Question

select value1 as value from T1 where id=10;

if id does not exist in T1 – execute another query:

select value2 as value from T2 where id=10;

So, I want to join these queries and return a single value (value1 or value2). Is it possible?

SOLUTION:

My solution:

select ifnull(value1, value2) as value from T1 left join T2 using(id) where id=10;

Answer 1

TRY (tested)

SELECT COALESCE(t1.value1, t2.value2) AS Value FROM t1
INNER JOIN t2 USING(id)
WHERE id=10

this will always check first the table t1 for id=10, if there is no value then see table t2 for the same id

Quoted FROM

The single result column that replaces two common columns is defined
using the coalesce operation. That is, for two t1.a and t2.a the
resulting single join column a is defined as a = COALESCE(t1.a, t2.a)