MySQL - How to select rows where value is in array?

Question

Ok, normally I know you would do something like this if you knew the array values (1,2,3 in this case):

SELECT * WHERE id IN (1,2,3)

But I

Answer 1

If you use the FIND_IN_SET function:

FIND_IN_SET(a, columnname) yields all the records that have "a" in them, alone or with others

AND

FIND_IN_SET(columnname, a) yields only the records that have "a" in them alone, NOT the ones with the others

So if record1 is (a,b,c) and record2 is (a)

FIND_IN_SET(columnname, a) yields only record2 whereas FIND_IN_SET(a, columnname) yields both records.