SQL联接查询

SELECT C.ID D.NAME

FROM dbo.test as c

CROSS JOIN dbo.testt as d;
//把多个表的列放在一起

交叉联接

SELECT E.empid, E.firstname,E.lastname,O.orderid
FROM HR.Employess AS E
   JOIN Sale.orders AS O
    ON E.empid=O.empid;
//两个相同的地方内联接查询

内联接

SELECT E.empid, E.firstname,E.lastname,O.orderid
FROM HR.Employess AS E
   JOIN Sale.orders AS O
    ON E.empid=O.empid;
    AND E.ID=O.ID
//多个相同的地方内联接查询

组合联接

SELECT E.empid, E.firstname,E.lastname,O.orderid
FROM HR.Employess AS E
   JOIN Sale.orders AS O
    ON E.empid<O.empid;
//与内联接相同，而内联接是等值联接，包含等号以外的运算符的联接是不等联接

不等联接

SELECT
c.custid,c.companyname,o.orderid,
OD.productid,OD.qty
FROM Sales.customers AS c
 JOIN SALES.Orders as o
 ON c.custid=o.custid
 JOIN Sales.OrderDetails AS OD
 ON o.orderid=OD.orderid;

多表联接

seclect c.custid,c.companyname
from sales.customers as c
left outer join sales.orders as o
On c.custid=o.custid
where o.orderis is null;
//查询没有订单的客户
//左外联接 left outer join 保留左边表和相同部分
//右外联接 right outer join 保留右边表和相同部分
// full outer join 保留左右两边表和相同部分
//PS：外联接保留内部行和外部行，而内联接只返回内部行

外联接

select orderid,orderdate,empid
from sales.orders
where orderid=(select max(o.orderid) from sales.orders as o);
//独立标量子查询，对于有效的标量子查询，返回值不能超过一个
//独立多值子查询，将=换成in 返回值可以多个
//独立的意思是子查询都可以单独拿出来用 不依赖外部查询

独立子查询

select custid,orderid,orderdate,empid
from sales.orders as o1
where orderid=
(select max(o2.orderid)
from sales.orders as o2
where o2.custid=o1.custid);
//子查询会为每个外部行单独计算一次
//类似两层For循环