How to perform CASE on values from subquery - sql

Question

This is going to be difficult to explain, but here goes.
I am looking to perform a CASE condition in a SELECT clause that will use the results of two calculations to determine which calculation value to return for a column value.
Maybe a code sample will help:
this works:
SELECT
A.[COLUMN1]
, B.[COLUMN1]
, CASE
WHEN A.[COLUMN2] + A.[COLUMN3] >= B.[COLUMN2] + B.[COLUMN3] THEN A.[COLUMN2] + A.[COLUMN3]
ELSE B.[COLUMN2] + B.[COLUMN3]
FROM
[TABLE_A] A
INNER JOIN [TABLE_B] B INNER JOIN ON A.ID = B.ID
The problem here is that the query above, in the case statement, is forced to perform the calculation twice. Once for the WHEN clause and again for the THEN clause.
I want to do something like this, but SQL is not happy with it.
SELECT
A.[COLUMN1]
, B.[COLUMN1]
, CASE
WHEN AB.X >= AB.Y THEN AB.X
ELSE AB.Y
END
FROM ((A.[COLUMN2] + A.[COLUMN3]) X, (B.[COLUMN2] + B.[COLUMN3]) Y)
FROM
[TABLE_A] A
INNER JOIN [TABLE_B] B INNER JOIN ON A.ID = B.ID
Is this even possible? In the second example, I am calculating the values only once and referring to them in the case statement, both for the WHEN and the THEN clauses.

Answer 1

I would much prefer to push the calculations down into each table. This keeps the structure of the query quite similar. So, a syntactically correct (or almost correct) version would be:
SELECT A.[COLUMN1], B.[COLUMN1],
(CASE WHEN a.col_2_3 >= b.col_2_3 THEN a.col_2_3
ELSE b.col_2_3
end)
FROM (select a.*, (A.[COLUMN2] + A.[COLUMN3]) as col_2_3
from [TABLE_A] a
) a INNER JOIN
(select b.*, (B.[COLUMN2] + B.[COLUMN3]) as col_2_3
from [TABLE_B] b
)b
ON a.ID = b.ID
There are so many important factors in performance, and overhead for simple calculations is just not one of them. Reading the data and the join are way, way more expensive than simple calculations.
However, moving variables into subqueries is useful for a few reasons. First, the calculations could be more expensive (using subqueries, say). It also helps with readability and hence maintainability.
Finally, a SQL engine could decide to evaluate those expressions just once. In practice, I'm guessing that none make that trivial optimization.

Answer 2

You could reformulate it as this
SELECT a_column1,
b_column1,
CASE
WHEN x >= y THEN x
ELSE y
END AS foo
FROM (SELECT A.[column1] A_COLUMN1,
B.[column1] B_COLUMN1,
( A.[column2] + A.[column3] ) X,
( B.[column2] + B.[column3] ) Y
FROM [table_a] A
INNER JOIN [table_b] B
ON A.id = B.id)t
But I'm not sure it will make a difference since the operations may be performed once per row anyway