SQL 左连接到由内部 SELECT 的结果确定的表

Question

I'm just wondering if this is possible. I'm trying show all results from one table and grab more details from respective group(table)

+--------------------------+     +--------------------------+
| table group_a            |     | table group_b            |
+--------------------------+     +--------------------------+
| id      name        age  |     | id      name        age  |
| s01     John        10   |     | s11     Clark       11   |
| s02     Jane        11   |     | s12     Cherry      09   |
+--------------------------+     +--------------------------+

+----------------------------+
| table result_1             |
+----------------------------+
| id      result     group   |
| s01     9          a       |
| s12     10         b       |
| s11     9          b       |
| s02     7          a       |
+----------------------------+

I was hoping to get this output

  id      name     age     result
+------------------------------------+
  s12     Cherry   09      10
  s01     John     10      9
  s11     Clark    11      9
  s02     Jane     11      7

I'm kind of stuck on how to point my query to different tables. Anyway I just want to know if this is possible or I should go for different approach.

B'rgrds,

Answer 1

You can use left join twice:

select r.id, coalesce(a.name, b.name) as name, coalesce(a.age, b.age) as age,
       r.result
from result_1 r left join
     group_a a
     on r.group = 'a' and r.id = a.id left join
     group_b b
     on r.group = 'b' and r.id = b.id;

I would expect the above to have the better performance, but you can also use union all before the join:

select r.id, ab.name, ab.age, r.result
from result_1 r left join
     ((select id, name, age, 'a' as group
       from group_a a
      ) union all
      (select id, name, age, 'b' as group
       from group_b b
      )
     ) ab
     on r.group = ab.group and r.id = as.id ;

Note: It is better to have all the groups in a single table rather than splitting them among multiple tables.