-- 学生表 Student

建表

  create table Student(SId varchar(10),Sname varchar(10),Sage datetime,Ssex varchar(10));

  insert into Student values('01' , '赵雷' , '1990-01-01' , '男');

  insert into Student values('02' , '钱电' , '1990-12-21' , '男');

  insert into Student values('03' , '孙风' , '1990-05-20' , '男');

  insert into Student values('04' , '李云' , '1990-08-06' , '男');

  insert into Student values('05' , '周梅' , '1991-12-01' , '女');

  insert into Student values('06' , '吴兰' , '1992-03-01' , '女');

  insert into Student values('07' , '郑竹' , '1989-07-01' , '女');

  insert into Student values('09' , '张三' , '2017-12-20' , '女');

  insert into Student values('10' , '李四' , '2017-12-25' , '女');

  insert into Student values('11' , '李四' , '2017-12-30' , '女');

  insert into Student values('12' , '赵六' , '2017-01-01' , '女');

  insert into Student values('13' , '孙七' , '2018-01-01' , '女');

   -- 成绩表 SC

  create table SC(SId varchar(10),CId varchar(10),score decimal(18,1));

  insert into SC values('01' , '01' , 80);

  insert into SC values('01' , '02' , 90);

  insert into SC values('01' , '03' , 99);

  insert into SC values('02' , '01' , 70);

  insert into SC values('02' , '02' , 60);

  insert into SC values('02' , '03' , 80);

  insert into SC values('03' , '01' , 80);

  insert into SC values('03' , '02' , 80);

  insert into SC values('03' , '03' , 80);

  insert into SC values('04' , '01' , 50);

  insert into SC values('04' , '02' , 30);

  insert into SC values('04' , '03' , 20);

  insert into SC values('05' , '01' , 76);

  insert into SC values('05' , '02' , 87);

  insert into SC values('06' , '01' , 31);

  insert into SC values('06' , '03' , 34);

  insert into SC values('07' , '02' , 89);

  insert into SC values('07' , '03' , 98);

 inner join-内连接

内连接中and和where没有区别，都是取连接后的结果进行条件筛选。

不加条件

 

select * from student inner join sc on sc.SId = student.SId

mysql连接查询中and与where的区别_表数据

select * from student inner join sc on sc.SId = student.SId and student.SId = '01'
select * from student inner join sc on sc.SId = student.SId and student.SId = '01'


mysql连接查询中and与where的区别_内连接_02

left join - 左外连接

select * from student inner join sc on sc.SId = student.SId

mysql连接查询中and与where的区别_内连接_03

and 条件

left join中以左表全匹配进行连接，之后使用and进行筛选；不符合条件的左表数据保留，右表数据为null

select * from student inner join sc on sc.SId = student.SId where student.SId = '01'

mysql连接查询中and与where的区别_内连接_04

 where 条件

在left join 中以左表全匹配进行连接，之后以where进行筛选；只筛选符合条件的数据。

select * from student inner join sc on sc.SId = student.SId where student.SId = '01'

mysql连接查询中and与where的区别_表数据_05

 总结

  • 所有连接，使用where是对连接后符合条件的数据行进行再次的条件筛选，只保留符合条件的数据行；
  • left join连接时，使用and以左表为主，左表数据全部保留，不符合条件的数据行右表数据为null；
  • right join连接时，使用and以右表为主，右表数据全部保留，不符合条件的数据行左表数据为null；