ALTER TABLE statement
Use the ALTER TABLE
statement to change the structure of an existing table in a database file. You can modify only one column in each statement. The formats of the ALTER TABLE
statement are:
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ALTER TABLE table_name ADD [COLUMN] column_definition
ALTER TABLE table_name DROP [COLUMN] unqualified_column_name
ALTER TABLE table_name ALTER [COLUMN] column_definition SET DEFAULT expr
ALTER TABLE table_name ALTER [COLUMN] column_definition DROP DEFAULT
You must know the table’s structure and how you want to modify it before using the ALTER TABLE
statement.
To |
Sample SQL |
Add columns |
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|
Remove columns |
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|
Set the default value for a column |
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|
Remove the default value for a column |
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|
Note SET DEFAULT
and DROP DEFAULT
do not affect existing rows in the table, but change the default value for rows that are subsequently added to the table.