Sequencing an Unsequenced Table
Problem
You forgot to include a sequence column when you created a table. Is it too late to sequence the table rows?
Solution
No, just add an
AUTO_INCREMENT
column using ALTER
TABLE
. MySQL will create the column and
number the rows automatically.
Discussion
To add a sequence to a table that doesn’t currently contain one,
use ALTER
TABLE
to create an AUTO_INCREMENT
column. Suppose that 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 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 ...
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