Home Next Previous SQL ALTER TABLE Statement The ALTER TABLE Statement is used :To Add a new Column in an existing table.To Delete the existing column from a table.To Modify the column data type.To Add a new constraint on an existing table.To drop a constraint from a table. Syntax:              ALTER TABLE  table_name           ADD column_name datatype; Example:  Objective: In this example, we will add a new column Dept to to the Emp table.           ALTER TABLE Tbl_Emp          Add Dept varchar(20); Syntax:              ALTER TABLE  table_name           DROP COLUMN column_name ; Example:  Objective: In this example, we will drop dept column from the emp table.           ALTER TABLE Tbl_Emp          DROP COLUMN Dept ; Syntax:              ALTER TABLE  table_name           ALTER COLUMN column_name datatype ; Example:  Objective: In this example, we will change/modify the datatype of dept column from varchar to nvarchar.           ALTER TABLE Tbl_Emp          ALTER COLUMN Dept nvarchar(20); Syntax:             ALTER TABLE  table_name          ADD CONSTRAINT ConstraintName          UNIQUE (column_name); Example:         ALTER TABLE Tbl_Emp         ADD CONSTRAINT unique_const         UNIQUE (Dept) Syntax:             ALTER TABLE  table_name          ADD CONSTRAINT ConstraintName          CHECK (condition); Example:         ALTER TABLE Tbl_Emp         ADD CONSTRAINT check_const         CHECK (Len(Dept)>5); Syntax:             ALTER TABLE  table_name          ADD CONSTRAINT ConstraintName          Primary Key (column_name); Example:         ALTER TABLE Tbl_Emp         ADD CONSTRAINT pk_const         PRIMARY KEY (ecode); Syntax:             ALTER TABLE  table_name          ADD CONSTRAINT ConstraintName          Foreign Key (column_name) REFERENCES table_name(column_name); Example:         ALTER TABLE Tbl_Emp         ADD CONSTRAINT fk_const         FOREIGN KEY (ecode) REFERENCES Tbl_Project(Ecode)