一.实验前准备

1.location表格创建

use class;
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');

2.store_info表格创建

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');

二.常用查询

1. select（显示查询）

显示表格中一个或数个字段的所有数据记录

select "字段" from "表名";
例:select store_name from store_info;

例:mysql> select store_name from store_info;
+-------------+
| store_name  |
+-------------+
| Los Angeles |
| Houston     |
| Los Angeles |
| Boston      |
+-------------+
4 rows in set (0.00 sec)

2. distinct（不重复查询）

select distinct "字段" from "表名";
例:select distinct store_name from store_info;

例:mysql> select distinct store_name from store_info;
+-------------+
| store_name  |
+-------------+
| Los Angeles |
| Houston     |
| Boston      |
+-------------+
3 rows in set (0.00 sec)

3. where（有条件查询）

select distinct store_name "字段" from "表名";
例:select distinct store_name from store_info;

例:mysql> select distinct store_name from store_info;
+-------------+
| store_name  |
+-------------+
| Los Angeles |
| Houston     |
| Boston      |
+-------------+
3 rows in set (0.00 sec)

4. and/or（且/或）

select "字段" from "表名" where "条件1" {[and|or] "条件2"} + ;
例:select store_name from store_info where sales > 1000 or ( sales < 500and sales > 200 );

例:mysql> select store_name from store_info where sales > 1000;
+-------------+
| store_name  |
+-------------+
| Los Angeles |
+-------------+
1 row in set (0.00 sec)

5. in （显示已知值的字段）

在已知的字段数据取值范围内取值

select "字段" from "表名" where "字段" in ('值1', '值2', ...);
例:select * from store_info where store_name in ('Los Angeles', 'Houston');

例:mysql> select * from store_info where store_name in ('Los Angeles','Hoston');
+-------------+-------+------------+
| Store_Name  | Sales | Date       |
+-------------+-------+------------+
| Los Angeles |  1500 | 2020-12-05 |
| Los Angeles |   300 | 2020-12-08 |
+-------------+-------+------------+
2 rows in set (0.00 sec)

6. between（显示两个值范围内的字段）

在两个字段数据值之间取值，包含两边字段的数据

select "字段" from "表名" where "字段"  between '值1' and '值2';
例:select * from store_info where Date between '2020-12-06' and '2020-12-10';

例:mysql> select * from store_info where date between '2020-12-06'
and '2020-12-10';
+-------------+-------+------------+
| Store_Name  | Sales | Date       |
+-------------+-------+------------+
| Houston     |   250 | 2020-12-07 |
| Los Angeles |   300 | 2020-12-08 |
| Boston      |   700 | 2020-12-08 |
+-------------+-------+------------+
3 rows in set (0.00 sec)

7. 通配符的使用

通常通配符都是与like配合使用的
% ：百分号表示零个、一个或多个字符
_ ：下划线表示单个字符

名称	作用	举例
‘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	‘ABZ’ 和 'A2Z’符合而 ‘AKKZ’ 不符合
‘ABC%’	所有以 ‘ABC’ 起头的字符串	‘ABCD’ 和 ‘ABCABC’
‘%XYZ’	所有以 ‘XYZ’ 结尾的字符串	‘WXYZ’ 和 ‘ZZXYZ’
‘%AN%’	所有含有 'AN’这个模式的字符串	‘LOS ANGELES’ 和 ‘SAN FRANCISCO’
‘_AN%’	所有第二个字母为 ‘A’ 和第三个字母为 ‘N’ 的字符串	‘SAN FRANCISCO’ 符合而 ‘LOS ANGELES’ 不符合

8. like （匹配关键字查询）

select "字段" from "表名" where "字段" like {模式};
例:select * from store_info where store_name like ‘%os%’;

例:mysql> select * from store_info where store_name like '%os%';
+-------------+-------+------------+
| Store_Name  | Sales | Date       |
+-------------+-------+------------+
| Los Angeles |  1500 | 2020-12-05 |
| Los Angeles |   300 | 2020-12-08 |
| Boston      |   700 | 2020-12-08 |
+-------------+-------+------------+
3 rows in set (0.00 sec)

