SQL | external connection

External connection (OUTER JOIN) Divided into three
1. The left outer join (LEFT OUTER JOIN or LEFT JOIN）： All the records in the left table will be shown , The right table will only display the records that meet the search criteria , The missing records in the right table are NULL
2. Right connection (RIGHT OUTER JOIN or RIGHT JOIN)： And left ( Outside ) The connection is the opposite , Right ( Outside ) Connect , All the records in the right table will be shown , The left table will only display the records that meet the search criteria , All the places that are not recorded in the left table are NULL
3. Full outer join (FULL OUTER JOIN or FULL JOIN)： There are no restrictions on the left and right tables , All the records show , The two meters are not enough null fill
MySQL All external connections are not supported in , have access to UNION To merge two or more SELECT The result set of the statement