Compatibility
CASE WHEN b1 THEN v1 END
Engine	OK	Alternative
ingres	Yes
mysql	Yes
oracle	Yes
postgres	Yes
sqlserver	Yes

CASE

CASE allows you to return different values under different conditions.

If there no conditions match (and there is not ELSE) then NULL is returned.

  CASE WHEN condition1 THEN value1 
       WHEN condition2 THEN value2  
       ELSE def_value 
  END

SELECT name, population
      ,CASE WHEN population<1000000 
            THEN 'small'
            WHEN population<10000000 
            THEN 'medium'
            ELSE 'large'
       END
  FROM bbc