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ALTER TABLE
Syntax
ALTER [IGNORE] TABLE tbl_name alter_specification [, alter_specification] ... alter_specification: ADD [COLUMN] create_definition [FIRST | AFTER column_name ] | ADD [COLUMN] (create_definition, create_definition,...) | ADD INDEX [index_name] [index_type] (index_col_name,...) | ADD [CONSTRAINT [symbol]] PRIMARY KEY [index_type] (index_col_name,...) | ADD [CONSTRAINT [symbol]] UNIQUE [index_name] [index_type] (index_col_name,...) | ADD FULLTEXT [index_name] (index_col_name,...) | ADD [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (index_col_name,...) [reference_definition] | ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT} | CHANGE [COLUMN] old_col_name create_definition [FIRST | AFTER column_name] | MODIFY [COLUMN] create_definition [FIRST | AFTER column_name] | DROP [COLUMN] col_name | DROP PRIMARY KEY | DROP INDEX index_name | DISABLE KEYS | ENABLE KEYS | RENAME [TO] new_tbl_name | ORDER BY col | CHARACTER SET character_set_name [COLLATE collation_name] | table_options |
ALTER TABLE
allows you to change the structure of an existing table.
For example, you can add or delete columns, create or destroy indexes, change
the type of existing columns, or rename columns or the table itself. You can
also change the comment for the table and type of the table.
See section CREATE TABLE
.
If you use ALTER TABLE
to change a column specification but
DESCRIBE tbl_name
indicates that your column was not changed, it is
possible that MySQL ignored your modification for one of the reasons
described in 13.2.5.1 Silent Column Specification Changes. For example, if you try to change
a VARCHAR
column to CHAR
, MySQL will still use
VARCHAR
if the table contains other variable-length columns.
ALTER TABLE
works by making a temporary copy of the original table.
The alteration is performed on the copy, then the original table is
deleted and the new one is renamed. This is done in such a way that
all updates are automatically redirected to the new table without
any failed updates. While ALTER TABLE
is executing, the original
table is readable by other clients. Updates and writes to the table
are stalled until the new table is ready.
Note that if you use any other option to ALTER TABLE
than
RENAME
, MySQL will always create a temporary table, even
if the data wouldn't strictly need to be copied (such as when you change the
name of a column). We plan to fix this in the future, but as one doesn't
normally do ALTER TABLE
that often this isn't that high on our TODO.
For MyISAM tables, you can speed up the index recreation part (which is the
slowest part of the recreation process) by setting the
myisam_sort_buffer_size
variable to a high value.
ALTER TABLE
, you need ALTER
, INSERT
,
and CREATE
privileges on the table.
IGNORE
is a MySQL extension to SQL-92.
It controls how ALTER TABLE
works if there are duplicates on
unique keys in the new table.
If IGNORE
isn't specified, the copy is aborted and rolled back.
If IGNORE
is specified, then for rows with duplicates on a unique
key, only the first row is used; the others are deleted.
ADD
, ALTER
, DROP
, and
CHANGE
clauses in a single ALTER TABLE
statement. This is a
MySQL extension to SQL-92, which allows only one of each clause
per ALTER TABLE
statement.
CHANGE col_name
, DROP col_name
, and DROP
INDEX
are MySQL extensions to SQL-92.
MODIFY
is an Oracle extension to ALTER TABLE
.
COLUMN
is a pure noise word and can be omitted.
ALTER TABLE tbl_name RENAME TO new_name
without any other
options, MySQL simply renames the files that correspond to the table
tbl_name
. There is no need to create the temporary table.
See section RENAME TABLE
.
create_definition
clauses use the same syntax for ADD
and
CHANGE
as for CREATE TABLE
. Note that this syntax includes
the column name, not just the column type.
See section CREATE TABLE
.
CHANGE old_col_name create_definition
clause. To do so, specify the old and new column names and the type that
the column currently has. For example, to rename an INTEGER
column
from a
to b
, you can do this:
mysql> ALTER TABLE t1 CHANGE a b INTEGER; |
If you want to change a column's type but not the name, CHANGE
syntax still requires an old and new column name, even if they are the same.
For example:
mysql> ALTER TABLE t1 CHANGE b b BIGINT NOT NULL; |
However, as of MySQL Version 3.22.16a, you can also use MODIFY
to change a column's type without renaming it:
mysql> ALTER TABLE t1 MODIFY b BIGINT NOT NULL; |
CHANGE
or MODIFY
to shorten a column for which
an index exists on part of the column (for instance, if you have an index
on the first 10 characters of a VARCHAR
column), you cannot make
the column shorter than the number of characters that are indexed.
CHANGE
or MODIFY
,
MySQL tries to convert data to the new type as well as possible.
FIRST
or
ADD ... AFTER col_name
to add a column at a specific position
within a table row. The default is to add the column last.
