高级查询

实验准备：
第一张表：
create table location (Region char(20),Store_Name char(20));
insert into location values('East','Boston');
insert into location values('East','New York');
insert into location values('West','Los Angeles');
insert into location values('West','Houston');
 
 
第二张表：
create table store_info (Store_Name char(20),Sales int(10),Date char(10));
insert into store_info values('Los Angeles','1500','2020-12-05');
insert into store_info values('Houston','250','2020-12-07');
insert into store_info values('Los Angeles','300','2020-12-08');
insert into store_info values('Boston','700','2020-12-08');

select

---- SELECT ----显示表格中一个或数个字段的所有数据记录
语法：SELECT "字段" FROM "表名";
 
 
eg：
在store_info表中查找store_name的记录
select Store_Name from store_info;
+-------------+
| Store_Name  |
+-------------+
| Los Angeles |
| Houston     |
| Los Angeles |
| Boston      |
+-------------+
 
 
在store_info表中查找store_name，sales,date的记录(字段顺序无所谓)
select store_name,sales,date from store_info;
+-------------+-------+------------+
| store_name  | sales | date       |
+-------------+-------+------------+
| Los Angeles |  1500 | 2020-12-05 |
| Houston     |   250 | 2020-12-07 |
| Los Angeles |   300 | 2020-12-08 |
| Boston      |   700 | 2020-12-08 |
+-------------+-------+------------+

DISTINCT

---- DISTINCT ----不显示重复的数据记录
语法：SELECT DISTINCT "字段" FROM "表名";
 
 
eg：
select distinct store_name from store_info;
+-------------+
| store_name  |
+-------------+
| Los Angeles |
| Houston     |
| Boston      |
+-------------+
 
select distinct store_name,date from store_info;
+-------------+------------+
| store_name  | date       |
+-------------+------------+
| Los Angeles | 2020-12-05 |
| Houston     | 2020-12-07 |
| Los Angeles | 2020-12-08 |
| Boston      | 2020-12-08 |
+-------------+------------+

WHERE

---- WHERE ----有条件查询
语法：SELECT "字段" FROM "表名" WHERE "条件";
 
eg：
查找sales大于1000的store_name
select store_name from store_info where sales>1000;
+-------------+
| store_name  |
+-------------+
| Los Angeles |
+-------------+
 
 
查找store_name为“Houston”的记录
select store_name,date,sales from store_info where store_name='Houston';
+------------+------------+-------+
| store_name | date       | sales |
+------------+------------+-------+
| Houston    | 2020-12-07 |   250 |
+------------+------------+-------+

AND OR

---- AND OR ----且 或
语法：SELECT "字段" FROM "表名" WHERE "条件1" {[AND|OR] "条件2"}+ ;
 
eg:
查找sales大于500且sales小于1000的store_name,sales,date
select store_name,sales,date from store_info where sales >500 and sales<1000;
+------------+-------+------------+
| store_name | sales | date       |
+------------+-------+------------+
| Boston     |   700 | 2020-12-08 |
+------------+-------+------------+
 
查找sales大于500且sales小于1000，或者sales大于1000的store_name,sales,date（注意把括号里的当成一个整体，并且优先执行）
select store_name,sales,date from store_info where (sales >500 and sales<1000) or sales > 1000;
+-------------+-------+------------+
| store_name  | sales | date       |
+-------------+-------+------------+
| Los Angeles |  1500 | 2020-12-05 |
| Boston      |   700 | 2020-12-08 |
+-------------+-------+------------+

IN

---- IN ----显示已知的值的数据记录
语法：SELECT "字段" FROM "表名" WHERE "字段" IN ('值1', '值2', ...);
 
 
eg:
在store_info表中查找store_name为Los Angeles或者Houston的字段
select * from store_info where store_name in ('Los Angeles','Houston');
+-------------+-------+------------+
| Store_Name  | Sales | Date       |
+-------------+-------+------------+
| Los Angeles |  1500 | 2020-12-05 |
| Houston     |   250 | 2020-12-07 |
| Los Angeles |   300 | 2020-12-08 |
+-------------+-------+------------+
 
 
在store_info表中查找store_name不为Los Angeles或者Houston的字段
select * from store_info where store_name  not in ('Los Angeles','Houston');
+------------+-------+------------+
| Store_Name | Sales | Date       |
+------------+-------+------------+
| Boston     |   700 | 2020-12-08 |
+------------+-------+------------+

BETWEEN

---- BETWEEN ----显示两个值范围内的数据记录
语法：SELECT "字段" FROM "表名" WHERE "字段" BETWEEN '值1' AND '值2';
 
eg:
在store_info表中查找时间在2020-12-06到2020-12-11的数据
select * from store_info where date between '2020-12-06' and '2020-12-11';

