1.内连接

1.1 概念

• 内连接根据两个表的共同列进行匹配，两表一般存在主外键关系。通常使用“=”比较运算符来判断两列数据是否相等，通过使用inner join关键字进行表之间的关联

1.2 实例：两张表连接

对sc和student表进行内连接，显示学生的学号，姓名，课程号，分数。语法1：select student.sno,student.sname,sc.sno,sc.grage from sc join student on sc.sno=student.sno语法2：select student.sno,student.sname,sc.sno,sc.grage from sc,student where sc.sno=student.sno

1.3 实例：三张表连接

显示学生的学号，姓名，课程名，考试分数语法1：select student.sno,student.sname,course.cname,sc.grage from sc join student on sc.sno=student.sno join course on sc.sno=course.sno语法2：select student.sno,student.sname,course.cname,sc.grage from sc,student,course where sc.sno=student.sno and sc.sno=course.sno 

2.外连接

2.1 左外连接(left join)

让student表和sc表进行左外连接，即不管是学生是否有选修课程，该学生的信息的都会显示出来select student.sno,student.sname,sc.sno,sc.grade from student left outer join sc on student.sno=sc.sno

2.2 右外连接(right join)

让sc表和teacher表进行右外连接，显示教师编号，教师姓名，讲师教授的课程号select teacher.tno,teacher.tname,sc.cno from sc right outer join teacher on sc.tno=teachaer.tno

2.3 全外连接(full join)

让sc表和teacher表进行全外连接，显示教师编号，教师姓名，讲师教授的课程号。teacher.tno,teacher.tname,sc.cno from sc full outer join teacher on sc.tno=teacher.tno 

2.4 交叉连接:表之间没有任何关联

让学生和课程两张表进行交叉连接select *from student cross join course