python 之 數據庫（多表查詢之連接查詢、子查詢、pymysql模塊的使用）

10.10 多表連接查詢

10.101 內連接

把兩張表有對應關系的記錄連接成一張虛擬表

select * from emp,dep；                                  #連接兩張表的笛卡爾積
select * from emp,dep where emp.dep_id = dep.id;            # 不推薦用where連接表
select * from emp inner join dep on emp.dep_id = dep.id;    #推薦
+----+-----------+--------+------+--------+------+--------------+
| id | name      | sex    | age  | dep_id | id   | name         |
+----+-----------+--------+------+--------+------+--------------+
|  1 | egon      | male   |   18 |    200 |  200 | 技術         |
|  2 | alex      | female |   48 |    201 |  201 | 人力資源     |
|  3 | wupeiqi   | male   |   38 |    201 |  201 | 人力資源     |
|  4 | yuanhao   | female |   28 |    202 |  202 | 銷售         |
|  5 | liwenzhou | male   |   18 |    200 |  200 | 技術         |
+----+-----------+--------+------+--------+------+--------------+

#應用：
select * from emp,dep where emp.dep_id = dep.id and dep.name = "技術"; 
select * from emp inner join dep on emp.dep_id = dep.id where dep.name = "技術";
+----+-----------+------+------+--------+------+--------+
| id | name      | sex  | age  | dep_id | id   | name   |
+----+-----------+------+------+--------+------+--------+
|  1 | egon      | male |   18 |    200 |  200 | 技術   |
|  5 | liwenzhou | male |   18 |    200 |  200 | 技術   |
+----+-----------+------+------+--------+------+--------+

10.102 左連接

在內連接的基礎上，保留左邊沒有對應關系的記錄

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 | 技術         |
|  5 | liwenzhou  | male   |   18 |    200 |  200 | 技術         |
|  2 | alex       | female |   48 |    201 |  201 | 人力資源     |
|  3 | wupeiqi    | male   |   38 |    201 |  201 | 人力資源     |
|  4 | yuanhao    | female |   28 |    202 |  202 | 銷售         |
|  6 | jingliyang | female |   18 |    204 | NULL | NULL         |
+----+------------+--------+------+--------+------+--------------+

10.103 右連接

在內連接的基礎上，保留右邊沒有對應關系的記錄

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 | 技術         |
|    2 | alex      | female |   48 |    201 |  201 | 人力資源     |
|    3 | wupeiqi   | male   |   38 |    201 |  201 | 人力資源     |
|    4 | yuanhao   | female |   28 |    202 |  202 | 銷售         |
|    5 | liwenzhou | male   |   18 |    200 |  200 | 技術         |
| NULL | NULL      | NULL   | NULL |   NULL |  203 | 運營         |
+------+-----------+--------+------+--------+------+--------------+

10.104 全連接

在內連接的基礎上，保留左、右邊沒有對應關系的記錄，並去重

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 | 技術         |
|    5 | liwenzhou  | male   |   18 |    200 |  200 | 技術         |
|    2 | alex       | female |   48 |    201 |  201 | 人力資源     |
|    3 | wupeiqi    | male   |   38 |    201 |  201 | 人力資源     |
|    4 | yuanhao    | female |   28 |    202 |  202 | 銷售         |
|    6 | jingliyang | female |   18 |    204 | NULL | NULL         |
| NULL | NULL       | NULL   | NULL |   NULL |  203 | 運營         |
+------+------------+--------+------+--------+------+--------------+

補充：多表連接可以不斷地與虛擬表連接

#查找各部門最高工資
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 子查詢

把一個查詢語句用括號括起來，當做另外一條查詢語句的條件去用，稱為子查詢

#查詢技術部員工的名字
select emp.name from emp inner join dep on emp.dep_id = dep.id where dep.name="技術";#連接查詢
select name from emp where dep_id =(select id from dep where name="技術");          #子查詢
+-----------+
| name      |
+-----------+
| egon      |
| liwenzhou |
+-----------+
#查詢平均年齡在25歲以上的部門名                                                       #子查詢
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                         #連接查詢
    group by dep.name
    having avg(age) > 25;
+--------------+
| name         |
+--------------+
| 人力資源      |
| 銷售          |
+--------------+
#查詢每個部門最新入職的那位員工
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   #拿到最大雇佣時間
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 | 張野   | operation| 2016-03-11 | operation| 2016-03-11 |
+----+--------+-----------------------------------------+-----

exists( )：括號內的值存在時滿足條件

select * from emp where exists (select id from dep where id > 3);       #找到所有

10.12 pymysql模塊的使用

10.121 pymysql查

import pymysql              #pip3 install pymysql
conn=pymysql.connect(        #連接
    host='127.0.0.1',
    port=3306,
    user='root',
    password='',
    database='db2',
    charset='utf8')
cursor=conn.cursor(pymysql.cursors.DictCursor)#以字典形式顯示表的記錄
rows=cursor.execute('show tables;')           #1 顯示受影響的行數(row),此處為有表的條數
print(rows)
rows=cursor.execute('select * from emp;')      #18 此處rows為emp表內有記錄的條數
print(rows)
​
print(cursor.fetchone())     #查看一條記錄 一個字典{key:value}
print(cursor.fetchmany(2))   #查看多條記錄 [{key:value},]
#print(cursor.fetchall())    #查看所有記錄 強調：下一次查找是接着上一次查找的位置繼續
​
cursor.scroll(0,'absolute')  #絕對移動，以0位置為參照顯示
print(cursor.fetchone())
​
cursor.scroll(1,'relative')  #相對移動，相對當前位置移動1條記錄
print(cursor.fetchone())
​
cursor.close()#光標
conn.close()

10.122 防止sql注入問題

在服務端防止sql注入問題：不要自己拼接字符串，讓pymysql模塊去拼接，pymysql拼接時會過濾非法字符

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('用戶名>>：').strip() #inp_user=""
inp_pwd=input('密碼>>：').strip() #inp_pwd=""
sql="select * from user where username=%s and password=%s"
print(sql)
            
rows=cursor.execute(sql,(inp_user,inp_pwd))#輸入的用戶名和密碼中的非法字符會被過濾掉
if rows:
    print('登錄成功')
else:
    print('登錄失敗')
cursor.close()
conn.close()

10.123 pymysql增刪改

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)'         #插入單行記錄
rows=cursor.execute(sql,('EGON','123456'))              
print(rows)
print(cursor.lastrowid)                                      #顯示當前最后一行的id
​
sql='insert into user(username,password) values(%s,%s)'         #一次插入多行記錄
rows=cursor.executemany(sql,[('lwz','123'),('evia','455'),('lsd','333')])
print(rows)
​
rows=cursor.execute('update user set username="alexSB" where id=2')#修改記錄
print(rows)
​
conn.commit() # 只有commit提交才會完成真正的修改
cursor.close()
conn.close()