9. order by (按关键字排序)

select "字段" from "表名" [where “条件”] order by "字段" [ASC, DESC];   #ASC默认按照升序排序的；DESC是按降序排序的
例:select store_name,sales,Date from store_info order by sales DESC;

例:mysql> select store_name,sales,date 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 |
+-------------+-------+------------+
4 rows in set (0.00 sec)

三.常用函数

1. 数学函数

关键字	含义
`abs(x)`	返回 x 的绝对值
`rand()`	返回 0 到 1 的随机数
`mod(x,y)`	返回 x 除以 y 以后的余数
`power(x,y)`	返回 x 的 y 次方
`round(x)`	返回离 x 最近的整数
`round(x,y)`	保留 x 的 y 位小数四舍五入后的值
`sqrt(x)`	返回 x 的平方根
`truncate(x,y)`	返回数字 x 截断为 y 位小数的值
`ceil(x)`	返回大于或等于 x 的最小整数
`floor(x)`	返回小于或等于 x 的最大整数
`greatest(x1,x2...)`	返回集合中最大的值，也可以返回多个字段的最大的值
`least(x1,x2…)`	返回集合中最小的值，也可以返回多个字段的最小的值

例1

select abs(-1), rand(), mod(5,3), power(2,3), round(1.89);

例:mysql> select abs(-1), rand(), mod(5,3), power(2,3), round(1.89);
+---------+---------------------+----------+------------+-------------+
| abs(-1) | rand()              | mod(5,3) | power(2,3) | round(1.89) |
+---------+---------------------+----------+------------+-------------+
|       1 | 0.04296933304795002 |        2 |          8 |           2 |
+---------+---------------------+----------+------------+-------------+
1 row in set (0.00 sec)

例2

select round(1.8937,3), truncate(1.235,2), ceil(5.2), floor(2.1), least(1.89,3,6.1,2.1);

例:mysql> 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) | 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 |
+-----------------+-------------------+-----------+------------+-----------------------+
1 row in set (0.00 sec)

2. 聚合函数

关键字	含义
`avg()`	返回指定列的平均值
`count()`	返回指定列中非 NULL 值的个数
`min()`	返回指定列的最小值
`max()`	返回指定列的最大值
`sum(x)`	返回指定列的所有值之和

求平均值

select avg(sales) from store_info;

例:mysql> select avg(sales) from store_info;
+------------+
| avg(sales) |
+------------+
|   687.5000 |
+------------+
1 row in set (0.00 sec)

求表中城市数量

select count(store_name) from store_info;

例:mysql> select count(store_name) from store_info;
+-------------------+
| count(store_name) |
+-------------------+
|                 4 |
+-------------------+
1 row in set (0.00 sec)

统计数量，进行去重

select count(distinct store_name) from store_info;

例:mysql> select count(distinct store_name) from store_info;
+----------------------------+
| count(distinct store_name) |
+----------------------------+
|                          3 |
+----------------------------+
1 row in set (0.00 sec)

统计字段中最大值

select max(sales) from store_info;

例:mysql> select max(sales) from store_info;
+------------+
| max(sales) |
+------------+
|       1500 |
+------------+
1 row in set (0.00 sec)

统计字段中最小值

select min(sales) from store_info;

例:mysql> select min(sales) from store_info;
+------------+
| min(sales) |
+------------+
|        250 |
+------------+
1 row in set (0.00 sec)

统计字段中的总和

select sum(sales) from store_info;

例:mysql> select sum(sales) from store_info;
+------------+
| sum(sales) |
+------------+
|       2750 |
+------------+
1 row in set (0.00 sec)

3. 字符串函数

关键字	含义
`trim()`	返回去除指定格式的值
`concat(x,y)`	将提供的参数 x 和 y 拼接成一个字符串
`substr(x,y)`	获取从字符串 x 中的第 y 个位置开始的字符串，跟substring()函数作用相同
`substr(x,y,z)`	获取从字符串 x 中的第 y 个位置开始长度为 z 的字符串
`length(x)`	返回字符串 x 的长度
`replace(x,y,z)`	将字符串 z 替代字符串 x 中的字符串 y
`upper(x)`	将字符串 x 的所有字母变成大写字母
`lower(x)`	将字符串 x 的所有字母变成小写字母
`left(x,y)`	返回字符串 x 的前 y 个字符
`right(x,y)`	返回字符串 x 的后 y 个字符
`repeat(x,y)`	将字符串 x 重复 y 次
``space(x)`	返回 x 个空格
`strcmp(x,y)`	比较 x 和 y，返回的值可以为-1,0,1
`reverse(x)`	将字符串 x 反转

