MySQL Alter table add column that is unique together with another column

Question

I have such a table:

+---------+--------------+------+-----+-------------------+-----------------------------+
| Field   | Type         | Null | Key | Default           | Extra                       |
+---------+--------------+------+-----+-------------------+-----------------------------+
| id      | int(11)      | NO   | PRI | NULL              | auto_increment              |
| url     | varchar(255) | YES  | UNI | NULL              |                             |
| ts      | timestamp    | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
| content | longblob     | YES  |     | NULL              |                             |
| source  | varchar(255) | YES  |     | NULL              |                             |
| state   | int(11)      | NO   |     | 0                 |                             |
+---------+--------------+------+-----+-------------------+-----------------------------+

I'd like the id to stay the only PRIMARY KEY and I'd like to add field "VERSION" which will be unique.

What I want is to create unique pair (url, version) unique together but not separately. How can I do that? Should I add field version just like that, alter url so it's not unique and then add constraint?

Thanks in advance!

Answer 1

If what you're looking for is to store multiple versions of the same URL together in the table, then yes, what you need to do is:

1. Drop the unique constraint on URL
2. Add non-unique column version (assume integer here)
3. Create unique constraint or index on (url, version). I would suggest an index since I think that should make the unique checks faster.