Changes the data type of a table’s column.

Example: In the ‘users’ table, make the column ‘incept_date’ into a

‘datetime’ type.

ALTER TABLE users

ALTER COLUMN incept_date datetime;

ALTER TABLE table_name 

ALTER COLUMN column_name new_data_type(size);

​

#SQL Server

​

ALTER TABLE Customers

ALTER COLUMN age VARCHAR(2);

​

#MySQL

​

ALTER TABLE Customers

MODIFY COLUMN age VARCHAR(2);

​

#Oracle

​

ALTER TABLE Customers

MODIFY age VARCHAR(2);

​

#PostgreSQL

​

ALTER TABLE Customers

ALTER COLUMN age TYPE VARCHAR(2);

-- Change column type

ALTER TABLE table_name

ALTER COLUMN some_col VARCHAR(250);

​

-- Change column name

ALTER TABLE table_name

RENAME COLUMN old_column_name TO new_column_name;

​

-- Change column values

UPDATE table_name

SET col_name=LOWER(col_name);

​

-- Add column

ALTER TABLE table_name

ADD COLUMN new_col INTEGER;

​

-- DELETE column

ALTER TABLE table_name

DROP COLUMN col_name;

ALTER TABLE table_name MODIFY column_name datatype NOT NULL;

ALTER TABLE table_name MODIFY COLUMN column_name datatype;

ALTER TABLE table_name 

ADD CONSTRAINT MyUniqueConstraint CHECK (CONDITION);

ALTER TABLE table_name 

ADD CONSTRAINT MyPrimaryKey PRIMARY KEY (column1, column2...);

-- With check if field already exists

​

IF NOT EXISTS(SELECT 1 FROM sys.columns 

          WHERE Name = N'Email'

          AND Object_ID = Object_ID(N'dbo.Customers'))

BEGIN    

    ALTER TABLE Customers

    ADD Email varchar(255);

END

ALTER TABLE table_name 

DROP PRIMARY KEY;