字段拼接

示例1

select concat(Region, store_name) from localtion where store_name = 'Boston';

例:mysql> select concat(Region, store_name) from location where store_name = 'Boston';
+----------------------------+
| concat(Region, store_name) |
+----------------------------+
| EastBoston                 |
+----------------------------+
1 row in set (0.01 sec)

示例2

select Region || ' ' || store_name from localtion where store_name = 'Boston';

例:mysql> select Region || ' ' || store_name from location where store_name = 'Boston';
+-----------------------------+
| Region || ' ' || store_name |
+-----------------------------+
| East Boston                 |
+-----------------------------+
1 row in set (0.00 sec)

示例3

select substr(store_name,3) from location where store_name = 'Los Angeles';

例:mysql> select substr(store_name,3) from location where store_name = 'Los Angeles';
+----------------------+
| substr(store_name,3) |
+----------------------+
| s Angeles            |
+----------------------+
1 row in set (0.00 sec)

示例4

select substr(store_name,2,4) from location where store_name = 'New York';

例:mysql> select substr(store_name,2,4) from location where store_name = 'New York';
+------------------------+
| substr(store_name,2,4) |
+------------------------+
| ew Y                   |
+------------------------+
1 row in set (0.00 sec)

字段移除

示例1

select trim ([ [位置] [要移除的字符串] from ] 字符串);
例：select trim(leading 'Ne ' from 'New York');

例:mysql> select TRIM(LEADinG 'Ne' from 'New York');
+------------------------------------+
| TRIM(LEADinG 'Ne' from 'New York') |
+------------------------------------+
| w York                             |
+------------------------------------+
1 row in set (0.00 sec)

示例2

select Region,length(store_name) from location;

例:mysql> select Region,length(store_name) from location;
+--------+--------------------+
| Region | length(store_name) |
+--------+--------------------+
| East   |                  6 |
| East   |                  8 |
| West   |                 11 |
| West   |                  7 |
+--------+--------------------+
4 rows in set (0.00 sec)

示例3

select REPLACE(Region,'ast','astern')from location;

例:mysql> select REPLACE(Region,'ast','astern')from location;
+--------------------------------+
| REPLACE(Region,'ast','astern') |
+--------------------------------+
| Eastern                        |
| Eastern                        |
| West                           |
| West                           |
+--------------------------------+
4 rows in set (0.01 sec)

四.查询与函数

1. GROUP by（汇总分组）

对GROUP by后面的栏位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，就是 select 后面的所有列中，没有使用聚合函数的列，必须出现在GROUP by后面

select "字段1", SUM("字段2") from "表名" GROUP by "字段1";
例：select store_name,sum(sales) from store_info group by store_name order by sales desc;

例:mysql> select store_name, SUM(sales) from store_info GROUP by store_name order by sales desc;
+-------------+------------+
| store_name  | SUM(sales) |
+-------------+------------+
| Los Angeles |       1800 |
| Boston      |        700 |
| Houston     |        250 |
+-------------+------------+
3 rows in set (0.00 sec)

2. HAVinG （过滤返回值）

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

select "字段1", SUM("字段2") from "表格名" GROUP by "字段1" HAVinG (函数条件);
例：select store_name, SUM(sales) from store_info group up store_name having SUM(sales) > 1500;

例:mysql> select store_name, SUM(sales) from store_info GROUP by store_name HAVinG SUM(sales) > 1500;
+-------------+------------+
| store_name  | SUM(sales) |
+-------------+------------+
| Los Angeles |       1800 |
+-------------+------------+
1 row in set (0.00 sec)

3. 别名（字段别名表格别名）

select "表格別名"."字段1" [AS] "字段別名" from "表格名" [AS] "表格別名";
例：select A.store_name Store, SUM(A.sales) "Total sales" from store_info A group by A.store_name;

