[lmm]

It's easier to constrain your objects in the same language they're written in. Say I have an object where my constraint is that either fielda is set, or fieldb and fieldc are set, but not both (ignoring for the moment that that's a stupid object to have). I can trivially enforce that in a constructor, but it would take me quite a while to work out how to express that in SQL, if it's even possible.

[autarch]

I think this reflects more on you than on SQL.

This is fairly trivial to express as a table-level constraint. I've done very similar things in Postgres. I have no idea if you can do this in MySQL, but it's quite crippled.