Question

I have such query. It returns ColA and ColB from TableA and UserName from table Users. Then it displays several fields from TableB as additional columns to results. It works but is there any better way than using these multiple LEFT JOINS ?

SELECT a.COlA, a.ColB, u.UserName,
b1.Value,
b2.Value,
b3.Value,
b4.Value,

FROM TableA a JOIN Users u ON a.UserId = u.UserId
    LEFT JOIN TableB b1 ON a.EventId = b1.EventId AND b1.Code = 5
    LEFT JOIN TableB b2 ON a.EventId = b2.EventId AND b2.Code = 15
    LEFT JOIN TableB b3 ON a.EventId = b3.EventId AND b3.Code = 18
    LEFT JOIN TableB b4 ON a.EventId = b4.EventId AND b4.Code = 40

WHERE (a.UserId = 3) ORDER BY u.UserName ASC

TableB looks like:

Id | EventId | Code | Value
----------------------------
1  |    1    |  5   | textA
2  |    1    |  15  | textB
3  |    1    |  18  | textC

Sometimes Code is missing but for each event there are no duplicated Codes (so each LEFT JOIN is just another cell in the same result record).

Answer 1

I cannot understand why you want to change something that is working, but here’s another way (which does those LEFT joins, but in a different way):

SELECT a.COlA, a.ColB, u.UserName,
  ( SELECT b.Value FROM TableB b WHERE a.EventId = b.EventId AND b.Code = 5 ),
  ( SELECT b.Value FROM TableB b WHERE a.EventId = b.EventId AND b.Code = 15 ),
  ( SELECT b.Value FROM TableB b WHERE a.EventId = b.EventId AND b.Code = 18 ),
  ( SELECT b.Value FROM TableB b WHERE a.EventId = b.EventId AND b.Code = 40 )

FROM TableA a JOIN Users u ON a.UserId = u.UserId

WHERE (a.UserId = 3) 

ORDER BY u.UserName ASC