inner join，left outer join，rignt outer join

#anti-forget

Inner join (inner join) is similar to natural join, except that natural join can only query multiple tables with the same column attributes.

And inner join can specify different column attributes of table A and table B to connect, (there must be equal data in the column) Example: Ac = Bd

left outer join (left outer join) A table left outer join B table, that is, use A table as the main table, and connect with the specified column in B table, if there is equal column data, join, if there is no, add Null.

right outer join same.