标签：左右 href link 结果 incr _id 连接查询 左连接 ref
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准备表和记录
#建表 create table dep( id int, name varchar(20) ); create table emp( id int primary key auto_increment, name varchar(20), sex enum(‘male‘,‘female‘) not null default ‘male‘, age int, dep_id int ); #插入数据 insert into dep values (200,‘技术‘), (201,‘人力资源‘), (202,‘销售‘), (203,‘运营‘)
(205,‘美工‘); insert into emp(name,sex,age,dep_id) values (‘jason‘,‘male‘,18,200), (‘egon‘,‘female‘,48,201), (‘kevin‘,‘male‘,18,201), (‘nick‘,‘male‘,28,202), (‘owen‘,‘male‘,18,203), (‘jerry‘,‘female‘,18,204);
#重点：外链接语法 SELECT 字段列表 FROM 表1 INNER|LEFT|RIGHT|union JOIN 表2 ON 表1.字段 = 表2.字段;
""" inner join 内连接 left join 左连接 right join 右连接 union 全连接 """
mysql> select * from dep,emp; # 结果 笛卡尔积，是把员工表中的每条记录和部门表中的每条记录映射一遍 +------+--------------+----+-------+--------+------+--------+ | id | name | id | name | sex | age | dep_id | +------+--------------+----+-------+--------+------+--------+ | 200 | 技术 | 1 | jason | male | 18 | 200 | | 201 | 人力资源 | 1 | jason | male | 18 | 200 | | 202 | 销售 | 1 | jason | male | 18 | 200 | | 203 | 运营 | 1 | jason | male | 18 | 200 | | 200 | 技术 | 2 | egon | female | 48 | 201 | | 201 | 人力资源 | 2 | egon | female | 48 | 201 | | 202 | 销售 | 2 | egon | female | 48 | 201 | | 203 | 运营 | 2 | egon | female | 48 | 201 | | 200 | 技术 | 3 | kevin | male | 18 | 201 | | 201 | 人力资源 | 3 | kevin | male | 18 | 201 | | 202 | 销售 | 3 | kevin | male | 18 | 201 | | 203 | 运营 | 3 | kevin | male | 18 | 201 | | 200 | 技术 | 4 | nick | male | 28 | 202 | | 201 | 人力资源 | 4 | nick | male | 28 | 202 | | 202 | 销售 | 4 | nick | male | 28 | 202 | | 203 | 运营 | 4 | nick | male | 28 | 202 | | 200 | 技术 | 5 | owen | male | 18 | 203 | | 201 | 人力资源 | 5 | owen | male | 18 | 203 | | 202 | 销售 | 5 | owen | male | 18 | 203 | | 203 | 运营 | 5 | owen | male | 18 | 203 | | 200 | 技术 | 6 | jerry | female | 18 | 204 | | 201 | 人力资源 | 6 | jerry | female | 18 | 204 | | 202 | 销售 | 6 | jerry | female | 18 | 204 | | 203 | 运营 | 6 | jerry | female | 18 | 204 | +------+--------------+----+-------+--------+------+--------+ 24 rows in set (0.00 sec) mysql> #那如何low的得到两个表拼接的内容 mysql> select * from emp,dep where emp.dep_id = dep.id; +----+-------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+-------+--------+------+--------+------+--------------+ | 1 | jason | male | 18 | 200 | 200 | 技术 | | 2 | egon | female | 48 | 201 | 201 | 人力资源 | | 3 | kevin | male | 18 | 201 | 201 | 人力资源 | | 4 | nick | male | 28 | 202 | 202 | 销售 | | 5 | owen | male | 18 | 203 | 203 | 运营 | +----+-------+--------+------+--------+------+--------------+ 5 rows in set (0.00 sec) mysql>
# 只拼接两张表中公有的数据部分
mysql> select * from emp inner join dep on emp.dep_id = dep.id; +----+-------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+-------+--------+------+--------+------+--------------+ | 1 | jason | male | 18 | 200 | 200 | 技术 | | 2 | egon | female | 48 | 201 | 201 | 人力资源 | | 3 | kevin | male | 18 | 201 | 201 | 人力资源 | | 4 | nick | male | 28 | 202 | 202 | 销售 | | 5 | owen | male | 18 | 203 | 203 | 运营 | +----+-------+--------+------+--------+------+--------------+ 5 rows in set (0.00 sec) mysql>
