MySQL INNER JOIN with SubQuery (well detailed)

Question

I'm trying to add a third INNER JOIN to a SELECT Query, where this third INNER JOIN needs to look for a value in that third Table, which value needs to be coupled with the outcome of a MAX + GROUP BY construct in the main SELECT Query. This may not be possible, I don't know. I can't get it to work :-)

Example

TableA
User - Score
userA - 10
userB - 42
userC - 32
userB - 42
userB - 18
userD - 12
userB - 65

.

Table B

User - Color

userA - Green

userB - Yellow

userC - Blue

On these two tables I use a

SELECT 
MAX(TableA.Score) AS MaxScore,
TableB.Color

FROM TableA

INNER JOIN TableB
ON TableA.User = TableB.User

GROUP BY TableA.User

The output here works nicely and is

User - MaxScore - Color

UserA  - 10 - Green

UserB  - 65 - Yellow

UserC  - 32 - Blue

Now, I have a separate table where all the Scores get names.

TableC

Score - Name

5 - Quite Low

10 - OK

25 - Not bad

32 - Fairly good

50 - Well done

65 - Excellent

What I try to do is get a QUERY result which reads…

User - MaxScore - Name - Color

UserA - 10 - OK - Green

UserB - 65 - Excellent - Yellow

UserC - 32 - Fairly Good - Blue

My QUERY would look something like this…

SELECT 
TableA.User
MAX(TableA.Score) AS MaxScore,
TableC.Name,
TableB.Color

FROM TableA

INNER JOIN TableB
ON TableA.User = TableB.User

INNER JOIN TableC
ON TableC.Score = (SELECT MAX(TableA.Score) AS MaxScore FROM Table A GROUP BY TableA.User) ThisSubQueryName

GROUP BY TableA.User

My problem is clearly with the syntax of the INNER JOIN of Table C, which INNER JOIN carries that subquery. I have no idea how to do this, or if it is even possible.

Looking forward to your wisdom.

Thanks :-)

Dutch

Answer 1

You should use a dinamic table(t) and then the inner join on TableC

  select t.User, t.MaxScore, c.Name, t.Color from (
          SELECT 
          TableA.User as User 
          MAX(TableA.Score) AS MaxScore,
          TableB.Color as Color

          FROM TableA

          INNER JOIN TableB
          ON TableA.User = TableB.User
          GROUP BY TableA.User
          ) t 
  INNER JOIN TableC on t.MaxScore = TableC.Score 

Answer 2

Try this instead:

SELECT mainQ.*, c.Name
FROM (
   SELECT a.`User`, MAX(a.Score) AS MaxScore, b.Color
   FROM TableA AS a
   INNER JOIN TableB AS b ON a.`User` = b.`User`
   GROUP BY a.`User`
) AS mainQ
INNER JOIN TableC AS c
ON mainQ.MaxScore = c.Score

Though your subquery could've worked with some modifications (involving table aliases and replacing it's GROUP BY with a WHERE ), correlated subqueries are relatively expensive and often unnecessary.

Edit: Wow, after all that, I just realized I basically duplicated scaisEdge's answer's query; amazing how much a couple newlines can throw off your skimming ability.