mysql - Is there any rules that which part of sql statement clause can be used in condition/case expression? -

the select clause/where clause in select statement seems can use if/case expression

which have see many times.

can statement used if/case expression?

alter table drop foreign key xxx 

if not then, there rules sql statement/clause can used in if/case expression?

any expression returns boolean value i.e. true / false can used in or if/else clause.