如果子查询的结果集列值为NULL，则显示特定值

Question

如果返回的列的值为NULL，我会尝试显示＆＃39;否＆＃39; 的值 我正在使用子查询从数据库中的另一个表中选择列。

我已经尝试IS NULL，但无法获得我想要的结果。

SELECT r.*,
CASE (SELECT isComplete FROM admission_mt WHERE registration_id = 1) 
WHEN NULL THEN 'No' END AS isAdmissionComplete
FROM registration_mt r
WHERE r.schoolyear_yearfrom = 2018;

我得到的isAdmissionComplete列是NULL而不是＆＃39;否＆＃39;

我需要能够显示一个值，这样当我做类似的事情时 resultSet.getString("isAdmissionComplete")然后我会得到至少一些值，而不是NULL值，这可能会导致Java中的NULLPointerException

谢谢。

Answer 1

尝试

WHEN IS NULL THEN

请注意添加的IS

Answer 2

  

选择r。*，当isAdmissionComplete为null时的情况为＆＃39;否＆＃39;什么时候   isAdmissionComplete不为null然后＆＃39;是＆＃39;以...结束   ＆＃34; isAdmissionComplete＆＃34; FROM registration_mt r WHERE   r.schoolyear_yearfrom = 2018;

Result Pic

结果文字

date_registred isAdmissionComplete isAdmissionComplete

2018-01-23（null）否

Answer 3

有几种方法，您可以检查null并替换为某个值。

IF(<column_name/expr> IS NULL,'NO','YES') 

或

CASE 
    WHEN <column_name/expr> IS NULL
    THEN 'NO'
    ELSE 'YES'
END

Answer 4

SELECT r.*,
 ifnull(a.isComplete,'No') AS isAdmissionComplete
FROM registration_mt r
left outer join admission_mt a
  on a.registration_id = r.registration_id
WHERE r.schoolyear_yearfrom = 2018;

Answer 5

我意识到registration_id表中没有匹配的admission_mt，这就是我无法获取值的原因。感谢大家的意见和建议。我意识到我只需要LEFT JOIN然后应用CASE WHEN

SELECT r.*,
CASE 
WHEN a.isComplete IS NULL THEN 'No' 
WHEN a.isComplete = 1 THEN 'Yes' END AS isAdmissionComplete
FROM registration_mt r 
LEFT JOIN admission_mt a ON r.registration_id = a.registration_id;