今天给大家介绍SQLServer中交叉联接的用法，希望对大家能有所帮助！

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

select * from t1 cross join t2;--常用写法select * from t1, t2;-- SQL:1989的规范select * from t1 cross join t2where t1.col1=t2.col2;--等价于内部联接select * from t1 inner join t2  on t1.col1=t2.col2

-- 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]GOSET 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0SELECT a.empName,b.name seasons ,isnull(c.salary,0) salary FROM EmpInfo a CROSS JOIN Seasons bLEFT OUTER JOIN SalaryInfo c ON a.empId=c.empId AND b.name=c.seasons 

针对一些情况可以采用交叉联接的方式替代子查询，通过减少子查询造成的多次表扫描，从而可以提高优化查询的性能。

4、总结

交叉联接虽然支持使用WHERE子句筛选行，由于笛卡儿积占用的资源可能会很多，如果不是真正需要笛卡儿积的情况下，则应当避免地使用CROSS JOIN。建议使用INNER JOIN代替，效率会更高一些。如果需要为所有的可能性都返回数据联接查询可能会非常实用。