Question

Scenario : Multiple product images, one main image.

Currently, I’ve got 2 tables to manage this. One for storing product images and another table for storing mainProductImageIds. In the table that stores a list of mainProductImageIds, I’ve got a unique index on prodid+isMain to enforce One Main Image Per Product Id.

My Question relates to the table below (if I was going to only use 1 table). How would I enforce 1 isMain=1 per prod? Is there any index setting that would error when I tried to set isMain=1 to image_id=2 since image=1 is already the main image? You can’t put a unique_key on prod+isMain.

|image_id |prod|isMain
|---------|----|------
|0        |1   |0
|1        |1   |1
|2        |1   |0
|3        |2   |1
|4        |3   |0
|5        |3   |1

Answer 1

The answer, I learned, is really simple and has to do with how MySQL indexes NULL values. Over Here they talk about:

For all engines, a UNIQUE index allows multiple NULL values for
columns that can contain NULL.

so as long as my isMain field allow enforces one value and NULL (maybe an ENUM), I’m all set. The unique_index will only be enforces when isMain is set to a distinct value and not when it is set to NULL.

The Updated Table Looks like This:

|image_id |prod|isMain
|---------|----|------
|0        |1   |null
|1        |1   |1
|2        |1   |null
|3        |2   |1
|4        |3   |null
|5        |3   |1