[MySQL] the difference between left join, right join and join

Link query join Regular use , But I haven't fully understood the differences between various usages ,left join、right join、join and inner join And so on join, What's the difference ？ The figure of summarizing properties given on the official website ：

This picture summarizes all join The difference between , If you just watch , Then I still can't understand it , You must try it yourself .

Set up two tables for connection query , The first table is called kemu, The second table is called score：

1. left join

seeing the name of a thing one thinks of its function , Namely “ Left connection ”, surface 1 Left link table 2, Mainly on the left , Denoted by table 1 Mainly , Associated with the above table 2 The data of , The results show all the data on the left , And then on the right is the data that intersects on the left . as follows ：

select  *

from kemu

left join score

on kemu.id = score.id

result ：

 

 

2. right join

“ The right connection ”, surface 1 Right join table 2, Mainly on the right , Denoted by table 2 Mainly , Associated query table 1 The data of , Look up table 2 All the data and tables 1 And table 2 Data with intersection , as follows ：

select
   *
from
   kemu
right join score on kemu.id = score.id

result ： 

 

 

3. join

join, In fact, that is “inner join”, It was written for the sake of brevity join, Two means one , Internal connection , It means the intersection of two tables , It turns out that there is an intersection between the two tables , The rest will not be displayed if there is no connection , This is also used in many cases , as follows

select
   *
from
   kemu
join score on kemu.id = score.id

result ：