SQL SERVER:CASE判断空，错误一例

 -----错误判断------------------------------------------------------------------------------------

SELECT

  policy_jq_no,policy_sy_no,

    CASE policy_jq_no

WHEN  '' THEN

    policy_sy_no

ELSE

    policy_jq_no

END policy_no

FROM

    ZY_SURVEY_VISIT order by FLOW_ID desc

 

---正确判断-----------------------------------------------------------------------------

  SELECT

  policy_jq_no,policy_sy_no,

    CASE

WHEN policy_jq_no IS NULL THEN

    policy_sy_no

ELSE

    policy_jq_no

END policy_no

FROM

    ZY_SURVEY_VISIT order by FLOW_ID desc