SQL連線可以分為內連線、外連線、交叉連線。

1.內連線：內連線查詢操作列出與連線條件匹配的資料行，它使用比較運算子比較被連線列的列值。

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

1. 外連線
   2.1 左外連線(簡單說，左表資料全顯示，右不匹配的顯示null)

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

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

2.3 全連線(當某行在另一個表中沒有匹配行時，則另一個表的選擇列表列包含空值。如果表之間有匹配行，則整個結果集行包含基表的資料值)

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

3.左表中的每一行與右表中的所有行組合。交叉聯接也稱作笛卡爾積

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