11.13 Sequencing an Unsequenced Table
11.13.1 Problem
You forgot to include an
AUTO_INCREMENT column when you created a table. Is
it too late?
11.13.2 Solution
No, just add one using
ALTER TABLE. MySQL
will create the column and number the rows automatically.
11.13.3 Discussion
To add a sequence to a table that doesn't currently
contain one, use ALTER TABLE to
create an AUTO_INCREMENT column. Suppose you have
a table t that contains name
and age columns, but no sequence column:
+----------+------+
| name | age |
+----------+------+
| boris | 47 |
| clarence | 62 |
| abner | 53 |
+----------+------+
You can add a sequence column named id to the
table as follows:
mysql> ALTER TABLE t
-> ADD id INT NOT NULL AUTO_INCREMENT,
-> ADD PRIMARY KEY (id);
mysql> SELECT * FROM t ORDER BY id;
+----------+------+----+
| name | age | id |
+----------+------+----+
| boris | 47 | 1 |
| clarence | 62 | 2 |
| abner | 53 | 3 |
+----------+------+----+
MySQL numbers the rows for you automatically. It's
not necessary to assign the values yourself. Very handy.
By default, ALTER TABLE adds
new columns to the end of the table. To place a column at a specific
position, use FIRST or AFTER at
the end of the ADD clause. The following
ALTER TABLE statements are
similar to the one just shown, but would place the
id column first in the table or after the
name column, respectively:
ALTER TABLE t
ADD id INT NOT NULL AUTO_INCREMENT FIRST,
ADD PRIMARY KEY (id);
ALTER TABLE t
ADD id INT NOT NULL AUTO_INCREMENT AFTER name,
ADD PRIMARY KEY (id);
For MyISAM tables, you can specify the
initial value for a new sequence column by including an
AUTO_INCREMENT =
n clause in the ALTER
TABLE statement:
mysql> ALTER TABLE t
-> ADD id INT NOT NULL AUTO_INCREMENT FIRST,
-> ADD PRIMARY KEY (id),
-> AUTO_INCREMENT = 100;
mysql> SELECT * FROM t ORDER BY id;
+-----+----------+------+
| id | name | age |
+-----+----------+------+
| 100 | boris | 47 |
| 101 | clarence | 62 |
| 102 | abner | 53 |
+-----+----------+------+
|
