Jd.com interviewer asked: what is the difference between using on or where in the left join association table and conditions

Code friends went to jd.com for an interview before , Asked about LEFT JOIN Used in association table ON still WHERE What's the difference with the conditions , It was answered soon , But ask what caused this situation , I didn't answer for a while .

Now let's talk about it. , Want to pass A left B join on and The latter condition makes the two records found become one , But there are still two .

It turned out join on and The number of result records will not be filtered , Only according to and Whether or not the condition after is displayed B The record of the table ,A The record of the table is bound to show .

No matter and The back is A.id=1 still B.id=1, All show A All the records in the table , And associated display B In the corresponding A In the table id by 1 Or B In the table id by 1 The record of .

function sql :

select * from student s left join class c on s.classId=c.id order by s.id

function sql :

 select * from student s left join class c on s.classId=c.id and s.name=" Zhang San " order by s.id

function sql :

select * from student s left join class c on s.classId=c.id and c.name=" Class 3, grade 3 " order by s.id

When a database returns records by connecting two or more tables , Will generate a temporary table in the middle , Then the temporary table is returned to the user .

In the use of left jion when ,on and where The conditions differ as follows ：

1、 on The condition is the condition used to generate the temporary table , It doesn't care on Is the condition in true , Will return to the records in the table on the left .

2、where The condition is that after the temporary table is generated , Then filter the temporary table . There is no such thing as left join The meaning of （ You have to go back to the records in the table on the left ） 了 , If the conditions are not true, filter them out .

Suppose there are two tables ：

surface 1：tab2

id

size

1

10

2

20

3

30

surface 2：tab2

size

name

10

AAA

20

BBB

20

CCC

Two article SQL:

1、select * form tab1 left join tab2 on (tab1.size = tab2.size) where tab2.name='AAA'
2、select * form tab1 left join tab2 on (tab1.size = tab2.size and tab2.name='AAA')

Article 1 with a SQL The process of ：

1、 In the middle of table
on Conditions :
tab1.size = tab2.size

tab1.idtab1.sizetab2.sizetab2.name

1

10

10

AAA

2

20

20

BBB

2

20

20

CCC

3

30

(null)

(null)

|

|

2、 Then filter the middle table
where Conditions ：
tab2.name=’AAA’

tab1.idtab1.sizetab2.sizetab2.name

1

10

10

AAA

Second SQL The process of ：

1、 In the middle of table
on Conditions :
tab1.size = tab2.size and tab2.name=’AAA’
( If the condition is not true, the record in the left table will be returned )

tab1.idtab1.sizetab2.sizetab2.name

1

10

10

AAA

2

20

(null)

(null)

3

30

(null)

(null)

In fact, the key reason for the above results is left join,right join,full join The particularity of , No matter on Whether the condition on is true or not will be returned left or right The records in the table ,full It has left and right The union of the properties of . and inner jion It's not that special , The conditions are as follows on neutralization where in , The result set returned is the same .