The ALTER TABLE
statement is used to add, delete, or modify columns in an existing table.
The ALTER TABLE
statement is also used to add and drop various constraints on
an existing table.
To add a column in a table, use the following syntax:
ALTER TABLE table_name
ADD column_name datatype;
The following SQL adds an "Email" column to the "Customers" table:
ALTER TABLE Customers
ADD Email varchar(255);
To delete a column in a table, use the following syntax (notice that some database systems don't allow deleting a column):
ALTER TABLE table_name
DROP COLUMN column_name;
The following SQL deletes the "Email" column from the "Customers" table:
ALTER TABLE Customers
DROP COLUMN Email;
To change the data type of a column in a table, use the following syntax:
SQL Server / MS Access:
ALTER TABLE table_name
ALTER COLUMN column_name datatype;
My SQL / Oracle (prior version 10G):
ALTER TABLE table_name
MODIFY COLUMN column_name datatype;
Oracle 10G and later:
ALTER TABLE table_name
MODIFY column_name datatype;
Now we want to change the data type of the column named "DateOfBirth" in the "Persons" table.
We use the following SQL statement:
ALTER TABLE Persons
ALTER COLUMN DateOfBirth year;
Notice that the "DateOfBirth" column is now of type year and is going to hold a year in a two- or four-digit format.
Next, we want to delete the column named "DateOfBirth" in the "Persons" table.
We use the following SQL statement:
ALTER TABLE Persons
DROP COLUMN DateOfBirth;