MySQL two table connection principle (understand join buf)

summary

In general , We use mysql Both of them use two kinds of queries , One is single table query , One is multi table connection . The principle of multi meter connection is still very important , Especially the understanding of query optimization . How to connect multiple tables should be considered to be reasonable , It is necessary to know the principle of connection .

principle

1. Under normal conditions, the table is connected

1. Two tables  stu  and stu_score, That is to say, a form recording the basic information of students , A student record sheet 

2. Now need join  Two tables , hypothesis  on  Condition is  stu.id=stu_score.stu_id As a condition of connection 
  
3. be based on stu As a driving table , from id be equal to 1 Start of record , Then I draw it in the picture stu_score It's just 
  Cluster index , But the connection condition is stu_score.stu_id, Then only cluster indexes can be scanned , here stu
  There are five records , So full scan stu Watch once , Each record is for connection ,