SQL连接可以分为内连接、外连接、交叉连接。

1.内连接：内连接查询操作列出与连接条件匹配的数据行，它使用比较运算符比较被连接列的列值。

1.1 select * from Table1 as a, Table2 as b where a.id= b.id

1.2 select * from Table1 as a inner join Table2 as b on a.id = b.id

外连接
2.1 左外连接(简单说，左表数据全显示，右不匹配的显示null)

select * from Table1 as a left join Table2 as b on a.id = b.id
2.2 右外连接(同上相反)

select * from Table1 as a right join Table2 as b on a.id = b.id

2.3 全连接(当某行在另一个表中没有匹配行时，则另一个表的选择列表列包含空值。如果表之间有匹配行，则整个结果集行包含基表的数据值)

select * from Table1 as a full outer join Table2 as b on a.id = b.id

3.左表中的每一行与右表中的所有行组合。交叉联接也称作笛卡尔积

select * from Table1 as a cross join Table2 as b order b a.id