MYSQL Left Join 如何选择 NULL 值?
这是我上一个关于 MySQL 中表连接的问题的后续问题
This is a follow up question to my last question about table joins in MySQL
我需要能够从左连接表中选择 NULL 值.
I need to be able to select the NULL values from the left joined table.
这是我的加入:
table1.id | table1.name | table2.id | table2.surname
1 | John | 1 | Doe
2 | Michael | 2 | Anderson
3 | Anna | NULL | NULL
4 | Sue | NULL | NULL
我想select WHERE table2.surname = NULL
,但是这样的查询不起作用:
I would want to select WHERE table2.surname = NULL
, but a query like this doesn't work:
SELECT table1.*,table2.*
FROM table1
LEFT JOIN table2
ON table1.id=table2.id
WHERE table2.surname=NULL
我可以稍微理解它背后的逻辑,没有给我任何结果,但必须有一种方法来获取它们的结果?
I can somewhat understand the logic behind it not giving me any results, but there must be a way to grab them results?
感谢任何帮助.
推荐答案
要比较 NULL
值,您必须使用 IS NULL
谓词,如下所示:
To compare NULL
values you have to use the IS NULL
predicate, like this:
SELECT table1.*, table2.*
FROM table1
LEFT JOIN table2 ON table1.id=table2.id
WHERE table2.surname IS NULL
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