SQL Server CASE Statement Evaluate Expression Once - sql

Question

I may be missing something obvious! Thanks in advance for any help.
I am trying to use a CASE statement in an inline SQL Statement. I only want to evaluate the expression once, so I am looking to put the expression in the CASE section, and then evaluate the result in each WHEN. Here is the example:
SELECT
MyTable.ColumnA,
CASE DateDiff(d, MyTable.MyDate, getDate())
WHEN <= 0 THEN 'bad'
WHEN BETWEEN 1 AND 15 THEN 'reasonable'
ELSE 'good'
END as MyCalculatedColumn,
MyTable.SomeOtherColumn
I know I can do this:
CASE
WHEN DateDiff(d, MyTable.MyDate, getDate()) <= 0 THEN 'bad'
WHEN DateDiff(d, MyTable.MyDate, getDate()) BETWEEN 1 AND 15 THEN 'reasonable'
ELSE 'good'
END
But in my first example, SQL does not seem to like this statement:
WHEN <= 0 THEN 'bad'
Note that the statement is inline with other SQL, so I can't do something like:
DECLARE #DaysDiff bigint
SET #DaysDiff = DateDiff(d, MyTable.MyDate, getDate())
CASE #DaysDiff
WHEN <= 0 THEN 'bad'
WHEN BETWEEN 1 AND 15 THEN 'reasonable'
ELSE 'good'
END
My actual DateDiff expression is much more complex and I only want to maintain its logic, and have it evaluated, only once.
Thanks again...

Answer 1

You can use apply for this purpose:
SELECT MyTable.ColumnA,
(CASE WHEN day_diff <= 0 THEN 'bad'
WHEN BETWEEN 1 AND 15 THEN 'reasonable'
ELSE 'good'
END) as MyCalculatedColumn,
MyTable.SomeOtherColumn
FROM MyTable CROSS APPLY
(VALUES (DateDiff(day, MyTable.MyDate, getDate()))) v(day_diff)
APPLY is a very handy way to add calculated values into a statement. Because they are defined in the FROM clause, they can be used in SELECT, WHERE, and GROUP BY clauses where column aliases would not be recognized.

Answer 2

I see your problem. CASE expression WHEN value can only do an equality check
You could try using a CTE (Common Table Expression), do everything except the case statement in the CTE and then put the CASE in the final SELECT at the end. I'm not sure whether it will prevent the expression being evaluated twice - thats kindof the optimisers problem, not yours (thats how I like to think about it)
WITH cteMyComplexThing AS(
SELECT MyTable.ColumnA,
DateDiff(d, MyTable.MyDate, getDate()) as ComplexThing,
MyTable.SomeOtherColumn
FROM MyTable
)
SELECT
ColumnA,
CASE
WHEN ComplexThing <= 0 THEN 'bad'
WHEN ComplexThing BETWEEN 1 AND 15 THEN 'reasonable'
ELSE 'good'
END as MyCalculatedColumn,
SomeOtherColumn
FROM cteMyComplexThing

Answer 3

The WHEN clause in a CASE statement needs both sides of the condition. <=0 cannot stand by itself.
CASE #DaysDiff
WHEN ? <= 0 THEN 'bad'
WHEN BETWEEN 1 AND 15 THEN 'reasonable'
ELSE 'good'
END