1.条件查询 where

表内容：

import pymysql

conn = pymysql.connect(host='127.0.0.1',port=3306,user='root',passwd='root',charset='utf8',db='db26')
cursor = conn.cursor()
# sql2 = """
# create table info(
#     id int not null auto_increment primary key,
#     name varchar(16) not null,
#     email varchar(32) not null,
#     age int,
#     depart_id int
# )default charset=utf8;
# """
# cursor.execute(sql2)
#conn.commit()
#sql = "select * from info where age>30"
# sql = "select * from info where id!=3"
#sql = "select * from info where id between 2 and 4"
#sql = "select * from info where (id between 2 and 4) and age>10"
#sql = "select * from info where id in (1,2,4)"
#sql = "select * from info where id not in (1,2,4)"
# sql = "select * from info where id in (select id from depart)"
# sql = "select * from info where not exists (select * from depart where id=5)"
#sql = "select * from info where exists (select * from depart where id=2)"
#sql = "select * from (select * from info where id>2)as T where age>10"
#sql = "select * from (select * from info where id>2) as T where age>10"


#sql = "select * from info where name like '%wu%'"
#sql = "select * from info where email like '%@live.com'"
sql = "select * from info where email like '__peiqi@live.co_'"
cursor.execute(sql)
result = cursor.fetchall()
#print(result)
for i in result:
    print(i)
cursor.close()
conn.close()

2. 通配符与模糊查询

% 代表任意个任意字符
_ 代表单个任意字符

select * from info where name like "%沛%";
select * from info where name like "%沛";
select * from info where email like "%@live.com";
select * from info where name like "武%齐";
select * from info where name like "k%y";
select * from info where email like "wupeiqi%";


select * from info where email like "_@live.com";
select * from info where email like "_upeiqi@live.com";
select * from info where email like "__peiqi@live.com";
select * from info where email like "__peiqi_live.co_";

import pymysql

conn = pymysql.connect(host='127.0.0.1',port=3306,user='root',passwd='root',charset='utf8',db='db26')
cursor = conn.cursor()
#sql = "select * from info where name like '%wu%'"
#sql = "select * from info where email like '%@live.com'"
sql = "select * from info where email like '__peiqi@live.co_'"
cursor.execute(sql)
result = cursor.fetchall()
#print(result)
for i in result:
    print(i)
cursor.close()
conn.close()

3. 映射

• 映射是指什么？查找的内容单独建一列，

select * from info;

select id, name				from info;
select id, name as NM 		from info;
select id, name as NM, 123  from info;
注意：少些select * ,自己需求。

select 
	id,
	name,
	666 as num,
	( select max(id) from depart ) as mid, -- max/min/sum  只能返回一个值，max(id)是一个值
	( select min(id) from depart) as nid, -- max/min/sum
	age
from info;

select 
	id,
	name,
	( select title from depart where depart.id=info.depart_id) as x1
from info;

# 注意：效率很低

select 
	id,
	name,
	( select title from depart where depart.id=info.depart_id) as x1,
	-- 顺序：先查表info,取出id,name列，再查当列的depart_id是否等于depart.id,AS后是重命名
	( select title from depart where depart.id=info.id) as x2
from info;

select 
	id,
	name,
	case depart_id when 1 then "第1部门" end v1
from info;

select 
	id,
	name,
	case depart_id when 1 then "第1部门" else "其他" end v2
	-- case语句 如果depart_id为1，就写为第一部门，否则就其他  该列为v2
from info;

select 
	id,
	name,
	case depart_id when 1 then "第1部门" end v1,
	case depart_id when 1 then "第1部门" else "其他" end v2,
	case depart_id when 1 then "第1部门" when 2 then "第2部门" else "其他" end v3,
	-- case语句 如果depart_id为1，就写为第一部门，如果是2 就是第2部门 否则就其他  该列为v2
	case when age<18 then "少年" end v4,
	case when age<18 then "少年" else "油腻男" end v5,
	case when age<18 then "少年" when age<30 then "青年" else "油腻男" end v6
	-- case语句 如果age<18就是少年，age<30就是青年，否则就其他 就是油腻， 该列为v6
from info;

4. 排序 order_by

select * from info order by age desc; -- 倒序
select * from info order by age asc;  -- 顺序

select * from info order by id desc;
select * from info order by id asc;
select * from info order by age asc,id desc; -- 优先按照age从小到大；如果age相同则按照id从大到小。


select * from info where id>10 order by age asc,id desc;
select * from info where id>6 or name like "%y" order by age asc,id desc;

5. 取部分 limit 和offset

select * from info limit 5;   										-- 获取前5条数据
select * from info order by id desc limit 3;						-- 先排序，再获取前3条数据
select * from info where id > 4 order by id desc limit 3;			-- 先排序，再获取前3条数据

select * from info limit 3 offset 2;	-- 从位置2开始，向后获取前3数据

limit 配合offset可以用于分页操作，比如在数据表内有1000条数据，每页显示10个，可以分为100页

- 第一页：`select * from info limit 10 offset 0;`
- 第二页：`select * from info limit 10 offset 10;`
- 第三页：`select * from info limit 10 offset 20;`
- 第四页：`select * from info limit 10 offset 30;

6. 分组 group by

sql = "select depart_id,max(age),min(age),avg(age),count(id),sum(age) from info group by depart_id" 
-- 查询 按照depart_id分组后 按照分组后的每组找出聚合函数后的内容，
sql = "select depart_id,count(id) from info group by depart_id having count(id) > 2;"
-- 先分组 后having子查询
sql = "select count(id) from info"
sql = "select min(id) from info"
select * from info where id in (select max(id) from info group by age);
-- 先查找每个年龄的最大ID，后查询这些ID的信息
select age,count(id) from info group by age having count(id) > 2;
-- 
select age,count(id) from info where id > 4 group by age having count(id) > 2;  
-- 聚合条件放在having后面

-- 一个最复杂的例子
select age,count(id) from info where id > 2 group by age having count(id) > 1 order by age desc limit 1;
- 要查询的表info
- 条件 id>2
- 根据age分组
- 对分组后的数据再根据聚合条件过滤 count(id)>1
- 根据age从大到小排序
- 获取第1条

7.左右连表 left outer join … on

主表 left outer join 从表 on 主表.x = 从表.id 
-- 左外连接时左边的表是主表，根据某个条件进行连表，利用了一个主表和一个从表
select * from info left outer join depart on info.depart_id = depart.id;
sql = "select info.id,info.name,info.email,info.age,info.depart_id,depart.title from info left outer join depart on info.depart_id=depart.id"
-- 左连接并显示指定字段
sql = "select * from info right outer join depart on info.depart_id=depart.id"
-- 右连接 右边是主表  左边是从表
sql = "select * from info right join depart on info.depart_id=depart.id"
-- 谁是主表 就要展示谁为主，没有对应的展示那些元素为null，主要使用的是左外连接
select * from info left outer join depart on ....
select * from depart left outer join info on ....
简写  可以去掉outer  默认是外连接

内连接：表 inner join 表 on 条件

互相匹配，只有两者能关联上的数据才会显示，没有关联的数据就不会显示
select * from info inner join depart on info.depart_id=depart.id;

8. 联合查询 union

• 又称为上下连表 很少用到，可以用于多张表联合去重

select id,title from depart 
union
select id,name from info;


select id,title from depart 
union
select email,name from info;
-- 列数需相同
-- 数据类型不一致没问题

select id from depart 
union all
select id from info;

-- 保留所有，查询时不会自动去重

到目前为止SQL执行顺序：
    join/union
    on 
    where 
    group by
    having 
    order by
    limit