分组后条件查询

SELECT  [WS_ID] ,
        ISNULL(MAX(CASE 
                        WHEN [NODE_ID] = 'Go' THEN STEP_ID
                        WHEN [NODE_ID] = 'Dispose' THEN STEP_ID
                        ELSE NULL
                   END), MAX([STEP_ID]))
FROM    [WS_INST_STEP]
GROUP BY [WS_ID];