ALTER TABLE.md

SQL ALTER TABLE Statement

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.

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ALTER TABLE - ADD Column

To add a column in a table, use the following syntax:

ALTER TABLE table_name
ADD column_name datatype;

ALTER TABLE - DROP COLUMN

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:

Example

ALTER TABLE Customers
DROP COLUMN Email;

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ALTER TABLE - RENAME COLUMN

To rename a column in a table, use the following syntax:

ALTER TABLE table_name
RENAME COLUMN old_name to new_name;

ALTER TABLE - ALTER/MODIFY DATA TYPE

To change the data type of a column in a table, use the following syntax:

ALTER TABLE table_name
MODIFY COLUMN column_name datatype;

Change Data Type Example

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.

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DROP COLUMN Example

Next, we want to delete the column named "DateOfBirth" in the "Persons" table.

We use the following SQL statement:

LINKS

[[DDL (Data Definition Language)]]