Use comparison signs inside a sql case statement

问题:

I'm looking for a way to build case statements in a sql select query using less than and greater than signs. For example, I want to select a ranking based on a variable:

DECLARE @a INT
SET @a = 0

SELECT CASE 
         WHEN @a < 3 THEN 0
         WHEN @a = 3 THEN 1
         WHEN @a > 3 THEN 2
       END

I'd like to write it as:

DECLARE @a INT
SET @a = 0

SELECT CASE @a
         WHEN < 3 THEN 0
         WHEN 3 THEN 1
         WHEN > 3 THEN 2
       END

...but SQL doesn't let me use the < and > signs in this way. Is there a way that I can do this is SQL 2005, or do I need to use the code like in the first one.

The reason for only wanting the code there once is because it would make the code a lot more readable/maintainable and also because I'm not sure if SQL server will have to run the calculation for each CASE statement.

I'm looking for a VB.NET case statement equivelent:

Select Case i
    Case Is < 100
        p = 1
    Case Is >= 100
        p = 2
End Select

Maybe it's not possible in SQL and that's ok, I just want to confirm that.

回答1:

You can use the SIGN function as

DECLARE @a INT
SET @a = 0

SELECT CASE SIGN(@a - 3)
         WHEN -1 THEN 0
         WHEN 0 THEN 1
         WHEN 1 THEN 2
       END

If @a is smaller than 3, then @a - 3 results in a negative int, in which SIGN returns -1.

If @a is 3 or greater, then SIGN returns 0 or 1, respectively.

---

If the output you want is 0, 1 and 2, then you can simplify even more:

DECLARE @a INT
SET @a = 0

SELECT SIGN(@a - 3) + 1

回答2:

Using SIGN as suggested by @Jose Rui Santos seems a nice workaround. An alternative could be to assign the expression an alias, use a subselect and test the expression (using its alias) in the outer select:

SELECT
  …,
  CASE
    WHEN expr < 3 THEN …
    WHEN expr > 3 THEN …
  END AS …
FROM (
  SELECT
    …,
    a complex expression AS expr
  FROM …
  …
)

回答3:

SELECT 
CASE 
WHEN ColumnName >=1 and ColumnName <=1 THEN 'Fail'
WHEN ColumnName >=6 THEN 'Pass'
ELSE 'Test'
END
FROM TableName