a表 left join b表 时必定连接一张表
(a表永远保留 ,操作的全是b表)
(1)如果on条件作用在b表字段上,则不符合条件的时候,当前b表所有的内容都清空
(2)如果加on条件作用在a表字段上,则不符合条件的时候,清空当前b表的所有记录
(3)where条件是作用在连接后的复合表的记录(a、b表当成一个整体),不符合条件,直接删除这条组合记录
table : STUDENT(id, name); CLASS(id, class):
id name id class
1 "张三" 1 no.1
2 “李四” 2 no.2
3 “王五” 3 no.2
4 "赵六"
Sql语句1: SELECT a.id, a.name, b.class FROM STUDENT AS s LEFT JOIN CLASS AS c ON s.id=c.id AND s.name="张三";
id name class
1 "张三" no.1
2 “李四” (不符合条件清空)
3 “王五” (不符合条件清空)
4 "赵六" (不符合条件清空)
Sql语句2: SELECT a.id, a.name, b.class FROM STUDENT AS s LEFT JOIN CLASS AS c ON s.id=c.id AND c.class="no.2";
id name class
1 "张三" (不符合条件清空)
2 “李四” no.2
3 “王五” no.2
4 "赵六" (不符合条件清空)
Sql语句3: SELECT a.id, a.name, b.class FROM STUDENT AS s LEFT JOIN CLASS AS c ON s.id=c.id WHERE s.name="张三";
id name class
1 "张三" no.1
(不符合条件清空)
(不符合条件清空)
(不符合条件清空)
Sql语句4: SELECT a.id, a.name, b.class FROM STUDENT AS s LEFT JOIN CLASS AS c ON s.id=c.id WHERE c.class="no.2";
id name class
(不符合条件清空)
2 “李四” no.2
3 “王五” no.2
(不符合条件清空)
******************************************************
inner join 只有符合条件的数据才显示 ON 和WHERE 作用相同
Sql语句5: SELECT a.id, a.name, b.class FROM STUDENT AS s INNER JOIN CLASS AS c ON s.id=c.id AND s.name="张三";
id name class
1 "张三" no.1
(不符合条件清空)
(不符合条件清空)
(不符合条件清空)
Sql语句6: SELECT a.id, a.name, b.class FROM STUDENT AS s
INNER
JOIN CLASS AS c ON s.id=c.id AND c.class="no.2";
id name class
(不符合条件清空)
2 “李四” no.2
3 “王五” no.2
(不符合条件清空)
Sql语句7: SELECT a.id, a.name, b.class FROM STUDENT AS s
INNER
JOIN CLASS AS c ON s.id=c.id WHERE s.name="张三";
id name class
1 "张三" no.1
(不符合条件清空)
(不符合条件清空)
(不符合条件清空)
Sql语句8: SELECT a.id, a.name, b.class FROM STUDENT AS s
INNER
JOIN CLASS AS c ON s.id=c.id WHERE c.class="no.2";
id name class
(不符合条件清空)
2 “李四” no.2
3 “王五” no.2
(不符合条件清空)