SQL Alter Table

The SQL Server Alter Table Statement is used to alter Tables. The Alter Table includes Add New Columns, Modifying existing Columns, Deleting existing Column, Add or Removing Indexes, Dropping and adding Constraints like Primary Keys to the existing table.

SQL Alter Table examples

The following are some of the examples of Alter Table to add, modify and delete columns and constraints. Let us see how to use the SQL Alter Table statement to alter the table definition with a live example. For this SQL Query, We use the Customer table.

SQL Alter Table Add New Column

To add a new column to the existing table, please use the following syntax

-- SQL Alter table Add New Column
ALTER TABLE [Table_Name]
ADD [New_Column_Name] Data_Type (Length) NULL | NOT NULL

We are going to add the Education Column to the existing table customers

-- SQL Alter table Add New Column
ALTER TABLE [Customer]

ADD Education VARCHAR (50) NULL

From the above code SQL Server snippet, the Education Column belongs to varchar data type, the data length is 50, and it will allow NULL Values.

Alter Table Delete Column

To delete or drop the existing column from the SQL table, Please use the following syntax

-- SQL Alter table Drop Column
ALTER TABLE [Table_Name]
DROP COLUMN [Column_Name]

We will delete the existing column called Profession from the customer table.

-- SQL Alter table Delete Column
ALTER TABLE [Customer]

DROP COLUMN Profession

SQL Alter Table Change Column Data Type

To changer modify the existing columns Data Type in a table, use the following syntax

-- SQL Server Alter table change Column data type
ALTER TABLE [Table_Name]
ALTER COLUMN [Column_Name] Data_Type NULL |NOT NULL

Here, we change the Education column data type from Varchar to Nvarchar and length from 50 to 75

-- SQL Server Alter table change data type
ALTER TABLE [Customer]
ALTER COLUMN Education NVARCHAR(75)

To alter the NULL functionality to restrict the NULL or allow NULLS, use the following code.

-- SQL Server Alter table change Column data type
ALTER TABLE [Customer]
ALTER COLUMN Education NVARCHAR(75) NOT NULL

We successfully changed the data type, length, and NULL handling of the existing column present in the SQL Table.

Alter Table Add primary key

To add Primary key and a foreign key to existing columns, please use the following SQL Server alter table add constraint syntax

-- SQL Alter Table Add primary key
ALTER TABLE [Customer]
ADD CONSTRAINT [Constraint_Name] PRIMARY KEY ([Column_Name])

Add a Primary Key constraint to the Customer Key Column

-- SQL Alter Table Add primary key
ALTER TABLE [Customer]
ADD CONSTRAINT PrimaryKey PRIMARY KEY ( [CustomerKey] )

Alter Table Drop Constraint

To delete or Drop primary keys and foreign key on existing columns, use the Alter Table Drop constraint syntax

-- SQL Server ALTER Table Drop Constraint
ALTER TABLE [Customer]
DROP CONSTRAINT [Constraint_Name]

Delete the previously created Primary Key constraint on the Customer Key Column.

-- SQL Server ALTER Table Drop Constraint ALTER TABLE [Customer] DROP CONSTRAINT PrimaryKey