1.子查詢in

# 1、in
select * from emp where age=18 or age=38 or age=28;
select * from emp where age in (18,38,28);
​
# 子查詢的思路
select * from emp where dep_id in
(select id from dep where name="技術" or name="銷售");
​
# 連結串列的思路
select * from emp inner join dep
on emp.dep_id = dep.id
where dep.name in ("技術","銷售");
​
​
# not in不支援null
 
mysql> select * from dep;
+------+--------------+
| id  | name     |
+------+--------------+
| 200 | 技術     |
| 201 | 人力資源   |
| 202 | 銷售     |
| 203 | 運營     |
+------+--------------+
4 rows in set (0.00 sec)
mysql> insert into emp values(7,'lili','female',48,null);
Query OK, 1 row affected (0.03 sec)
​
mysql> select * from emp
 
   -> ;
+----+------------+--------+------+--------+
| id | name    | sex   | age  | dep_id |
+----+------------+--------+------+--------+
| 1 | egon    | male  |  18 |  200 |
| 2 | alex    | female |  48 |  201 |
| 3 | wupeiqi   | male  |  38 |  201 |
| 4 | yuanhao   | female |  28 |  202 |
| 5 | liwenzhou  | male  |  18 |  200 |
 
| 6 | jingliyang | female |  18 |  204 |
| 7 | lili    | female |  48 |  NULL |
+----+------------+--------+------+--------+
7 rows in set (0.00 sec)
​
mysql>
​
​
查詢出有員工的部門，
select * from dep where id in
(select distinct dep_id from emp);
​
查詢出沒有員工的部門，
select * from dep where id not in
(select distinct dep_id from emp);
​
​
select * from dep where id not in
(select distinct dep_id from emp where dep_id is not null);

2.子查詢any和all

# any後也跟子查詢語句，與in不一樣的地方在哪裡
#  in (子查詢語句)
#  in (值1,值2,值3)
#  而any只能跟子查詢語句
#  any必須跟比較運算子配合使用
​
select * from emp where dep_id in
(select id from dep where name in ("技術","人力資源"));
​
select * from emp where dep_id = any
(select id from dep where name in ("技術","人力資源"));
​
​
select * from emp where dep_id not in
(select id from dep where name in ("技術","人力資源"));
​
select * from emp where ! (dep_id = any(select id from dep where name in ("技術","人力資源")));
​
​
查詢出那些薪資比所有部門的平均薪資都高的員工=》薪資在所有部門平均線以上的狗幣資本家
select * from employee where salary > all
(select avg(salary) from employee where depart_id is not null group by depart_id);
​
查詢出那些薪資比所有部門的平均薪資都低的員工=》薪資在所有部門平均線以下的無產階級勞苦大眾
select * from employee where salary < all
(select avg(salary) from employee where depart_id is not null group by depart_id);
​
查詢出那些薪資比任意一個部門的平均薪資高的員工=》薪資在任一部門平均線以上的員工
select * from employee where salary > any
(select avg(salary) from employee where depart_id is not null group by depart_id);
​
​
select * from employee where salary < any
(select avg(salary) from employee where depart_id is not null group by depart_id);

3.子查詢exists

# 準備資料
create database db13;
​
use db13
​
create table student(
   id int primary key auto_increment,
   name varchar(16)
);
​
create table course(
   id int primary key auto_increment,
   name varchar(16),
  comment varchar(20)
);
​
create table student2course(
   id int primary key auto_increment,
   sid int,
   cid int,
  foreign key(sid) references student(id),
  foreign key(cid) references course(id)
);
​
​
insert into student(name) values
("egon"),
("lili"),
("jack"),
("tom");
​
insert into course(name,comment) values
("資料庫","資料倉庫"),
("數學","根本學不會"),
("英語","鳥語花香");
​
​
insert into student2course(sid,cid) values
(1,1),
(1,2),
(1,3),
(2,1),
(2,2),
(3,2);
​
準備資料