# 左表所有的数据都展示出来 没有对应的项就用NULL
mysql> select * from emp left join dep on emp.dep_id = dep.id; +----+-------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+-------+--------+------+--------+------+--------------+ | 1 | jason | male | 18 | 200 | 200 | 技术 | | 2 | egon | female | 48 | 201 | 201 | 人力资源 | | 3 | kevin | male | 18 | 201 | 201 | 人力资源 | | 4 | nick | male | 28 | 202 | 202 | 销售 | | 5 | owen | male | 18 | 203 | 203 | 运营 | | 6 | jerry | female | 18 | 204 | NULL | NULL | +----+-------+--------+------+--------+------+--------------+ 6 rows in set (0.00 sec) mysql>
# 右表所有的数据都展示出来 没有对应的项就用NULL
mysql> select * from emp right join dep on emp.dep_id = dep.id; +------+-------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +------+-------+--------+------+--------+------+--------------+ | 1 | jason | male | 18 | 200 | 200 | 技术 | | 2 | egon | female | 48 | 201 | 201 | 人力资源 | | 3 | kevin | male | 18 | 201 | 201 | 人力资源 | | 4 | nick | male | 28 | 202 | 202 | 销售 | | 5 | owen | male | 18 | 203 | 203 | 运营 | | NULL | NULL | NULL | NULL | NULL | 205 | 美工 | +------+-------+--------+------+--------+------+--------------+ 6 rows in set (0.00 sec) mysql>
select * from emp left join dep on emp.dep_id = dep.id
union
select * from emp right join dep on emp.dep_id = dep.id;
mysql> select * from emp left join dep on emp.dep_id = dep.id -> union -> select * from emp right join dep on emp.dep_id = dep.id; +------+-------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +------+-------+--------+------+--------+------+--------------+ | 1 | jason | male | 18 | 200 | 200 | 技术 | | 2 | egon | female | 48 | 201 | 201 | 人力资源 | | 3 | kevin | male | 18 | 201 | 201 | 人力资源 | | 4 | nick | male | 28 | 202 | 202 | 销售 | | 5 | owen | male | 18 | 203 | 203 | 运营 | | 6 | jerry | female | 18 | 204 | NULL | NULL | | NULL | NULL | NULL | NULL | NULL | 205 | 美工 | +------+-------+--------+------+--------+------+--------------+ 7 rows in set (0.00 sec) mysql>
#示例1：以内连接的方式查询emp和dep表，并且emp表中的age字段值必须大于25,即找出年龄大于25岁的员工以及员工所在的部门 mysql> select emp.name as "员工姓名",dep.name as "部门名称" from emp inner join dep on emp.dep_id = dep.id where age > 25; +--------------+--------------+ | 员工姓名 | 部门名称 | +--------------+--------------+ | egon | 人力资源 | | nick | 销售 | +--------------+--------------+ 2 rows in set (0.00 sec) mysql> #示例2：以内连接的方式查询employee和department表，并且以age字段的升序方式显示 mysql> select emp.id,emp.name,emp.age,dep.name from emp,dep -> where emp.dep_id = dep.id -> and age > 25 -> order by age asc; +----+------+------+--------------+ | id | name | age | name | +----+------+------+--------------+ | 4 | nick | 28 | 销售 | | 2 | egon | 48 | 人力资源 | +----+------+------+--------------+ 2 rows in set (0.00 sec) mysql>
""" 子查询就是我们平时解决问题的思路 分步骤解决问题 第一步 第二步 ... 将一个查询语句的结果当做另外一个查询语句的条件去用 """
# 查询部门是技术或者人力资源的员工信息 1 先获取部门的id号 2 再去员工表里面筛选出对应的员工 select id from dep where name=‘技术‘ or name = ‘人力资源‘; select name from emp where dep_id in (200,201); #上面两步等同于子查询： select * from emp where dep_id in (select id from dep where name=‘技术‘ or name = ‘人力资源‘); mysql> select * from emp where dep_id in (select id from dep where name=‘技术‘ or name = ‘人力资源‘); +----+-------+--------+------+--------+ | id | name | sex | age | dep_id | +----+-------+--------+------+--------+ | 1 | jason | male | 18 | 200 | | 2 | egon | female | 48 | 201 | | 3 | kevin | male | 18 | 201 | +----+-------+--------+------+--------+ 3 rows in set (0.00 sec) mysql>
总结：
表的查询结果可以作为其他表的查询条件 也可以通过起别名的方式把它作为一个张虚拟表根其他表关联 """ 多表查询就两种方式 先拼接表再查询 子查询 一步一步来 """
标签：左右 href link 结果 incr _id 连接查询 左连接 ref
原文地址：https://www.cnblogs.com/baicai37/p/12837838.html
