SQL Server中交叉联接的用法详解

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2022-05-23 0 618

1、交叉联接(cross join)的概念

交叉联接是联接查询的第一个阶段，它对两个数据表进行笛卡尔积。即第一张数据表每一行与第二张表的所有行进行联接，生成结果集的大小等于T1*T2。

select * from t1 cross join t2

2、交叉联接的语法格式

SQL Server中交叉联接的用法详解

select * from t1 cross join t2;--常用写法 
select * from t1, t2;-- SQL:1989的规范 
select * from t1 cross join t2 
where t1.col1=t2.col2;--等价于内部联接 
select * from t1 inner join t2  on t1.col1=t2.col2

3、交叉查询的使用场景

3.1 交叉联接可以查询全部数据

— 示例

-- 员工表 
CREATE TABLE [dbo].[EmpInfo]( 
  [empId] [int] IDENTITY(1,1) NOT NULL, 
  [empNo] [varchar](20) NULL, 
  [empName] [nvarchar](20) NULL, 
 CONSTRAINT [PK_EmpInfo] PRIMARY KEY CLUSTERED  
( 
  [empId] ASC 
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF 
, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] 
) ON [PRIMARY] 
 
-- 奖金表 
CREATE TABLE [dbo].[SalaryInfo]( 
  [id] [int] IDENTITY(1,1) NOT NULL, 
  [empId] [int] NULL, 
  [salary] [decimal](18, 2) NULL, 
  [seasons] [varchar](20) NULL, 
 CONSTRAINT [PK_SalaryInfo] PRIMARY KEY CLUSTERED  
( 
  [id] ASC 
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF 
, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] 
) ON [PRIMARY] 
-- 季度表 
CREATE TABLE [dbo].[Seasons]( 
  [name] [nchar](10) NULL 
) ON [PRIMARY] 
 
GO 
SET IDENTITY_INSERT [dbo].[EmpInfo] ON  
 
INSERT [dbo].[EmpInfo] ([empId], [empNo], [empName]) VALUES (1, N'A001', N'王强') 
INSERT [dbo].[EmpInfo] ([empId], [empNo], [empName]) VALUES (2, N'A002', N'李明') 
INSERT [dbo].[EmpInfo] ([empId], [empNo], [empName]) VALUES (3, N'A003', N'张三') 
 
INSERT [dbo].[SalaryInfo] ([id], [empId], [salary], [seasons]) 
 VALUES (1, 1, CAST(3000.00 AS Decimal(18, 2)), N'第一季度') 
INSERT [dbo].[SalaryInfo] ([id], [empId], [salary], [seasons]) 
 VALUES (2, 3, CAST(5000.00 AS Decimal(18, 2)), N'第一季度') 
INSERT [dbo].[SalaryInfo] ([id], [empId], [salary], [seasons]) 
 VALUES (3, 1, CAST(3500.00 AS Decimal(18, 2)), N'第二季度') 
INSERT [dbo].[SalaryInfo] ([id], [empId], [salary], [seasons]) 
 VALUES (4, 3, CAST(3000.00 AS Decimal(18, 2)), N'第二季度 ') 
INSERT [dbo].[SalaryInfo] ([id], [empId], [salary], [seasons]) 
 VALUES (5, 2, CAST(4500.00 AS Decimal(18, 2)), N'第二季度') 
 
INSERT [dbo].[Seasons] ([name]) VALUES (N'第一季度') 
INSERT [dbo].[Seasons] ([name]) VALUES (N'第二季度') 
INSERT [dbo].[Seasons] ([name]) VALUES (N'第三季度') 
INSERT [dbo].[Seasons] ([name]) VALUES (N'第四季度') 
 
-- 查询每个人每个季度的奖金情况 如果奖金不存在则为0 
SELECT a.empName,b.name seasons ,isnull(c.salary,0) salary  
FROM EmpInfo a  
CROSS JOIN Seasons b 
LEFT OUTER JOIN SalaryInfo c ON a.empId=c.empId AND b.name=c.seasons