例:mysql> select A.store_name store, SUM(A.sales) "Total sales" from store_info A GROUP by A.store_name;
+-------------+-------------+
| store       | Total sales |
+-------------+-------------+
| Boston      |         700 |
| Houston     |         250 |
| Los Angeles |        1800 |
+-------------+-------------+
3 rows in set (0.00 sec)

4. 子查询

连接表格，在where 子句或 HAVinG 子句中插入另一个 SQL 语句

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

查询两个表交集的城市

select * from location where store_name in (select store_name from store_info);

例:mysql> select * from location where store_name in (select store_name from store_info);
+--------+-------------+
| Region | Store_Name  |
+--------+-------------+
| West   | Los Angeles |
| West   | Houston     |
| East   | Boston      |
+--------+-------------+
3 rows in set (0.00 sec)

查询在location表中西部城市的销售总和

select sum(sales) from store_info where store_name in (select store_name from location where region = 'West');

例:mysql> select SUM(sales) from store_info where store_name in 
    -> (select store_name from location where Region = 'West');
+------------+
| SUM(sales) |
+------------+
|       2050 |
+------------+
1 row in set (0.00 sec)

通过多表查询，查询两个表中有交集的值

select sum(A.sales) from store_info A where A.store_name in 
(select store_name from location B where B.store_name = A.store_name);

例:mysql> select SUM(A.sales) from store_info A where A.store_name in 
    -> (select store_name from location B where B.store_name = A.store_name);
+--------------+
| SUM(A.sales) |
+--------------+
|         2750 |
+--------------+
1 row in set (0.00 sec)

5. EXISTS（类查询）

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

select "字段1" from "表格1" where EXISTS (select * from "表格2" where "条件");
例：select SUM(sales) from store_info where exists (select * from location where Region = 'West');

例：mysql> select SUM(sales) from store_info where EXISTS (select * from location where Region = 'West');
+------------+
| SUM(sales) |
+------------+
|       2750 |
+------------+
1 row in set (0.00 sec)

五.连接查询

1.inner join(内连接)

只返回两个表中联结字段相等的行
示例一

select * from location A inner join store_info B on A.store_name = B.store_name ;

例：mysql> select * from location A inner join store_info B on A.store_name = B.store_name ;
+--------+-------------+-------------+-------+------------+
| Region | Store_Name  | Store_Name  | Sales | Date       |
+--------+-------------+-------------+-------+------------+
| West   | Los Angeles | Los Angeles |  1500 | 2020-12-05 |
| West   | Houston     | Houston     |   250 | 2020-12-07 |
| East   | Boston      | Boston      |   700 | 2020-12-08 |
+--------+-------------+-------------+-------+------------+
3 rows in set (0.00 sec)

示例二

select * from location A, store_info B where A.store_name = B.store_name;

例：mysql> select * from location A, store_info B where A.store_name = B.store_name;
+--------+-------------+-------------+-------+------------+
| Region | Store_Name  | Store_Name  | Sales | Date       |
+--------+-------------+-------------+-------+------------+
| West   | Los Angeles | Los Angeles |  1500 | 2020-12-05 |
| West   | Houston     | Houston     |   250 | 2020-12-07 |
| East   | Boston      | Boston      |   700 | 2020-12-08 |
+--------+-------------+-------------+-------+------------+
3 rows in set (0.00 sec)

示例三

select A.region region, sum(B.sales) sales from location A, store_info B 
where A.store_name = B.store_name group by region;

例：mysql> select A.region region, sum(B.sales) sales from location A, store_info B 
    -> where A.store_name = B.store_name group by region;
+--------+-------+
| region | sales |
+--------+-------+
| East   |   700 |
| West   |  1750 |
+--------+-------+
2 rows in set (0.00 sec)

2.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

select * from location A left join store_info B on A.store_name = B.store_name ;

例：mysql> select * from location A left join store_info B on A.store_name = B.store_name ;
+--------+-------------+-------------+-------+------------+
| Region | Store_Name  | Store_Name  | Sales | Date       |
+--------+-------------+-------------+-------+------------+
| West   | Los Angeles | Los Angeles |  1500 | 2020-12-05 |
| West   | Houston     | Houston     |   250 | 2020-12-07 |
| East   | Boston      | Boston      |   700 | 2020-12-08 |
| East   | New York    | NULL        |  NULL | NULL       |
+--------+-------------+-------------+-------+------------+
4 rows in set (0.00 sec)

