join表连接

select  o.CustomerID,c.Name,c.Age

from T_orders as o join T_Customers as c on o.CustomerID=c.id

where c.Age>15

--一个客户订单表。一个客户信息表。唯一联系的是客户信息表，客户订单表 ID=CustomerID

--有关联就可以查

--思路

--select ？？ from T_order as o join T_Customers as c on o.Customer=c.id   俩个表做关联

--关联之后查要找的信息

select  o.CustomerID,c.Name,c.Age

from T_orders as o join T_Customers as c on o.CustomerID=c.id

where c.Age>15

select  o.CustomerID,c.Name,c.Age,o.billno,o.id,c.id

from T_orders as o join T_Customers as c on o.CustomerID=c.id

where c.Age>15

select  o.CustomerID,c.Name,c.Age,o.billno,o.id,c.id

from T_orders as o join T_Customers as c on o.CustomerID=c.id

where

age>(select AVG(age) from T_Customers)

--查询结果可以当作一个表来处理。可以当作表用。用法一样

--例子

select  * from 

(select * from T_Customers) as o