simplify SQL statement by using CTE

问题

I have a query like the following:

SELECT A.a, A.b, B.c,
(CASE WHEN ... THEN ... ELSE ... END) AS CalculatedValue,
B.d
FROM    dbo.TableA A INNER JOIN
        dbo.TableB B ON (...)
WHERE (CASE WHEN ... THEN ... ELSE ... END) BETWEEN @DayStart AND @DayEnd
GROUP BY A.a, (CASE WHEN ... THEN ... ELSE ... END), B.c

to avoid repeating many times the exact same expression: (CASE WHEN ... THEN ... ELSE ... END) I wanted to define a CTE and query such table using in the select, where and group by the expression CalculatedValue

unfortunately this does not work because the select needs to already include the group by when creating the CTE

is there any other way I could use to not repeat the CASE WHEN... so many times?

回答1:

Use CROSS APPLY, which can be used to define aliased fields and then refer to them:

SELECT A.a, 
       A.b, 
       B.c,
       CalculatedValue,
       B.d
FROM    
       dbo.TableA A 
INNER JOIN
        dbo.TableB B 
        ON (...)
CROSS APPLY 
        (SELECT (CASE WHEN ... THEN ... ELSE ... END)) CxA(CalculatedValue)
WHERE CalculatedValue BETWEEN @DayStart AND @DayEnd
GROUP BY A.a, CalculatedValue, B.c

The CxA is just an alias and you can name it whatever you like.

回答2:

For above, I think you could just do two layers of CTE's. The first would do Calculate for all, the second would select from first CTE and filter based on the calculated value. The final query would join in the second layer CTE.

Just a thought.

来源：https://stackoverflow.com/questions/9198062/simplify-sql-statement-by-using-cte