# exists  vs in
# in的效果 高於 exists
# 見部落格：https://www.cnblogs.com/linhaifeng/articles/7267596.html#_label4
​
select * from 表1 where exists (select * from 表2);
​
# 例如：查詢有員工的部門=》
select * from dep where exists (select * from emp where dep.id=emp.dep_id);
​
​
# not exists的效果 高於 not in
select * from dep where not exists (select * from emp where 203=emp.dep_id);
​
​
​
# 例：查詢選修了所有課程的學生id、name:
​
# 實現方式一：選修了三門課程的學生就是選修了所有課程的學生
select s.id,s.name from student as s inner join student2course as sc
on s.id = sc.sid
group by sc.sid
having count(sc.cid) = (select count(id) from course);
​
# 實現方式二：找到這樣的學生，該學生不存在沒有選修過的課程
select * from student as s where not exists (
  select * from course as c where not exists (
    select * from student2course as sc where sc.sid = s.id and sc.cid = c.id
   )
);
​
​
select * from student as s where not exists (
  select * from course as c where not exists (
    select * from student2course as sc where sc.sid = s.id and sc.cid = c.id
   )
);
​
​
​
學生記錄可以過濾出來，一定是子查詢內沒有記錄
​
for 學生: # s.id=2
  for 課程: # c.id=1
    for 學生2課程: # sc.sid = 2 and sc.cid = 1
       pass
​
==================================
for sid in [1,2,3,4]:
  for cid in [1,2,3]:
     (sid,cid)
​
​
最外層迴圈一次
# (1,1)
# (1,2)
# (1,3)
最外層迴圈二次
# (2,1)
# (2,2)
# (2,3)
最外層迴圈三次
# (3,1)
# (3,2)
# (3,3)
最外層迴圈四次
# (4,1)
# (4,2)
# (4,3)
​
===================================
# 例2、查詢沒有選擇所有課程的學生，即沒有全選的學生。=》找出這樣的學生，存在沒有選修過的課程
select * from student as s where exists (
  select * from course as c where not exists (
    select * from student2course as sc where sc.sid = s.id and sc.cid = c.id
   )
);
​
# 例3、查詢一門課也沒有選的學生=》找出這樣的學生，不存在選修過的課程
select * from student as s where not exists (
  select * from course as c where exists (
    select * from student2course as sc where sc.sid = s.id and sc.cid = c.id
   )
);
​
# 例4、查詢至少選修了一門課程的學生=》找出這樣的學生，存在選修過課程
select * from student as s where exists (
  select * from course as c where exists (
    select * from student2course as sc where sc.sid = s.id and sc.cid = c.id
   )
);

4.pymysql模組

import pymysql # pip3 install pymysql
​
conn = pymysql.connect(host="127.0.0.1", port=3306, user="root", password="123", db="db13", charset="utf8mb4")
cursor = conn.cursor(cursor=pymysql.cursors.DictCursor)
​
# 記錄=》增刪改
# cursor.execute("insert into student values(%s,%s)",[5,'kkk'])
# cursor.executemany("insert into student values(%s,%s)",[(6,'xxx'),(7,'yyy'),(8,'zzzz')])
​
cursor.execute("insert into student(name) values(%s)", ['zzz'])
print(cursor.lastrowid)
​
# cursor.execute("delete from student where id > 6;")
​
# 記錄=》查
# rows = cursor.execute('select * from student;')
# print(rows)
​
# line = cursor.fetchone()
# print(line)
# line = cursor.fetchone()
# print(line)
#
# lines = cursor.fetchmany(2)
# print(lines)
# lines = cursor.fetchall()
# print(lines)
​
# for line in cursor:
#   print(line)
​
​
# cursor.scroll(3,mode='absolute')  # 相對絕對位置移動
# print(cursor.fetchone())
# print(cursor.fetchall())
# cursor.scroll(0,mode='absolute')  # 相對絕對位置移動
# print(cursor.fetchall())
​
​
# cursor.fetchone()
# cursor.scroll(1,mode='relative') # 相對當前位置移動
# cursor.scroll(0,mode='absolute') # 相對當前位置移動
# print(cursor.fetchone())
​
conn.commit()
cursor.close()
conn.close()