col 可能 dad 財經大學 插入數據 not var engine blog

工作中我們經常會使用表與表關聯來查詢數據，如果對join 不熟悉，可能會得到我們不想要的節過，這裏就來介紹下join的幾種常用方法：
建表及插入數據，
CREATE TABLE school (
sch_id int(11) NOT NULL AUTO_INCREMENT,
sch_name varchar(50) NOT NULL,
sch_addr varchar(100) DEFAULT NULL,
PRIMARY KEY (sch_id)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8;

CREATE TABLE student (
st_id int(11) NOT NULL AUTO_INCREMENT,

st_name varchar(20) NOT NULL,
age smallint(6) DEFAULT NULL,
hight int(5) DEFAULT NULL,
sch_id int(11) DEFAULT NULL,
PRIMARY KEY (st_id),
KEY sch_id (sch_id)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8 ;

INSERT INTO school VALUES (1,‘南開大學‘,‘南開‘),(2,‘中央財經大學‘,‘北京‘),(3,‘香港理工大學‘,‘香港‘),(4,‘西安交通大學‘,‘西安‘),(5,‘悉尼大學‘,‘悉尼‘),(6,‘曼徹斯特大學‘,‘曼徹斯特‘),(8,‘延安抗日軍政大學‘,‘延安‘);

INSERT INTO student VALUES (1,‘王曉陽‘,26,168,6),(2,‘王楠‘,28,162,2),(3,‘楊振宇‘,30,178,1),(4,‘苗昕‘,28,162,3),(5,‘張詩雨‘,27,171,5),(8,‘李倩‘,28,162,4),(9,‘蔣結石‘,26,178,7);

1.左關聯：以左表為中心，查出左表的全部數據，關聯字段值不相等則右表查出的數據顯示為空；
select * from school a left join student b on a.sch_id=b.sch_id;

2.右關聯：以右表為中心，查出右表的全部數據，關聯字段值不相等則左表查出的數據顯示為空；

select * from school a right join student b on a.sch_id=b.sch_id;

3.內關聯：查出兩表關聯字段等值的數據
select * from school a inner join student b on a.sch_id=b.sch_id;

4.查出只屬於左表的數據
select * from school a left join student b on a.sch_id=b.sch_id where b.st_id is null;

5.查出只屬於右表的數據
select * from school a right join student b on a.sch_id=b.sch_id where a.sch_id is null;

6.查出全部數據
select from school a left join student b on a.sch_id=b.sch_id union select from school a right join student b on a.sch_id=b.sch_id;

7.查出左表和右表關聯不相等的數據
select from school a left join student b on a.sch_id=b.sch_id where b.st_id is null union select from school a right join student b on a.sch_id=b.sch_id where a.sch_id is null;

MySQL表關聯的幾種常用方式