Analysis of query results using both left join on and where in MySQL

Used to use it before oracle, When dealing with left connections and links , Only need sql Add (+) That's all right. , But in mysql in , take left join perhaps right join And on and where When used in combination , Different combinations , The results are different . Now take a note of .

 

Suppose there is a left table tb_oder, The table has order_id and user_id Field ; There is a right table tb_user, The table has user_id and user_name Two fields , Two tables use user_id Association .

 

SQL1：

select o.order_id,o.user_id,u.user_id,u.user_name

from tb_order o LEFT JOIN tb_user u

on o.user_id = u.user_id;

This is the most common usage , If there is no corresponding data in the right table , Then the result set is empty .

 

SQL2：

select o.order_id,o.user_id,u.user_id,u.user_name

from tb_order o LEFT JOIN tb_user u

on o.user_id = u.user_id

and u.user_id > 10;

After filtering the right table, it is associated with the left table , therefore , Even in the right table and the left table user_id<10 The data of ,user_name The display result of is also empty .

 

SQL3

select o.order_id,o.user_id,u.user_id,u.user_name

from tb_order o LEFT JOIN tb_user u

on o.user_id = u.user_id

and o.order_id > 10;

 

SQL4：

select o.order_id,o.user_id,u.user_id,u.user_name

from tb_order o LEFT JOIN tb_user u

on o.user_id = u.user_id

and o.user_id > 10;

These two statements seem to be after filtering the left table , Then associated with the right table , But he is not like this . actually , in any case , The left table tb_order All the data in will be displayed , Under the condition of limiting the left table , The content in the corresponding right table that meets the association conditions is forcibly set to empty . The effect of these two statements is similar to SQL2 It's the same , But the effect is hard to understand , It is generally not recommended to write .

 

SQL5：

select o.order_id,o.user_id,u.user_id,u.user_name

from tb_order o LEFT JOIN tb_user u

on o.user_id = u.user_id

where u.user_id > 10;

 

SQL6：

select o.order_id,o.user_id,u.user_id,u.user_name

from tb_order o LEFT JOIN tb_user u

on o.user_id = u.user_id

where o.user_id > 10;

The result of these two statements is more interesting , Their effect is the same , Namely , Use left join on The result set of association is then according to where Conditions for screening .

 

Follow the above analysis , Seems to be , In this way SQL in , None of them is easy to understand , First, filter the contents of the left table , Then match with the right table . A better way is , Filter the left table with sub query , If you can't understand the above left join on where Medium where How to use , You can also filter the right table with sub queries .

SQL as follows ：

select o.order_id,o.user_id,u.user_id,u.user_name

from ( select * from tb_order where order_id > 10 ) o LEFT JOIN tb_user u

on o.user_id = u.user_id;

 

select o.order_id,o.user_id,u.user_id,u.user_name

from ( select * from tb_order where order_id > 10 ) o LEFT JOIN (select * from tb_user where user_id > 20) u

on o.user_id = u.user_id;