Question

Hi I want to know if it is possible to make two primary keys in one table in MySQL. If so, please explain the concept behind this. I am asking because I have seen a table in which two primary keys are there with no auto increment set.

Answer 1

you can only have 1 primary key, but:

• you can combine more than one column to be the primary key (maybe it’s this what you have seen)
• the primary key don’t needs to be an auto-increment, it just has to be unique
• you can add more than one index to one or more colums to speed up SELECT-statements (but slow down INSERT / UPDATE)
• those indexes can be marked as unique, wich means they don’t let you insert a second row with the same content in the index-fields (just like a primary key)