自關聯操作
多表連接查詢 inner join 內查詢 left join 左查詢 right join 右查詢
select student.id,student.name,classes.name from student inner join classes on student.cls_id = classes.id;
inner join內連 left join左連 right join右連 表名join 表名 on 連接字段
select student.name,course_student.cid from student left join course_student on student.id = course_student.sid;
以student.id與course_student.sid連接匹配,打印student.name,course_student.cid
select student.name,course.name from student left join course_student on student.id = course_student.sid left join course on course_student.cid =course.id; 三表(2明細表+1關系表)相連取交集打印學生名字與課程名字
select * from areas as p inner join areas as a on p.aid = a.pid where p.atitle = '河南省';
省、市，兩表查詢在一張表里(as起別名)用上級id打印，
select * from areas as p inner join areas as c on c.pid = p.aid inner join areas as a on a.pid = c.aid where a.atitle = '二七區';
省、市、區，三表查詢在一張表里(as起別名)用上級id打印區表title為‘二七區’的數據
select * from from areas as p inner join areas as c on c.pid = p.aid inner join areas as as a on a.pid = c.aid where p.atitle = '河南省';
打印河南省所有的市和所有的區