There are three kinds of SQL connections: internal connection, external connection and cross connection

Prepare two sheets and PersonId Related to .

One 、 Cross connect （CROSS JOIN）

Cross connect returns the Cartesian product of all data rows of the connected two tables .

//  These two sentences sql Completely equivalent 
select * from person CROSS JOIN address
select * from person, address

Query results ：

Two 、 Internal connection （INNER JOIN）

The internal connection can obtain the records of the common part of the two tables , That is to use conditional expressions to eliminate some data rows of cross connection .

//  These two sentences sql Completely equivalent 
select * from person INNER JOIN address ON person.PersonId = address.PersonId 
select * from person, address WHERE person.PersonId = address.PersonId 

Query results ： Notice here PersonId = 4 My little grandson didn't export
Therefore, the inner connection only queries the data contained in both tables

3、 ... and 、 External connection （LEFT JOIN、RIGHT JOIN、FULL JOIN）

Be careful ： When connecting outside , Attention should be paid to where And on The difference between

• 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 .
• 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

1. The left outer join

The result set of the left out join includes all rows of the specified left table , Not just the rows that the join columns match . If a row in the left table does not match a row in the right table , All selection list columns in the right table in the associated result set row are null （null）

select * from person LEFT JOIN address ON person.PersonId = address.PersonId 

Query results ： Notice here PersonId = 4 Of ‘ Little grandson ’ Also output and due to ‘ Little grandson ’ There is no corresponding address, therefore address Data set to null

2. Right connection

The right outer join is the reverse join of the left outer join . All rows of the right table will be returned . If a row in the right table does not match a row in the left table , Null value will be returned for the left table （null）

select * from person RIGHT JOIN address ON person.PersonId = address.PersonId 

Query results ： Notice here AddressId = 6 Of ‘ guangdong ’ Also output and due to ‘ guangdong ’ There is no corresponding person, therefore preson Data set to null

3. Complete external connection

Full outer join returns all rows in the left and right tables . When a row has no matching row in another table , The selection list column of another table contains null values （null）
Mysql China does not support it. FULL JOIN, So here we use UNION Joint query instead of

select * from person LEFT JOIN address ON person.PersonId = address.PersonId 
UNION
select * from person RIGHT JOIN address ON person.PersonId = address.PersonId 

Query results ： At the same time, it outputs ' Little grandson ' as well as ' guangdong '