3.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

select * from location A right join store_info B on A.store_name = B.store_name ;

例：mysql> select * from location A right join store_info B on A.store_name = B.store_name ;
+--------+-------------+-------------+-------+------------+
| Region | Store_Name  | Store_Name  | Sales | Date       |
+--------+-------------+-------------+-------+------------+
| East   | Boston      | Boston      |   700 | 2020-12-08 |
| West   | Los Angeles | Los Angeles |  1500 | 2020-12-05 |
| West   | Houston     | Houston     |   250 | 2020-12-07 |
| NULL   | NULL        | Washington  |   300 | 2020-12-08 |
+--------+-------------+-------------+-------+------------+
4 rows in set (0.00 sec)

4.create view （视图虚拟表或存储查询）

视图跟表格的不同是，表格中有实际储存数据记录，而视图是建立在表格之上的一个架构，它本身并不实际储存数据记录
临时表在用户退出或同数据库的连接断开后就自动消失了，而视图不会消失
视图不含有数据，只存储它的定义，它的用途一般可以简化复杂的查询。比如你要对几个表进行连接查询，而且还要进行统计排序等操作，写SQL语句会很麻烦的，用视图将几个表联结起来，然后对这个视图进行查询操作，就和对一个表查询一样，很方便

create view "视图表名" as "select 语句";
例：create view V_region_sales as select A.region region,sum(B.sales) sales from location A inner join store_info B on A.store_name = B.store_name group by region;
select * from V_region_sales;
drop view V_region_sales;

例：mysql> create view V_region_sales as select A.region region,sum(B.sales)sales from location A
    -> inner join store_info B on A.store_name = B.store_name group by region;
Query OK, 0 rows affected (0.01 sec)
mysql> select * from V_region_sales;
+--------+-------+
| region | sales |
+--------+-------+
| East   |   700 |
| West   |  1750 |
+--------+-------+
2 rows in set (0.00 sec)
mysql> drop view V_region_sales;
Query OK, 0 rows affected (0.00 sec)

5.union（联集）

将两个SQL语句的结果合并起来，两个SQL语句所产生的字段需要是同样的数据记录种类
生成结果的数据记录值将没有重复，且按照字段的顺序进行排序

① union 用法

[select 语句 1] union [select 语句 2];
select store_name from location union select store_name from store_info;

例：mysql> select store_name from location union select store_name from store_info;
+-------------+
| store_name  |
+-------------+
| Boston      |
| New York    |
| Los Angeles |
| Houston     |
| Washington  |
+-------------+
5 rows in set (0.00 sec)

② union all 用法

[select 语句 1] union all [select 语句 2];
select store_name from location union all select store_name from store_info;

例：mysql> select store_name from location union all select store_name from store_info;
+-------------+
| store_name  |
+-------------+
| Boston      |
| New York    |
| Los Angeles |
| Houston     |
| Los Angeles |
| Houston     |
| Washington  |
| Boston      |
+-------------+
8 rows in set (0.00 sec)

6.交集值

取两个SQL语句结果的交集

select A.store_name from location A inner join store_info B on A.store_name = B.store_name;

例：mysql> select A.store_name from location A inner join store_info B on A.store_name = B.store_name;
+-------------+
| store_name  |
+-------------+
| Los Angeles |
| Houston     |
| Boston      |
+-------------+
3 rows in set (0.00 sec)

select A.store_name from location A inner join store_info B using(store_name);

例：mysql> select A.store_name from location A inner join store_info B using(store_name);
+-------------+
| store_name  |
+-------------+
| Los Angeles |
| Houston     |
| Boston      |
+-------------+
3 rows in set (0.00 sec)

取两个SQL语句结果的交集，且没有重复

select DISTINCT A.store_name from location A inner join store_info B using(store_name);

select DISTINCT store_name from location where (store_name) IN (select store_name from store_info);

select DISTINCT A.store_name from location A left join store_info B using(store_name) where B.store_name IS NOT NULL;

select A.store_name from (select B.store_name from location B inner join store_info C on B.store_name = C.store_name) A 
group by A.store_name;

