Question

people know that we can use if statement to configure a query in the select statement like this

select if(var=1,amount,amount/2) from mytable;

But what if I want to achieve something like this:

select amount from if(var=1,mytable1,mytable2);

Is there any way to configure the table at run time?

Answer 1

SELECT amount FROM mytable1 WHERE @var = 1
UNION
SELECT amount FROM mytable2 WHERE @var = 0

UPD: Here’s what MySQL EXPLAIN looks like for the part of the query which has a condition evaluating to FALSE:

Note the Impossible WHERE part. MySQL recognizes that the expression in WHERE is constantly evaluating to FALSE, so it doesn’t even try executing the query. Hence, no performance overhead when using this approach.