How to auto increment on unique value - mysql

Question

Let's say I have this table, with both columns as primary keys, in mysql workbench and Innodb engine:
+--------+---------+
| grp | name |
+--------+---------+
| fish | lax |
| mammal | dog |
| mammal | bat |
| mammal | whale |
| bird | bat |
| bird | ostrich |
+--------+---------+
How can I add a column, behaving like this with grp column:
+--------+----+---------+
| grp | id | name |
+--------+----+---------+
| fish | 1 | lax |
| mammal | 2 | dog |
| mammal | 2 | cat |
| mammal | 2 | whale |
| bird | 3 | penguin |
| bird | 3 | ostrich |
+--------+----+---------+
Note that the actual table I want to alter is much bigger. Also note this is not what auto incrementing with MyISAM does and not what is asked here(but the answers could be helpful): How to auto increment on different foreign keys?

Answer 1

CREATE TABLE Groups (
id INT UNSIGNED NOT NULL AUTO_INCREMENT,
grp VARCHAR...
PRIMARY KEY(id),
UNIQUE(grp)
) ENGINE=InnoDB
SELECT DISTINCT grp FROM YourTable;
That should give you the mapping between grp and id.
At this point, you should not generate a table with 3 columns (grp+id+name), but instead use the above Groups as a "normalization", which you seem to be doing. Then do
CREATE TABLE New LIKE YourTable;
ALTER TABLE New
ADD COLUMN id INT UNSIGNED NOT NULL,
DROP COLUMN grp;
# Note: if you have an index on grp, something else may need to be done.
INSERT INTO New
SELECT g.id, y.name
FROM Groups g
JOIN YourTable y ON y.grp = g.grp;
# At this point, verify that `New` has what you want. Then switch over:
RENAME TABLE YourTable TO Old, New TO YourTable;
DROP TABLE Old;
Edit I chose to create a new table rather than update the existing table -- because it will be (I think) considerably faster. With UPDATE, all the rows would need to be locked. With INSERT..SELECT, they are simply created.