Mysql：选择与左联接不能按预期方式

Question

I have a query of the following type:

select * from tbl_1 where [cond] as aliasA LEFT JOIN tbl_2 as aliasB 
 ON (aliasA.id = aliasB.id) WHERE aliasB.id IS NULL

It appears that it's working except that it's ignoring the last WHERE aliasB.id IS NULL . So, it's just returning the results of:

select * from tbl_1 where cond as aliasA LEFT JOIN tbl_2 as aliasB 
 ON (aliasA.id = aliasB.id)

How would I change the above query to get the results of querying tbl_1 where [cond] displaying only the rows that are not in tbl_2?

Thanks in advance!

Answer 1

aliasB.id = NULL will always be false. The correct way is aliasB.id IS NULL or the uglier MySQL-specific aliasB.id <=> NULL

Answer 2

Orignal Ans at : How do I decide when to use right joins/left joins or inner joins Or how to determine which table is on which side?

Best way i found is have look to below image clear out you doubt

Check the case 1 and 2 in image will give you your answer

and change you where condition to WHERE aliasB.id is NULL

Answer 3

You could try:

SELECT * FROM tbl_1 aliasA LEFT JOIN tbl_2 aliasB
ON aliasA.id = aliasB.id
WHERE condA 
  AND aliasB.id IS NULL

Answer 4

http://dev.mysql.com/doc/refman/5.0/en/working-with-null.html

You need to use "IS NULL" instead of "= NULL"

Answer 5

您需要使用IS NULL来检查NULL值，而不是= NULL ：

WHERE aliasB.id IS NULL