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

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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.


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