What is inner connection and outer connection? What are the uses and benefits

Inner join is the same data of left table and right table .

The external connection is divided into ： The left outer join 、 Right connection 、 Full outer join .
The left outer connection is based on the left table , To match the right table , How many pieces of data are there in the left table , The result is how many pieces of data . The right outer connection is the opposite of the left outer connection , Subject to the right table , To match the left table , How many pieces of data are there in the right table , The result is how many pieces of data . The number of total external connection data is not necessarily , Equivalent to left outer connection And right outer connection The synthesis of .