the value NULL into column 'SALARY', table 'TaskTrack.dbo.Employee'; column does not allow nulls. INSERT fails.
The statement has been terminated.

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