Not Null Constraint

NOT NULL is integrity constraint

NOT NULL constraints in Microsoft SQL Server allow you to specify that a column may not contain NULL values.

By default, a table column can hold NULL values.

USE AdventureWorksGO
-- NOT NULL Constraint when Table is created
CREATE TABLE ConstraintTable
(ID INT, ColSecond INT NOT NULL)

GO

-- NOT NULL Constraint after Table is created
ALTER TABLE ConstraintTable
ALTER COLUMN ID INT NOT NULL

GO

SQL Server checks the column’s current contents for any NULL values. If the column currently contains NULL values, the constraint creation fails.

Altering a column: null to not null

You will have to do it in two steps:

1.     Update the table so that there are no nulls in the column.  

      UPDATE MyTable SET MyNullableColumn = 0 WHERE MyNullableColumn IS NULL

2.     Alter the table to change the property of the column

     ALTER TABLE MyTable

     ALTER COLUMN MyNullableColumn Datatype NOT NULL