如果子查询（内查询）返回空值给外查询怎么处理

Question

I have a vertical table like this:

id	Profile_id	feature_id	value
1	1	1	Rick
2	1	2	Novak
3	5	3	5428
4	5	1	Joe
...	...	...	...

(above table is short part of profile_features table)

I have a query:

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

but if one of the subqueries returns null, query have not any output.

I want a row result with TelNum, NCode, Fname, Lname either they are Null or not null.

Answer 1

You could use scalar subqueries in the select-list instead of your current Cartesian product:

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;

Alternatively, you could learn to use 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;