MySQL - Retrieve row value from different table depending on value of row in a table

问题

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.

回答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

回答2:

Select M.member_name + ' is a ' 
    + gender_attributes.sex_name 
    + ' ' 
    + gender_attributes.build
From members As M
    Join general_table As G
        On G.member_id = M. member_id   
    Join    (
            Select 'm' As sex, 'male' As sex_name, male_id as id, member_id, male_build As build
            From males_table    
            Union All 
            Select 'f' As sex, 'female', female_id, member_id, female_build
            From females_table
            ) As gender_attributes
        On gender_attributes.sex = G.sex
            And gender_attributes.member_id = M.member_id

回答3:

I'd just have a build_types table and a foreign key on your members table. It may be an edge case, but you can get muscular women, and thin guys.

来源：https://stackoverflow.com/questions/10353813/mysql-retrieve-row-value-from-different-table-depending-on-value-of-row-in-a-t

标签

mysql

case

if-statement