sql语句表关联
1.基础表student-学生表,class--班级表
2.内部关联,两种方式
2.1.使用“=”或者“<>”符号
select class.name as class_name ,student.name as student_name from class,student where class.id=student.class_id
2.2.使用inner join进行表关联
select class.name as class_name ,student.name as student_name from class inner join student on class.id=student.class_id
返回结果相同为:
分析结果:两张表进行笛卡尔积运算之后筛选出条件相符的数据。
说明:笛卡尔积运算(a,b)(1,2,3)运算结果为(a,1)(a,2),(a,3)(b,1)(b,2)(b,3),其中两个基础表的笛卡尔积结果为
3.左关联 left join
select class.name as class_name ,student.name as student_name from class left join student on class.id=student.class_id
运行结果
分析结果:左边表数据全部显示,当在右边表找不到结果是,字段内容为null
4.右关联 right join
select class.name as class_name ,student.name as student_name from class right join student on class.id=student.class_id
分析结果:右边表数据全部显示,当在左边表找不到结果是,字段内容为null
注:自我理解,如若不对请指教。