例：mysql> select distinct A.store_name from location A inner join store_info B using(store_name);
+-------------+
| store_name  |
+-------------+
| Los Angeles |
| Houston     |
| Boston      |
+-------------+
3 rows in set (0.01 sec)

mysql> select distinct store_name from location where (store_name) in (select store_name from store_info);
+-------------+
| store_name  |
+-------------+
| Los Angeles |
| Houston     |
| Boston      |
+-------------+
3 rows in set (0.00 sec)

mysql> select distinct A.store_name from location A left join store_info B using(store_name)where B.store_name is not null;
+-------------+
| store_name  |
+-------------+
| Boston      |
| Los Angeles |
| Houston     |
+-------------+
3 rows in set (0.01 sec)

mysql> select A.store_name from (select B.store_name from location B inner join store_info C on B.store_name = C.store_name)A
    -> group by A.store_name;
+-------------+
| store_name  |
+-------------+
| Boston      |
| Houston     |
| Los Angeles |
+-------------+
3 rows in set (0.00 sec)

mysql> select A.store_name from 
    -> (select distinct store_name from location union all select distinct store_name from store_info)A
    -> group by A.store_name having count(*) > 1;
+-------------+
| store_name  |
+-------------+
| Boston      |
| Houston     |
| Los Angeles |
+-------------+
3 rows in set (0.01 sec)

7.无交集值

显示第一个SQL语句的结果，且与第二个SQL语句没有交集的结果，且没有重复

select DISTINCT store_name from location where (store_name) NOT IN (select store_name from store_info);

例：mysql> select distinct store_name from location where (store_name) not in (select store_name from store_info);
+------------+
| store_name |
+------------+
| New York   |
+------------+
1 row in set (0.00 sec)

select DISTINCT A.store_name from location A left join store_info B using(store_name) where B.store_name IS NULL;

例：mysql> select distinct A.store_name from location A left join store_info B using(store_name) where B.store_name is null;
+------------+
| store_name |
+------------+
| New York   |
+------------+
1 row in set (0.00 sec)

select A.store_name from 
(select DISTINCT store_name from location union all select DISTINCT store_name from store_info) A 
group by A.store_name HAVING COUNT(*) = 1;

例：mysql> select A.store_name from 
    -> (select distinct store_name from location union all select distinct store_name from store_info)A
    -> group by A.store_name having count(*) = 1;
+------------+
| store_name |
+------------+
| New York   |
| Washington |
+------------+
2 rows in set (0.00 sec)

8.Case

是 SQL 用来做为 if-then-ELSE 之类逻辑的关键字

select Case ("字段名")
  when "条件1" then "结果1"     # "条件" 可以是一个数值或是公式。 ELSE 子句则并不是必须的。
  when "条件2" then "结果2"
  ...
  [ELSE "结果N"]
  end
from "表名";
例：select store_name, Case store_name 
  when 'Los Angeles' then sales * 2 
  when 'Boston' then 2000
  ELSE sales 
  end 
"New sales",Date    #"New sales" 是用于 Case 那个字段的字段名。
from store_info;

例：mysql> select store_name, Case store_name 
    ->   WHEN 'Los Angeles' THEN sales * 2 
    ->   WHEN 'Boston' THEN 2000
    ->   ELSE sales 
    ->   END 
    -> "New sales",Date 
    -> from store_info;
+-------------+-----------+------------+
| store_name  | New sales | Date       |
+-------------+-----------+------------+
| Los Angeles |      3000 | 2020-12-05 |
| Houston     |       250 | 2020-12-07 |
| Washington  |       300 | 2020-12-08 |
| Boston      |      2000 | 2020-12-08 |
+-------------+-----------+------------+
4 rows in set (0.00 sec)

