Using IN within a CASE statement without repeating input expression? - sql-server

Question

When using a CASE statement, I realise that as per the documentation I can either use a simple CASE expression, with an input_expression and pairs of when_expression and result_expression values (assume in these examples that values in myField are always positive integers):
CASE [input_expression]
WHEN [when_expression] THEN [result_expression]
...
ELSE [result_expression]
END
CASE myField
WHEN 1 THEN 'One'
WHEN 2 THEN 'Two'
WHEN 3 THEN 'Three'
ELSE 'More than three'
END
or I can use a searched CASE expression, with pairs of Boolean_expression and result_expression values:
CASE
WHEN [Boolean_expression] THEN [result_expression]
...
ELSE [result_expression]
END
CASE
WHEN myField = 1 THEN 'One'
WHEN myField = 2 THEN 'Two'
WHEN myField = 3 THEN 'Three'
ELSE 'More than three'
END
I also know that I can use the IN operator as part of any Boolean_expression when writing a searched CASE expression:
CASE
WHEN myField IN (1,2) THEN 'One or two'
WHEN myField = 3 THEN 'Three'
ELSE 'More than three'
END
What would be really useful would be to be able to use the IN operator when writing a simple CASE expression, like:
CASE myField
WHEN IN (1,2,3) THEN 'Between one and three'
WHEN IN (4,5,6) THEN 'Between four and six'
WHEN IN (7,8,9) THEN 'Between seven and nine'
ELSE 'More than nine'
END
But this will not run and gives the error:
Incorrect syntax near the keyword 'IN'.
Obviously my example above is contrived, but is there any way to write a simple CASE expression and also use the IN operator?
I understand from reading the syntax on MSDN that it appears this is not possible, because of how the simple CASE expression is formed - comparing the input_expression to each when_expression for equality - but I just wondered if there's any kind of way to achieve what I'm asking as it would be a handy to save writing the same field name over and over again within each WHEN clause of a searched CASE expression.

Answer 1

No, this is not possible. The documentation differentiates between the when_expression and the Boolean_expression. The when_expression always uses the equality operator.
Aaron Bertrand described this in his Dirty Secrets of the CASE expression that an expression like this
SELECT CASE #variable
WHEN 1 THEN 'foo'
WHEN 2 THEN 'bar'
END
evaluates to
SELECT
WHEN #variable = 1 THEN 'foo'
WHEN #variable = 2 THEN 'bar'
END
So there's no way to replace the comparison operator using the when_expression. This is only possible with the Boolean_expression like you already mentioned:
SELECT
WHEN #variable IN (1,2) THEN 'foo'
WHEN #variable in (3,4) THEN 'bar'
END