SQLite LEFT JOIN&WHERE子句混淆

Question

我有两张桌子:

表1:问题:

QuestionId NUMERIC
Title      TEXT

问题表中的测试数据:

QuestionId     Title
1              What is your name?
2              What is your age?

表2:答案:

AnswerId     NUMERIC
PersonId     NUMERIC
QuestionId   NUMERIC
Answer       TEXT

如果Answers表中没有数据,则下面的查询返回正确的结果(2行):

SELECT  q.QuestionId, q.Title, a.Answer
FROM    Questions q
    LEFT OUTER JOIN Answers a ON q.QuestionId = a.QuestionId
WHERE    a.PersonId = 2 OR a.PersonId IS null

但是,如果我在答案表中有例如1条记录,例如:

AnswerId     1
PersonId     1
QuestionId   1
Answer       'My Name is Yaqub'

以下查询工作正常:

SELECT  q.QuestionId, q.Title, a.Answer
FROM    Questions q
    LEFT OUTER JOIN Answers a ON q.QuestionId = a.QuestionId
WHERE    a.PersonId = 1 OR a.PersonId IS null

但是这个查询只返回一行:

SELECT  q.QuestionId, q.Title, a.Answer
FROM    Questions q
    LEFT OUTER JOIN Answers a ON q.QuestionId = a.QuestionId
WHERE    a.PersonId = 2 OR a.PersonId IS null

上面的查询只返回一行是错误的,因为我在问题表中有两个问题,上面的查询应该返回它们两个因为条件'或a.PersonId IS null'.

为什么它没有返回两行？

期望的结果:

我想获得所有问题(2行)和PersonId具有某些值的答案,例如Person 2的两行,因为我在答案表中没有他的数据.

Answer 1

我找到了解决方案:

SELECT q.QuestionId, q.Title, a.Answer
FROM Questions q
 LEFT OUTER JOIN Answers a ON q.QuestionId = a.QuestionId AND 
(a.PersonId = 2 OR a.PersonId IS null)

此查询将列出所有问题,WHERE子句将过滤掉PersonID具有特定值或为null的答案.因此,只要有问题的答案,我就不会获得PersonID的NULL值.

Answer 2

您可以这样做：

SELECT q.QuestionId, q.Title, a.Answer
FROM Questions q
 LEFT OUTER JOIN Answers a ON q.QuestionId = a.QuestionId AND 
a.PersonId = 2

由于外部联接，查询仍然返回PersonId = null的记录。