SQL Alter Table

The Alter Table statement is used to modify Tables. The SQL Server Alter Table includes Add New Columns, Modifying existing Columns, Deleting existing Columns, Adding or Removing Indexes, and 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 this statement to change the table definition with a live example. For this Query, We use the Customer.

Alter Table Add New Column

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

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

ALTER TABLE [Customer]

ADD Education VARCHAR (50) NULL

From the above code Server snippet, the Education Column belongs to the 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 table, Please use the following syntax.

ALTER TABLE [Table_Name]
DROP COLUMN [Column_Name]

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

ALTER TABLE [Customer]

DROP COLUMN Profession

Change Column Data Type

To Alter or modify the existing columns Data Type in a table, use the following syntax.

-- 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

-- change data type
ALTER TABLE [Customer]
ALTER COLUMN Education NVARCHAR(75)

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

-- 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.

Add primary key

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

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

Add a Primary Key constraint to the Customer Key Column

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

Drop Constraint

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

-- Drop Constraint
ALTER TABLE [Customer]
DROP CONSTRAINT [Constraint_Name]

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

--  Drop Constraint 
ALTER TABLE [Customer] DROP CONSTRAINT PrimaryKey