From MySQL Version 4.0.1, you can also use the FIRST
and
AFTER
keywords in CHANGE
or MODIFY
.
ALTER COLUMN
specifies a new default value for a column
or removes the old default value.
If the old default is removed and the column can be NULL
, the new
default is NULL
. If the column cannot be NULL
, MySQL
assigns a default value, as described in
CREATE TABLE
.
DROP INDEX
removes an index. This is a MySQL extension to
SQL-92. See section 13.2.7 DROP INDEX
Syntax.
DROP TABLE
instead.
DROP PRIMARY KEY
drops the primary index. (Prior to MySQL 4.1.2,
if no primary index exists, DROP PRIMARY KEY
drops the first
UNIQUE
index in the table.
MySQL marks the first UNIQUE
key as the PRIMARY KEY
if no PRIMARY KEY
was specified explicitly.)
If you add a UNIQUE INDEX
or PRIMARY KEY
to a table, this
is stored before any not UNIQUE
index so that MySQL can detect
duplicate keys as early as possible.
ORDER BY
allows you to create the new table with the rows in a
specific order. Note that the table will not remain in this order after
inserts and deletes. In some cases, it might make sorting easier for
MySQL if the table is in order by the column that you want to
order it by later. This option is mainly useful when you know that you
are mostly going to query the rows in a certain order; by using this
option after big changes to the table, you might be able to get higher
performance.
ALTER TABLE
on a MyISAM
table, all non-unique
indexes are created in a separate batch (like in REPAIR
).
This should make ALTER TABLE
much faster when you have many indexes.
ALTER TABLE ... DISABLE KEYS
makes MySQL to stop updating
non-unique indexes for MyISAM
table.
ALTER TABLE ... ENABLE KEYS
then should be used to re-create missing
indexes. As MySQL does it with a special algorithm which is much
faster then inserting keys one by one, disabling keys could give a
considerable speedup on bulk inserts.
mysql_info()
, you can find out how many
records were copied, and (when IGNORE
is used) how many records were
deleted due to duplication of unique key values.
FOREIGN KEY
, CHECK
, and REFERENCES
clauses don't
actually do anything, except for InnoDB type tables which support
... ADD [CONSTRAINT [symbol]] FOREIGN KEY (...) REFERENCES ... (...)
and ... DROP FOREIGN KEY ...
.
See section 14.4.5.2 FOREIGN KEY
Constraints.
The syntax for other table types is provided only for compatibility,
to make it easier to port code from other SQL servers and to run applications
that create tables with references.
See section 1.8.5 MySQL Differences Compared to SQL-92.
ALTER TABLE
ignores the DATA DIRECTORY
and INDEX
DIRECTORY
table options.
ALTER TABLE table_name CHARACTER SET character_set_name; |
Note that the following command will change only the default character
set
for a table:
ALTER TABLE table_name DEFAULT CHARACTER SET character_set_name; |
The default character set
is the character set that is used if
you don't specify the character set for a new column you add to a table
(for example with ALTER TABLE ... ADD column
).
Here is an example that shows some of the uses of ALTER TABLE
. We
begin with a table t1
that is created as shown here:
mysql> CREATE TABLE t1 (a INTEGER,b CHAR(10)); |
To rename the table from t1
to t2
:
mysql> ALTER TABLE t1 RENAME t2; |
To change column a
from INTEGER
to TINYINT NOT NULL
(leaving the name the same), and to change column b
from
CHAR(10)
to CHAR(20)
as well as renaming it from b
to
c
:
mysql> ALTER TABLE t2 MODIFY a TINYINT NOT NULL, CHANGE b c CHAR(20); |
To add a new TIMESTAMP
column named d
:
mysql> ALTER TABLE t2 ADD d TIMESTAMP; |
To add an index on column d
, and make column a
the primary key:
mysql> ALTER TABLE t2 ADD INDEX (d), ADD PRIMARY KEY (a); |
To remove column c
:
mysql> ALTER TABLE t2 DROP COLUMN c; |
To add a new AUTO_INCREMENT
integer column named c
:
mysql> ALTER TABLE t2 ADD c INT UNSIGNED NOT NULL AUTO_INCREMENT, ADD INDEX (c); |
Note that we indexed c
, because AUTO_INCREMENT
columns must be
indexed, and also that we declare c
as NOT NULL
, because
indexed columns cannot be NULL
.
When you add an AUTO_INCREMENT
column, column values are filled in
with sequence numbers for you automatically. You can set the first
sequence number by executing SET INSERT_ID=value
before
ALTER TABLE
or using the AUTO_INCREMENT=value
table option.
See section 7.5.6 SET
Syntax.
With MyISAM tables, if you don't change the AUTO_INCREMENT
column, the sequence number will not be affected. If you drop an
AUTO_INCREMENT
column and then add another AUTO_INCREMENT
column, the numbers will start from 1 again.
See section A.7.1 Problems with ALTER TABLE
..
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