Change a table definition

Changing the definition of a table allows you to add new columns, change an existing column definition (change its length, default value, and so on), drop existing columns, and add and remove constraints.

Table definitions are changed using the SQL ALTER TABLE statement.

You can add, change, or drop columns and add or remove constraints all with one ALTER TABLE statement. However, a single column can be referenced only once in the ADD COLUMN, ALTER COLUMN, and DROP COLUMN clauses. That is, you cannot add a column and then alter that column in the same ALTER TABLE statement.

Add a column
When you add a new column to a table, the column is initialized with its default value for all existing rows. If NOT NULL is specified, a default value must also be specified.
Change a column
You can change a column definition in a table using the ALTER COLUMN clause of the ALTER TABLE statement.
Allowable conversions
When you change the data type of an existing column, the old and new attributes must be compatible.
Delete a column
You can delete a column using the DROP COLUMN clause of the ALTER TABLE statement.
Order of operations for ALTER TABLE statement
An ALTER TABLE statement is performed as this set of steps shows.

Parent topic: Data definition language (DDL)

Related information

ALTER TABLE