中财网消防安全知识讲座主持:SqlServer数据库语句大全（二）
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2.3连接查询Table Joins
多表连接类型可分为三类(内/外/交叉连接)
主从表或者父子表进行多表连接多以主键和外键进行关联
Outer joins(LEFT OUTER, RIGHT OUTER, and FULL OUTER joins)
left outer join:查询的结果以左边表行数为准
right outer join:查询的结果以右边表行数为准
2.3.1.内连接inner join
功能:
语法:
SELECT select_list
FROM table_1
[INNER] JOIN table_2
ON join_condition_1
[[INNER] JOIN table_3
ON join_condition_2]...
代码:
//没有where子句的内连接
SELECT *
FROM Products
INNER JOIN Suppliers
ON Products.SupplierID = Suppliers.SupplierID
//有where子句的内连接
SELECT p.ProductID, s.SupplierID, p.ProductName, s.CompanyName
FROM Products p
INNER JOIN Suppliers s
ON p.SupplierID = s.SupplierID
WHERE p.ProductID < 4
-----------------------------------------------------------
2.3.2.外连接outer join
功能:包括三种连接LEFT OUTER, RIGHT OUTER, and FULL OUTER joins
left outer :查询的结果以左边表行数为准
right outer :查询的结果以右边表行数为准
语法:select ... from table1 [left/right/full outer join ]table2 where ...
代码:
//以Customers表行数为标准去连接Orders表
SELECT c.CustomerID, CompanyName
FROM Customers c
LEFT OUTER JOIN Orders o
ON c.CustomerID = o.CustomerID
WHERE o.CustomerID IS NULL
-----------------------------------------------------------
2.3.3.交叉连接cross join
功能:以主从表或者父子表之间的主键进行连接，最终以笛卡尔乘积运算的结果
语法:select ... from table1 cross join table2 where ...
代码:
//显示结果以表1行数*表2行数
假设Departments为4行记录
假设Jobs为3行记录
下面的显示结果为4*3=12行记录
SELECT deptname,jobdesc FROM Departments CROSS JOIN Jobs
//用关键字匹配的交叉连接
oc_head/oc_detail是主从表
oc_head(主键oc_number)
oc_detail(主键oc_number,item_number,ship_date)
SELECT h.customerid,d.item_number,d.ship_date
from oc_head as h CROSS JOIN oc_detail as d
where h.oc_number=d.oc_number