CASE IN statement with multiple values

Is there a way to make a CASE statement with an IN clause?

SELECT
CASE c.Number
IN ('1121231','31242323') THEN 1
IN ('234523','2342423') THEN 2
END AS Test
FROM tblClient c

Yes. You need to use the "Searched" form rather than the "Simple" form of the CASE expression

SELECT CASE
         WHEN c.Number IN ( '1121231', '31242323' ) THEN 1
         WHEN c.Number IN ( '234523', '2342423' ) THEN 2
       END AS Test
FROM   tblClient c  

You can return the same value from several matches:

SELECT
  CASE c.Number
    WHEN '1121231' THEN 1
    WHEN '31242323' THEN 1
    WHEN '234523' THEN 2
    WHEN '2342423' THEN 2
  END AS Test
FROM tblClient c

This will probably result in the same execution plan as Martins suggestion, so it's more a matter of how you want to write it.

The question is specific to SQL Server, but I would like to extend Martin Smith's answer.

SQL:2003 standard allows to define multiple values for simple case expression:

SELECT CASE c.Number
          WHEN '1121231','31242323' THEN 1
          WHEN '234523','2342423' THEN 2
       END AS Test
FROM tblClient c;

It is optional feature: Comma-separated predicates in simple CASE expression“ (F263).

Syntax:

CASE <common operand>
     WHEN <expression>[, <expression> ...] THEN <result>
    [WHEN <expression>[, <expression> ...] THEN <result>
     ...]
    [ELSE <result>]
END

As for know I am not aware of any RDBMS that actually supports that syntax.