pymysql模塊和多表查詢

pymysql模塊

• 建立鏈接 conn

• 獲取游標cur

• 執行sql execute(sql,(可迭代類型的參數集))

  • sql是查 -- 只涉及文件的讀操作

    • fetchone()

    • fetchmany(n)

    • fetvhall()

  • 增刪改 -- 涉及到文件的寫操作

    • conn.comimit()

• 關閉游標和鏈接

  • cur.close()

  • conn.close()

代碼實現：

import pymysql
conn = pymysql.conn(host = 'localhost', user = 'root', password = '123', database = 'demo')  # 返回的是一個對象
​
# cur游標 cur數據庫操作符
cur = conn.curson()  
sql = 'insert into employee values(%s, %s, %s, %s)'
# 操作文件
with open('add.file', 'r', encoding='utf-8') as f:
    for line in f:
        try:
            lst = line.strip().split('|')
            cur.execute(sql,lst)  # 會自動將數據補充
            conn.commit()
        except:
            conn.rollback()
cur.close()
conn.close()
​
​
# sql 注入
# 登錄注冊 + database
# 表 userinfo
​
cur = conn.curson()
name = input('請輸入用戶名：')
password = input('請輸入用戶密碼：')
sql = 'select * from userinfo where username = %s and password = %s;'
ret = cur.execute(sql,(name,password))
if ret:
    print('登陸成功')
else:
    print('登陸失敗')
​

多表查詢

連表

內連接(所有不在條件匹配內的數據，都會被踢出連表)

方式一：
select * from employee,department where dep_id = department.id;
​
方式二：
select * from employee inner join department on dep_id = department.id;
​
實例1：以內連接的方式查詢employee和department表，並且employee表中的age字段值必須大於25,即找出年齡大於25歲的員工以及員工所在的部門
select employee.name,department.name from eemployee inner join department on dep_id = department.id wwhere age > 25;
取別名簡寫代碼：
select e.name,d.name from employee e inner join department d on dep_id = d.id where age > 25;
​
 示例2：以內連接的方式查詢employee和department表，並且以age字段的升序方式顯示
select * from employee inner join department d on dep_id = d.id order by age;

外連接

左外連接 left join
select * from employee left join department on dep_id = department.id;
​
右外連接 right join
select * from employee right join department on dep_id = department.id;
​
全外連接 full join
select * from employee left join department on dep_id = department.id 
union
select * from eemployee right join department on dep_id = department.id;

子查詢

查詢平均年齡再25以上的部門名:
    先再employee中查詢平均年齡再25以上的部門id
    select dep_id from employee group by dep_id having avg(age) > 25;
    再去department中查詢部門名字
    select name from department where id in (select dep_id from employee group by dep_id having avg(age) > 25);
    
查看技術部員工姓名:
    先去department中查詢技術部的id
    select id from department where name = '技術':
    再去employee中查詢員工姓名
    select name from employee where dep_id = (select id from department where name = '技術');
    
查看不足1人的部門名(子查詢得到的是有人的部門id):
    先再employee中查看有人的部門id
    select distinct dep_id from employee;
    再去department中找什么id沒有再上面的表中
    select name from department where id not in (select distinct dep_id from employee);
    
查詢大於所有人平均年齡的員工名與年齡:
    先查所有人的平均年齡
    select avg()age from employee;
    在查詢大於平均年齡的員工名與年齡
    select name,age from employee where age > (select avg(age) from employee);
    
查詢大於部門內平均年齡的員工名、年齡:
    先查詢各部門的平均年齡
    select dep_id,avg(age) from employee group by dep_id;
    將上面所得到表與employee表連接起來
    select * from employee as e inner join (select dep_id,avg(age) from employee group by dep_id) as t on e.dep_id = t.dep_id;
    再查詢大於各部門中大於平均年齡的人
    select * from employee as e inner join (select dep_id,avg(age) from employee group by dep_id) as t on e.dep_id = t.dep_id where e.age > t.avg(age);