The 'ALTER' statement. is used to alter the structure of a table. It is a 'Data Definition Langauge' (DDL) command.
The ALTER Statement
The 'ALTER" statement modifies the structure of a table by adding/modifying/deleting column(s) to it. It can also
be used to add or drop a constraint on a table. Since it is DDL command, it is irrevocable.
SQL ALTER Syntax and Example
Alter table tablename add columnname datatype
Alter table Employees add JoinDate smalldatetime
The above command adds a new column to an existing table
Alter table tablename drop column columnname
Alter table Employees drop column JoinDate
The above command drops a column to an existing table
Alter table tablename Add constraint constraintname Primary key (columnname)
Alter table Designation Add constraint DesigPrimary Primary key (DesignationID)
The above command adds a primary key on a column in an existing table
Alter table tablename drop constraint constraintname
Alter table Designation drop constraint DesigPrimary
The above command drops a constraint on an existing table
Alter table tablename Alter column columnname datatype
Alter table Employees Alter column PhoneNo int
The above command changes the datatype of a column in an existing table