Question

I have a table in my database formatted like this:

customer   old_code   new_code
C1         A          X
C1         B          Y        
C2         C          Y
C2         D          Z

So the pair of keys customer and old_code map to a new_code. This is great as a way of storing the data, looks ups are very fast, but for human consumption the data would be better displayed like this:

      C1    C2
X     A
Y     B     C
Z           D

Is there an easy way using SQL to transform the data to the second view? Obviously there can be any number of customers although I could query for the unique set before hand. Currently I have < 50000 records and I expect that to be the norm but I’d like any solutions to scale up to a few hundred thousand if possible. My application currently targets MySQL.

Answer 1

One standard way would be:

SELECT CC.NEW_CODE,
       MAX(CASE CUSTOMER WHEN 'C1' THEN OLD_CODE ELSE NULL END) C1,
       MAX(CASE CUSTOMER WHEN 'C2' THEN OLD_CODE ELSE NULL END) C2
FROM CUSTOMER_CODE CC
GROUP BY CC.NEW_CODE
ORDER BY CC.NEW_CODE

Of course, this depends on some assumptions and you would have more information on the uniqueness of the columns. I tested this in Oracle, where you’d usually do DECODE; I think CASE should work for you.