How do I conditionally sort on a second column using a custom MySQL function?

Question

I've got a result set I have to sort in quite a complex way. There are two fields (A and B) which are employed in the sorting, and the algorithm is like this:

1. Sort by field A
2. Then, if the field A contains a substring K, sort by field B using a custom function F (see below). If the field A doesn't contain K, the built-in sorting for B is just fine.

More information:

• The field A is a VARCHAR.
• The field B is an INTEGER that can have the following values: {1, 2, 3, 4, 5}.
• Function F is simple as "sort values in this way: {4, 5, 1, 2, 3}".

How can I do this in MySQL through a SQL query? Am I forced to do it cient-side?

Answer 1

…
ORDER BY
  A,
  CASE WHEN A LIKE CONCAT('%', K, '%') THEN Func(B) ELSE B END