Choose field from joined table based on another field?

Question

Good morning!
Is it possible to select a field from a joined table row based on another field? Consider the following data in the CVALUES table:
ID  C1   C2    C3    C4    C5    C6
T1  9.6  10.3  12.4  19.6  19.6    0
T7  9.6  10.4     0     0     0    0
T3  8.2   9.9  11.4  19.6  19.6    0
T8  8.3  10.7  13.5  20.7     0    0
T5  9.4  10.3  12.7  22.9  19.6    0
...And the following data in the MDETAIL table:
ID           MDATE        CNUM     ...
T1  03/28/2012 18:00:00    4
T1  03/28/2012 18:00:00    1
T8  03/29/2012 06:00:00    6
T8  03/29/2012 06:00:00    5
...I want to get the column from CVALUES corresponding to the CNUM field in the MDETAIL table. I'm currently doing this:
SELECT m.t1, m.mdate,
       CASE WHEN m.cnum = 1 THEN c.c1
            WHEN m.cnum = 2 THEN c.c2
            WHEN m.cnum = 3 THEN c.c3
            WHEN m.cnum = 4 THEN c.c4
            WHEN m.cnum = 5 THEN c.c5
            WHEN m.cnum = 6 THEN c.c6 END AS cvalue
  FROM mdetail m
  JOIN cvalues c ON m.id = c.idThe records in the CVALUES table are unique; only one row per ID.
This method works, and it's fast but it's messy code and I think there has to be a better way to do it. I use the cvalue value in several calculations so every calculation I use it in has this giant glob of CASE in it!

Answer 1

Since you're comparing the same column against scalar values, the CASE statement could be simplified in a similar way :
CASE m.cnum
  WHEN 1 THEN c.c1
  WHEN 2 THEN c.c2
  WHEN 3 THEN c.c3
  WHEN 4 THEN c.c4
  WHEN 5 THEN c.c5
  WHEN 6 THEN c.c6
END AS cvalueSince performance is not the issue here, now it depends on what you consider "more readable".
Using a one-liner piece of code certainly makes the query looks less bulky, but I won't be so sure about readability and immediate understanding.