通配符与like

---- 通配符 ----通常通配符都是跟 LIKE 一起使用的
% ：百分号表示零个、一个或多个字符
_ ：下划线表示单个字符
 
'A_Z'：所有以 'A' 起头，另一个任何值的字符，且以 'Z' 为结尾的字符串。例如，'ABZ' 和 'A2Z' 都符合这一个模式，而 'AKKZ' 并不符合 (因为在 A 和 Z 之间有两个字符，而不是一个字符)。
'ABC%': 所有以 'ABC' 起头的字符串。例如，'ABCD' 和 'ABCABC' 都符合这个模式。
'%XYZ': 所有以 'XYZ' 结尾的字符串。例如，'WXYZ' 和 'ZZXYZ' 都符合这个模式。
'%AN%': 所有含有 'AN'这个模式的字符串。例如，'LOS ANGELES' 和 'SAN FRANCISCO' 都符合这个模式。
'_AN%'：所有第二个字母为 'A' 和第三个字母为 'N' 的字符串。例如，'SAN FRANCISCO' 符合这个模式，而 'LOS ANGELES' 则不符合这个模式。
 
---- LIKE ----匹配一个模式来找出我们要的数据记录
语法：SELECT "字段" FROM "表名" WHERE "字段" LIKE {模式};
 
 
eg：
在store_info表中查找所有包含‘on’的数据
select * from store_info where store_name like '%on%';

ORDER BY

---- ORDER BY ----按关键字排序
语法：SELECT "字段" FROM "表名" [WHERE "条件"] ORDER BY "字段" [ASC, DESC];
#ASC 是按照升序进行排序的，是默认的排序方式。
#DESC 是按降序方式进行排序。
 
 
eg:
如果不指定排序方式，默认对sales字段进行升序排列
select * from store_info order by sales;
+-------------+-------+------------+
| Store_Name  | Sales | Date       |
+-------------+-------+------------+
| Houston     |   250 | 2020-12-07 |
| Los Angeles |   300 | 2020-12-08 |
| Boston      |   700 | 2020-12-08 |
| Los Angeles |  1500 | 2020-12-05 |
+-------------+-------+------------+
 
 
对sales字段进行降序排列
select * from store_info order by sales desc;
+-------------+-------+------------+
| Store_Name  | Sales | Date       |
+-------------+-------+------------+
| Los Angeles |  1500 | 2020-12-05 |
| Boston      |   700 | 2020-12-08 |
| Los Angeles |   300 | 2020-12-08 |
| Houston     |   250 | 2020-12-07 |
+-------------+-------+------------+

数学函数

eg：
select abs(-1),rand(),mod(5,3),power(2,3),round(1.89);
abs(-1）绝对值
rand（）生成 0 到 1 的随机数
mod(5,3) 5除以3的余数
power（2,3）2的3次方
round（1.89）1.89四舍五入后的结果
+---------+--------------------+----------+------------+-------------+
| abs(-1) | rand()             | mod(5,3) | power(2,3) | round(1.89) |
+---------+--------------------+----------+------------+-------------+
|       1 | 0.4283285573388716 |        2 |          8 |           2 |
+---------+--------------------+----------+------------+-------------+
 
 
select round(1.8937,3),truncate(1.235,2),ceil(5.2),floor(2.1),least(1.89,3,6.1,2.1);
round(1.8937,3)四舍五入并保留三位小数
truncate(1.235,2)给1.235直接截断成2位小数
ceil(5.2)返回大于5.2的最小整数
floor(2.1)返回小于2.1的最大整数
least(1.89,3,6.1,2.1)返回这组数中最小的值
+-----------------+-------------------+-----------+------------+-----------------------+
| round(1.8937,3) | truncate(1.235,2) | ceil(5.2) | floor(2.1) | least(1.89,3,6.1,2.1) |
+-----------------+-------------------+-----------+------------+-----------------------+
|           1.894 |              1.23 |         6 |          2 |                  1.89 |
+-----------------+-------------------+-----------+------------+-----------------------+

聚合函数

eg：
找到最大的sales
mysql> select max(sales) from store_info;
+------------+
| max(sales) |
+------------+
|       1500 |
+------------+
 
 
找到最小的sales
mysql> select min(sales) from store_info;
+------------+
| min(sales) |
+------------+
|        250 |
+------------+
 
 
求sales的平均值
mysql> select avg(sales) from store_info;
+------------+
| avg(sales) |
+------------+
|   687.5000 |
+------------+
 
 
求sales的和
mysql> select sum(sales) from store_info;
+------------+
| sum(sales) |
+------------+
|       2750 |
+------------+
 

新建一个city表，用来演示count()函数
create table city (name char(20));
insert into city values('nanjing');
insert into city values('hangzhou');
insert into city values('shanghai');
insert into city values();
insert into city values();
insert into city values('beijing');
insert into city values();
insert into city values();
 
 
 
select count(name) from city; #count（name）仅记录值不为NULL的
+-------------+
| count(name) |
+-------------+
|           4 |
+-------------+
 
 
select count(*) from city;   #count（*）会将NULL值记录进去
+----------+
| count(*) |
+----------+
|        8 |
+----------+

