python 之 数据库(多表查询之连接查询子查询pymysql模块的使用)
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10.10 多表连接查询
10.101 内连接
把两张表有对应关系的记录连接成一张虚拟表
select * from emp,dep; #连接两张表的笛卡尔积 select * from emp,dep where emp.dep_id = dep.id; # 不推荐用where连接表 select * from emp inner join dep on emp.dep_id = dep.id; #推荐 +----+-----------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+-----------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技术 | +----+-----------+--------+------+--------+------+--------------+
#应用: select * from emp,dep where emp.dep_id = dep.id and dep.name = "技术"; select * from emp inner join dep on emp.dep_id = dep.id where dep.name = "技术"; +----+-----------+------+------+--------+------+--------+ | id | name | sex | age | dep_id | id | name | +----+-----------+------+------+--------+------+--------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技术 | +----+-----------+------+------+--------+------+--------+
10.102 左连接
在内连接的基础上,保留左边没有对应关系的记录
select * from emp left join dep on emp.dep_id = dep.id; +----+------------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+------------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 6 | jingliyang | female | 18 | 204 | NULL | NULL | +----+------------+--------+------+--------+------+--------------+
10.103 右连接
在内连接的基础上,保留右边没有对应关系的记录
select * from emp right join dep on emp.dep_id = dep.id; +------+-----------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +------+-----------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技术 | | NULL | NULL | NULL | NULL | NULL | 203 | 运营 | +------+-----------+--------+------+--------+------+--------------+
10.104 全连接
在内连接的基础上,保留左、右边没有对应关系的记录,并去重
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 | egon | male | 18 | 200 | 200 | 技术 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技术 | | 2 | alex | female | 48 | 201 | 201 | 人力资源 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 | | 4 | yuanhao | female | 28 | 202 | 202 | 销售 | | 6 | jingliyang | female | 18 | 204 | NULL | NULL | | NULL | NULL | NULL | NULL | NULL | 203 | 运营 | +------+------------+--------+------+--------+------+--------------+
补充:多表连接可以不断地与虚拟表连接
#查找各部门最高工资 select t1.* from emp as t1 inner join (select post,max(salary) as ms from emp group by post) as t2 on t1.post = t2.post where t1.salary = t2.ms;
10.11 子查询
把一个查询语句用括号括起来,当做另外一条查询语句的条件去用,称为子查询
#查询技术部员工的名字 select emp.name from emp inner join dep on emp.dep_id = dep.id where dep.name="技术";#连接查询 select name from emp where dep_id =(select id from dep where name="技术"); #子查询 +-----------+ | name | +-----------+ | egon | | liwenzhou | +-----------+ #查询平均年龄在25岁以上的部门名 #子查询 select name from dep where id in (select dep_id from emp group by dep_id having avg(age) > 25); select dep.name from emp inner join dep on emp.dep_id = dep.id #连接查询 group by dep.name having avg(age) > 25; +--------------+ | name | +--------------+ | 人力资源 | | 销售 | +--------------+ #查询每个部门最新入职的那位员工 select t1.id,t1.name,t1.post,t1.hire_date,t2.post,t2.max_date from (emp as t1) inner join (select post,max(hire_date) as max_date from emp group by post) as t2 #拿到最大雇佣时间 on t1.post = t2.post where t1.hire_date = t2.max_date; +----+--------+-----------------------------------------+---- | id | name | post | hire_date | post | max_date | +----+--------+-----------------------------------------+----- | 1 | egon | 外交大使 | 2017-03-01 | 外交大使 | 2017-03-01 | | 2 | alex | teacher | 2015-03-02 | teacher | 2015-03-02 | | 13 | 格格 | sale | 2017-01-27 | sale | 2017-01-27 | | 14 | 张野 | operation| 2016-03-11 | operation| 2016-03-11 | +----+--------+-----------------------------------------+-----
exists( ):括号内的值存在时满足条件
select * from emp where exists (select id from dep where id > 3); #找到所有
10.12 pymysql模块的使用
10.121 pymysql查
import pymysql #pip3 install pymysql conn=pymysql.connect( #连接 host=‘127.0.0.1‘, port=3306, user=‘root‘, password=‘‘, database=‘db2‘, charset=‘utf8‘) cursor=conn.cursor(pymysql.cursors.DictCursor)#以字典形式显示表的记录 rows=cursor.execute(‘show tables;‘) #1 显示受影响的行数(row),此处为有表的条数 print(rows) rows=cursor.execute(‘select * from emp;‘) #18 此处rows为emp表内有记录的条数 print(rows) ? print(cursor.fetchone()) #查看一条记录 一个字典key:value print(cursor.fetchmany(2)) #查看多条记录 [key:value,] #print(cursor.fetchall()) #查看所有记录 强调:下一次查找是接着上一次查找的位置继续 ? cursor.scroll(0,‘absolute‘) #绝对移动,以0位置为参照显示 print(cursor.fetchone()) ? cursor.scroll(1,‘relative‘) #相对移动,相对当前位置移动1条记录 print(cursor.fetchone()) ? cursor.close()#光标 conn.close()
10.122 防止sql注入问题
在服务端防止sql注入问题:不要自己拼接字符串,让pymysql模块去拼接,pymysql拼接时会过滤非法字符
import pymysql conn=pymysql.connect( host=‘127.0.0.1‘, port=3306, user=‘root‘, password=‘‘, database=‘db2‘, charset=‘utf8‘ ) cursor=conn.cursor(pymysql.cursors.DictCursor) ? inp_user=input(‘用户名>>:‘).strip() #inp_user="" inp_pwd=input(‘密码>>:‘).strip() #inp_pwd="" sql="select * from user where username=%s and password=%s" print(sql) rows=cursor.execute(sql,(inp_user,inp_pwd))#输入的用户名和密码中的非法字符会被过滤掉 if rows: print(‘登录成功‘) else: print(‘登录失败‘) cursor.close() conn.close()
10.123 pymysql增删改
import pymysql conn=pymysql.connect( host=‘127.0.0.1‘, port=3306, user=‘root‘, password=‘‘, database=‘db2‘, charset=‘utf8‘) cursor=conn.cursor(pymysql.cursors.DictCursor) sql=‘insert into user(username,password) values(%s,%s)‘ #插入单行记录 rows=cursor.execute(sql,(‘EGON‘,‘123456‘)) print(rows) print(cursor.lastrowid) #显示当前最后一行的id ? sql=‘insert into user(username,password) values(%s,%s)‘ #一次插入多行记录 rows=cursor.executemany(sql,[(‘lwz‘,‘123‘),(‘evia‘,‘455‘),(‘lsd‘,‘333‘)]) print(rows) ? rows=cursor.execute(‘update user set username="alexSB" where id=2‘)#修改记录 print(rows) ? conn.commit() # 只有commit提交才会完成真正的修改 cursor.close() conn.close()
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