9.空值(NULL) 和无值(‘’) 的区别

无值的长度为 0，不占用空间的；而 NULL 值的长度是 NULL，是占用空间的
IS NULL 或者 IS NOT NULL，是用来判断字段是不是为 NULL 或者不是 NULL，不能查出是不是无值的。
无值的判断使用='‘或者<>’'来处理。<> 代表不等于
在通过 count()指定字段统计有多少行数时，如果遇到 NULL 值会自动忽略掉，遇到无值会加入到记录中进行计算

select length(NULL), length(''), length('1');
select * from City where name IS NULL;
select * from City where name IS NOT NULL;
select * from City where name = '';
select * from City where name <> '';
select COUNT(*) from City;c
select COUNT(name) from City;

例：mysql> SELECT length(NULL), length(''), length('1');
+--------------+------------+-------------+
| length(NULL) | length('') | length('1') |
+--------------+------------+-------------+
|         NULL |          0 |           1 |
+--------------+------------+-------------+
1 row in set (0.00 sec)
mysql> SELECT * FROM city WHERE name IS NULL;
Empty set (0.00 sec)

mysql> SELECT * FROM city WHERE name IS NOT NULL;
+----------+
| name     |
+----------+
| beijing  |
| nanjing  |
| shanghai |
| <null>   |
| <null>   |
+----------+
5 rows in set (0.00 sec)

mysql> SELECT * FROM city WHERE name = '';
Empty set (0.00 sec)

mysql> SELECT * FROM city WHERE name <> '';
+----------+
| name     |
+----------+
| beijing  |
| nanjing  |
| shanghai |
| <null>   |
| <null>   |
+----------+
5 rows in set (0.00 sec)

mysql> SELECT COUNT(*) FROM city;
+----------+
| COUNT(*) |
+----------+
|        5 |
+----------+
1 row in set (0.00 sec)

mysql> SELECT COUNT(name) FROM city;
+-------------+
| COUNT(name) |
+-------------+
|           5 |
+-------------+
1 row in set (0.00 sec)

10.正则表达式

匹配模式	描述	实例
^	匹配文本的开始字符	‘^bd’ 匹配以 bd 开头的字符串
$	匹配文本的结束字符	‘qn$’ 匹配以 qn 结尾的字符串
.	匹配任何单个字符	‘s.t’ 匹配任何 s 和 t 之间有一个字符的字符串
*	匹配零个或多个在它前面的字符	‘fo*t’ 匹配 t 前面有任意个 o
+	匹配前面的字符 1 次或多次	‘hom+’ 匹配以 ho 开头，后面至少一个m 的字符串
字符串	匹配包含指定的字符串	‘clo’ 匹配含有 clo 的字符串
p1\|p2	匹配 p1 或 p2	‘bg\|fg’ 匹配 bg 或者 fg
[…]	匹配字符集合中的任意一个字符	‘[abc]’ 匹配 a 或者 b 或者 c
[^…]	匹配不在括号中的任何字符	‘[ ^ ab]’ 匹配不包含 a 或者 b 的字符串
{n}	匹配前面的字符串 n 次	‘g{2}’ 匹配含有 2 个 g 的字符串
{n,m}	匹配前面的字符串至少 n 次	至多m 次 ‘f{1,3}’ 匹配 f 最少 1 次，最多 3 次

select "字段" from "表名" where "字段" REGEXP {模式};
例：select * from store_info where store_name REGEXP 'os';
select * from store_info where store_name REGEXP '^[A-G]';
select * from store_info where store_name REGEXP 'Ho|Bo';

例：mysql> SELECT * FROM store_info WHERE Store_Name REGEXP 'os';
+-------------+-------+------------+
| Store_Name  | Sales | Date       |
+-------------+-------+------------+
| Los Angeles |  1500 | 2020-12-05 |
| Boston      |   700 | 2020-12-08 |
+-------------+-------+------------+
2 rows in set (0.00 sec)

mysql> SELECT * FROM store_info WHERE Store_Name REGEXP '^[A-G]';
+------------+-------+------------+
| Store_Name | Sales | Date       |
+------------+-------+------------+
| Boston     |   700 | 2020-12-08 |
+------------+-------+------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM store_info WHERE Store_Name REGEXP 'Ho|Bo';
+------------+-------+------------+
| Store_Name | Sales | Date       |
+------------+-------+------------+
| Houston    |   250 | 2020-12-07 |
| Boston     |   700 | 2020-12-08 |
+------------+-------+------------+
2 rows in set (0.00 sec)