FROM join_table join_type join_table[ON (join_condition)]

select * from T_student s,T_class c where s.classId = c.classId 

 select * from T_student s inner join T_class c on s.classId = c.classId 

select * from T_student s inner join T_class c on s.classId <> c.classId

 select s.*,c.className from T_student s inner join T_class c on s.classId = c.classId

select * from T_student s left join T_class c on s.classId = c.classId

 select * from T_student s right join T_class c on s.classId = c.classId

 select * from T_student s full join T_class c on s.classId = c.classId

 select * from T_student s cross join T_class c where s.classId = c.classId 

 --内连接：保罗包括等值连接，不等值连接，自然连接
 
 --等值连接;在连接条件中使用等号运算符，其查询结果中，
 --列出被连接表中的所有列,包括其中的重复列
 SELECT * FROM dbo.Territories,dbo.Region
 WHERE dbo.Region.RegionID=dbo.Territories.RegionID;
 
 SELECT * FROM dbo.Territories
 INNER JOIN dbo.Region
 ON dbo.Territories.RegionID=dbo.Region.RegionID;
 
 
 --不等值连接：在连接条件中，使用除等号之外的运算符
 SELECT * FROM dbo.Territories 
 INNER JOIN dbo.Region
 ON dbo.Region.RegionID<>dbo.Territories.RegionID;
 
 
 --自然连接
 
 --连接条件和等值连接条件相同,但是会删除表中的重复列
 SELECT T.*,R.RegionDescription FROM dbo.Territories AS T
 INNER JOIN dbo.Region AS R
 ON R.RegionID = T.RegionID;
 
 --外连接:分为左连接，右连接，全连接，或者叫，左外连接，右外连接，全外连接
 

 --左连接
 SELECT * FROM dbo.Region
 LEFT JOIN dbo.Territories
 ON Territories.TerritoryDescription = Region.RegionDescription;
 
 --右连接
 SELECT * FROM dbo.Region
 RIGHT JOIN dbo.Territories
 ON Territories.TerritoryDescription = Region.RegionDescription;
 
 
 --全连接
 SELECT * FROM dbo.Region
FULL JOIN dbo.Territories
ON Territories.TerritoryDescription = Region.RegionDescription;
 
 --交叉连接：也称笛卡尔积
 --不带where条件子句,将会返回被连接两个表的笛卡尔积,返回结果的行数等于两个表行数的乘积,如果带where，返回或显示匹配的行数
 
SELECT * FROM dbo.Region,dbo.Territories;
 
 --或者
SELECT * FROM dbo.Region CROSS JOIN dbo.Territories;