I was asked for help in optimizing a MySQL query where flags are stored in a database and references should be counted based on the flag value. Sounds not complicated at first, but there are several flags that should be counted and also just once per reference. A lot of food for GROUP BY you may think. Having said this, search and group for flags in this table would be really slow due to a very poor cardinality. But let's start with the actual problem. The example is fictitious, but I did my best to find a general use case for this problem.