setting MySQL auto_increment to be dependent on two other primary keys

Question

i'm trying to set up a MySQL database for storing biological data. I have to extract this data from a file and i have a perl script for that. The problem i have is that i need three primary keys in order for them to be unique, and i want one of them to be an auto increment integer. I would like, however, the auto-incremented value to reset each time the combination of the first two keys changes.

sequence1 | hit1      | 1
sequence1 | hit1      | 2
sequence1 | hit2      | 1
sequence2 | something | 1
sequence2 | something | 2
sequence2 | something | 3
sequence3 | something | 1

etc. etc.

is that possible or do i have to implement that directly into the script?

thank you

Answer 1

It is possible with MyISAM tables only and will not work in InnoDB or any other storage engine MySQL has.

Just create a primary key on (col1, col2, id) and set auto_increment flag on id column. And make sure there is no unique constraint on id alone. MyISAM will generate a new sequence of values per each unique pair of (col1, col2).