Question

I don’t think this is a simple LIMIT query.
I have organisers that have categories.
Each organiser can only have 3 categories.
My problem is that I have a page to edit their categories and if I use a regular INSERT, then it may exceed 3 categories.

1. Some organisers may have 2 categories, and then choose to update to have 3.
2. Some organisers can have 3 categories, and update to have 2.
3. An organiser must have at least 1 category (this is handled on the client side).

EDIT:
My current solution currently implemented solution is to delete all categories before inserting the modified ones, then using the client-side to restrict the limit to 3. I’m using MySQL. The database is normalised like so:

       organiser                                     organiser_categories
  +-------------------+                    +-------------------+--------------+
  |  organiser_email  |                    |  organiser_email  | category_id  |
  +-------------------+                    +-------------------+--------------+
  |  [e-mail]    |                    |   [e-mail]   |     1        |
  |  [e-mail]    |                    |   [e-mail]   |     2        |
  +-------------------+                    +-------------------+--------------+


                             categories
                 +---------------+---------------+
                 |  category_id  | category_name |
                 +---------------+---------------+
                 |      1        |    Leisure    |
                 |      2        |    History    |
                 +---------------+---------------+

Answer 1

I do think you could do the checking (limit to 3 category) at the app site instead of at mysql. That is part of the verification of data.

When updating to database, i do agree with you to delete all categories and inserting the categories.