如何在SQL中使用或条件连接3个表

Question

I have 3 tables: table1

• inner join with table2; or:
• inner join with table3

The query:

select table1.x
from table1 right outer join
     table2
     on table1.x = table2.x right outer join
     table3
     on table1.x = table3.x

but I can only see x values that are in both table2 and table3

Answer 1

Use left join , not right join , along with an appropriate filter condition:

select table1.x
from table1 left join
     table2
     on table1.x = table2.x left join
     table3
     on table1.x = table3.x
where table2.x is not null or table3.x is not null;

You might consider writing this using exists :

select t1.*
from table1 t1
where exists (select 1 from table2 t2 where t2.x = t1.x) or
      exists (select 1 from table3 t3 where t3.x = t1.x);

That seems like a more natural way to implement your logic -- and you won't get duplicates when x is duplicated in table2 or table3 .