字符串函数

返回去除指定格式的值

SELECT TRIM ([ [位置] [要移除的字符串] FROM ] 字符串);
#[位置]：的值可以为 LEADING (起头), TRAILING (结尾), BOTH (起头及结尾)。 
#[要移除的字符串]：从字串的起头、结尾，或起头及结尾移除的字符串。缺省时为空格。
 
select trim(leading 'Ne' from 'New York');

将提供的参数 x 和 y 拼接成一个字符串

#将zhang和san拼接起来
select concat('zhang','san');

对location表中的两个字段进行合并，注意不要加引号
select concat(region,store_name) from location;

#如sql_mode开启了PIPES_AS_CONCAT，"||"视为字符串的连接操作符而非或运算符，和字符串的拼接函数Concat相类似，这和Oracle数据库使用方法一样的
SELECT Region || ' ' || Store_Name FROM location WHERE Store_Name = 'Boston';

获取从字符串 x 中的第 y 个位置开始长度为 z 的字符串

#将region和store_name先进行拼接，并获取从第五个字符开始字符串
select substr(concat(region,store_name),5) from location;

#在store_name中从第五个字符开始，截取长度为6的字符串
select substr(store_name,5,6) from location where store_name='Los Angeles';

返回字符串 x 的长度

#返回store_name中，每个字段的的长度
select store_name,length(store_name) from location;

将字符串 z 替代字符串 x 中的字符串 y

#将st替换成stern
select replace(region,'st','stern') from location;

mysql进阶查询

GROUP BY

对GROUP BY后面的字段的查询结果进行汇总分组，通常是结合聚合函数一起使用的
GROUP BY 有一个原则，凡是在 GROUP BY 后面出现的字段，必须在 SELECT 后面出现；
凡是在 SELECT 后面出现的、且未在聚合函数中出现的字段，必须出现在 GROUP BY 后面
语法：SELECT "字段1", SUM("字段2") FROM "表名" GROUP BY "字段1";
 
#降序
select store_name,sum(sales) from store_info group by store_name order by sum(sales) desc;
 
#不指定默认升序
select store_name,sum(sales) from store_info group by store_name order by sum(sales);

HAVING

用来过滤由 GROUP BY 语句返回的记录集，通常与 GROUP BY 语句联合使用
HAVING 语句的存在弥补了 WHERE 关键字不能与聚合函数联合使用的不足。
语法：SELECT "字段1", SUM("字段2") FROM "表格名" GROUP BY "字段1" HAVING (函数条件);
 
 
#eg:
select store_name,sum(sales) from store_info group by store_name having sum(sales)>1000;

别名

字段別名 表格別名
语法：SELECT "表格別名"."字段1" [AS] "字段別名" FROM "表格名" [AS] "表格別名";
 
#对字段设置别名
select store_name,sum(sales) as total from store_info group by store_name having sum(sales)>1000;
 
 
#对表设置别名
select store_name,sum(sales) from store_info as a group by a.store_name;

子查询

连接表格，在WHERE 子句或 HAVING 子句中插入另一个 SQL 语句
语法：SELECT "字段1" FROM "表格1" WHERE "字段2" [比较运算符] 				#外查询
(SELECT "字段1" FROM "表格2" WHERE "条件");									#内查询
 
#可以是符号的运算符，例如 =、>、<、>=、<= ；也可以是文字的运算符，例如 LIKE、IN、BETWEEN
 
#括号里的查询语句作为where语句的查询条件
SELECT SUM(Sales) FROM store_info WHERE store_name IN  (SELECT Store_Name FROM location WHERE Region = 'West');

EXISTS

用来测试内查询有没有产生任何结果，类似布尔值是否为真
#如果有的话，系统就会执行外查询中的SQL语句。若是没有的话，那整个 SQL 语句就不会产生任何结果。
语法：SELECT "字段1" FROM "表格1" WHERE EXISTS (SELECT * FROM "表格2" WHERE "条件");
 
 
SELECT SUM(Sales) FROM store_info WHERE EXISTS (SELECT * FROM location WHERE Region = 'West');

连接查询

inner join(内连接)：只返回两个表中联结字段相等的行
left join(左连接)：返回包括左表中的所有记录和右表中联结字段相等的记录
right join(右连接)：返回包括右表中的所有记录和左表中联结字段相等的记录

inner join(内连接)

#连接两个表中字段记录相等的数据记录
方法一：
select * from location A inner join store_info B on A.store_name=B.store_name;
方法二：
select * from location A, store_info B where A.store_name=B.store_name;

left join(左连接)

SELECT * FROM location A LEFT JOIN store_info B on A.Store_Name = B.Store_Name ;

right join(右连接)

SELECT * FROM location A RIGHT JOIN store_info B on A.Store_Name = B.Store_Name ;

自我连接

#分组汇总后统计 score 字段的值是比自己本身的值小的以及 score 字段 和 name 字段都相同的数量
select A.name,A.score,count(A.score) rank from student as A,student as B where A.score<=B.score group by A.name,A.score order by rank asc;