Question

I have 3 tables: Table1, Table2, Table3

I want to make a query that use Table1’s result as Table2’s constrain, Table2’s result as Table3’s constrain

so my query now is something like:

SELECT tb3.id 
FROM tb3 
WHERE tb3.tag = (
                   SELECT tb2.num 
                   FROM tb2 
                   WHERE tb2.name = (
                                      SELECT tb1.user 
                                      FROM tb1 
                                      WHERE check = '0'
                                    )
                 )

for example,

tb1 has item ‘tb1.check’, ‘tb1.user’;

tb2 has item ‘tb2.num’, ‘tb2.name’;

tb3 has item ‘tb3.tag’, ‘tb3.id’;

first check tb1 for data where ‘tb1.check’= 0 , then get its ‘tb1.user’,

then check tb2 for data where ‘tb2.name’ = ‘tb1.user’ and get ‘tb2.num’,

and so, check tb3 for data where ‘tb3.tag’ = ‘tb2.num’ then get ‘tb3.id’

but all I get is empty, any help?

Answer 1

use INNER JOIN instead of subquery as:

SELECT tb3.id 
FROM tb3 
     INNER JOIN tb2  
         ON tb3.tag = tb2.num
     INNER JOIN Invitation inv
         ON tb2.name = inv.user AND
            inv.check = 0;