如果子查詢（內查詢）返回空值給外查詢怎么處理

Question

我有一個這樣的垂直表：

ID	Profile_id	特征標識	價值
1	1	1	瑞克
2	1	2	諾瓦克
3	5	3	5428
4	5	1	喬
...	...	...	...

（上表是 profile_features 表的一小部分）

我有一個查詢：

SELECT * FROM(SELECT `value` TelNum FROM `profile_features` WHERE `feature_id` IN (10, 64, 103) AND `profile_id` = 16752 LIMIT 1) as TelNum,
    (SELECT `value` NCode From `profile_features` WHERE `feature_id` IN (5, 61, 100) AND `profile_id` = 16752 LIMIT 1)  AS NCode,
    (SELECT `value` Fname From `profile_features` WHERE `feature_id` IN (1, 55, 86) AND `profile_id` = 16752 LIMIT 1) AS Fname,
    (SELECT `value` Lname From `profile_features` WHERE `feature_id` IN (2, 56, 95) AND `profile_id` = 16752 LIMIT 1) AS Lname

但如果其中一個子查詢返回 null，則查詢沒有任何輸出。

我想要一個帶有 TelNum、NCode、Fname、Lname 的行結果，它們是 Null 還是非 null。

Answer 1

您可以在選擇列表中使用標量子查詢，而不是當前的笛卡爾積：

SELECT (SELECT `value` FROM `profile_features` WHERE `feature_id` IN (10, 64, 103) AND `profile_id` = 16752 LIMIT 1) as TelNum,
  (SELECT `value` From `profile_features` WHERE `feature_id` IN (5, 61, 100) AND `profile_id` = 16752 LIMIT 1)  AS NCode,
  (SELECT `value` From `profile_features` WHERE `feature_id` IN (1, 55, 86) AND `profile_id` = 16752 LIMIT 1) AS Fname,
  (SELECT `value` From `profile_features` WHERE `feature_id` IN (2, 56, 95) AND `profile_id` = 16752 LIMIT 1) AS Lname;

或者，您可以學習使用 LEFT OUTER JOIN：

SELECT TelNum.TelNum, NCode.NCode, Fname.Fname, Lname.Lname
FROM (SELECT NULL AS dummy) AS dummy
LEFT OUTER JOIN (SELECT `value` TelNum FROM `profile_features` WHERE `feature_id` IN (10, 64, 103) AND `profile_id` = 16752 LIMIT 1) AS TelNum ON true
LEFT OUTER JOIN (SELECT `value` NCode From `profile_features` WHERE `feature_id` IN (5, 61, 100) AND `profile_id` = 16752 LIMIT 1) AS NCode ON true
LEFT OUTER JOIN (SELECT `value` Fname From `profile_features` WHERE `feature_id` IN (1, 55, 86) AND `profile_id` = 16752 LIMIT 1) AS Fname ON true
LEFT OUTER JOIN (SELECT `value` Lname From `profile_features` WHERE `feature_id` IN (2, 56, 95) AND `profile_id` = 16752 LIMIT 1) AS Lname ON true;