Question

I have a feeling CASE should be used here but I am not sure. I’m trying to take the sex (m/f) from the general table and depending on the sex, get the build from the corresponding table. For example, when the sex is m, the query should know that by going to the males table and selecting the male_build. I put together an output along with a sample query to give an idea of what I’m trying to accomplish:

members table
member_id | member_name
------------------------
     1         Herb
     2         Karen
     3         Megan
     4         Pablo

males table
male_id | member_id | male_build
---------------------------------
    1         1        muscular
    2         4          fat

females table
female_id | member_id | female_build
-------------------------------------
    1           2           thin
    2           3           fat 

general table
general_id | member_id | sex
-----------------------------
     1          1         m
     2          2         f
     3          3         f
     4          4         m

The output should be like this:

1. Herb is a muscular male.
2. Karen is a thin female.
3. Megan is a fat female.
4. Pablo is a fat male.
   -> query = "SELECT g.general_id, g.member_id, g.sex,   
              (CASE when g.sex=m THEN SELECT ma.male_build AS build 
                   FROM males ma WHERE ma.member_id=g.member_id,
              CASE when g.sex=f THEN SELECT fe.female_build AS build 
                   FROM females fe WHERE fe.member_id=g.member_id),
               m.member_name 
               FROM members m
               JOIN members m ON g.member_id=m.member_id 
               WHERE g.sex='m' OR g.sex='f'";

Looking for an efficient query for this task.

Answer 1

When you are having to do queries like this, it is a good idea to rethink your database design schemas. That being said, this should work:

SELECT mem.*, g.*, coalesce(m.male_build, f.female_build) as build
from members_table mem
inner join general g on mem.meber_id = g.member_id
left join males m on mem.member_id = m.member_id
left join females f on mem.member_id = f.member_id