How to golf evaluation of math expression in MySQL?
Score 69
select * from expressions
where elt(ord(operation)/2-20,a*b,a+b,a-b,a/b)=c

Takes inspiration from Marmite Bomber's use of elt, but uses a magic formula. Indexes into the list via ord(operation)2/-20, which buckets the four character codes across 1 to 4 by abusing elt's rounding behavior.
-1 thanks to Arnauld, who improved on the modulo reduction.
-1 thanks to Bubbler, who found a better non-modulo formula.
Score 73
select * from expressions
where elt(instr('*/-+',operation),a*b,a/b,a-b,a+b)=c

This is my best guess as to the 73 you mention other people have gotten in the question.