实验准备，数据表配置
mysql -uroot -p show databases; create database train_ticket; use train_ticket; create table REGION(region varchar(10),site varchar(20)); create table FARE(site varchar(20),money int(10),date varchar(15)); desc REGION; desc FARE; insert into REGION values ('south','changsha'); insert into REGION values ('south','nanchang'); insert into REGION values ('north','beijing'); insert into REGION values ('north','tianjin'); insert into FARE values ('changsha',1000,'2021-01-30'); insert into FARE values ('nanchang',700,'2021-01-30'); insert into FARE values ('beijing',1500,'2021-01-30'); insert into FARE values ('tianjin',1200,'2021-01-30'); insert into FARE values ('beijing',2200,'2021-02-05'); select * from REGION; select * from FARE;
MySQL 高阶SQL 语句
1、select
显示表格中一个或数个字段的所有资料
语法：SELECT 字段 FROM 表名
select region from REGION;
2、DISTINCT
不显示重复的资料（去重）
语法：SELECT DISTINCT 字段 FROM 表名
select distinct region from REGION;
3、WHERE
有条件查询
语法：SELECT 字段 FROM 表名 WHERE 条件
select site from FARE where money > 1000; select site from FARE where money < 1000; select site from FARE where money = 1000;
4、AND、OR
and（并且）、or（或者）
语法：SELECT 字段 FROM 表名 WHERE 条件1 ([AND|OR] 条件2)+；
select site from FARE where money > 1000 and (money < 1500); select site,money from FARE where money < 500 or (money < 1500 and money >= 700); select site,money,date from FARE where money >= 500 and (date < '2021-02-05' and money < 1000);
5、IN
显示已知的值的资料
语法：SELECT 字段 FROM 表名 WHERE 字段 IN (‘值1’,‘值2’,……);
select site,money from FARE where money in (700,1000);
6、BETWEEN
显示两个值范围内的资料
语法：SELECT 字段 FROM 表名 WHERE 字段 BETWEEN ‘值一’ and ‘值二’;
select * from FARE where money between 500 and 1000;
7、通配符、LIKE
通常通配符都是跟LIKE一起使用
%：百分号表示零个、一个或多个字符
_： 下划线表示单个字符
LIKE：用于匹配模式来查找资料
语法：SELECT 字段 FROM 表名 WHERE 字段 LIKE ‘模式’;
select * from FARE where site like 'be%'; select site,money from FARE where site like '%jin_';
8、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：按照升序进行排序，默认的排序方式
#DESC：按照降序进行排序
select * from FARE order by money desc; select date,money from FARE order by money desc;
函数：
1、数字函数：
select abs(-1),rand(),mod(5,3),power(2,3),round (1.579),round(1.734,2);
select sqrt(9),truncate(1.234,2),ceil(1.2),floor(1.9),greatest(1,2,3,4),least(1,2,3,4);
2、聚合函数：
select avg(money) from FARE; select count(money) from FARE; select min(money) from FARE; select max(money) from FARE; select sum(money) from FARE;
#count(*)包括所有列的行数，在统计结果时，不好忽略值为null #count(字段)只包括那一行的列数，在统计结果的时候，会忽略列值为null的值
3、字符串函数：
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 'na' from 'nanchang'); select trim(trailing '--' from 'nanchang--'); select trim(both '--' from '--nanchang--'); select concat(region,site) from REGION where region = 'south'; select concat(region,' ',site) from REGION where region = 'south'; select substr(money,1,2) from FARE; select length(site) from FARE; select replace(site,'ji','--') from FARE; select upper(site) from FARE; select lower('HAHAHA'); select left(site,2) from FARE; select right(site,3) from FARE; select repeat(site,2) from FARE; select space(2); select strcmp(100,200); select reverse(site) from FARE;
4、|| 连接符：
如果sql_mode开启开启了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
mysql -uroot -p use train_ticket; select region || ' ' || site from REGION where region = 'north'; select site || ' ' || money || ' ' || date from FARE;
5、GROUP BY ：
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。
语法：SELECT 字段1,SUM(字段2) FROM 表名 GROUP BY 字段1;
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。 语法：SELECT 字段1,SUM(字段2) FROM 表名 GROUP BY 字段1;
6、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。
如果被SELECT的只有函数栏，那就不需要GROUP BY子句。
语法：SELECT 字段1,SUM(字段2) FROM 表名 GROUP BY 字段1 HAVING(函数条件);
select site,count(money),sum(money),date from FARE group by site having sum(money) >=700;
7、别名 :
字段别名、表格别名
语法：SELECT “表格別名”.“字段1” [AS] “字段1別名” FROM “表格名” [AS] “表格別名”;
select RE.region AS reg, count(site) from REGION AS RE group by reg; select FA.site AS si,sum(money),count(money),date AS da from FARE AS FA group by si;
8、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] (SELECT 字段1 FROM 表格2 WHERE 条件)
可以是符号的运算符 例:=、>、<、>=、<=
也可以是文字的运算符 例:LIKE、IN、BETWEEN
select A.site,region from REGION AS A where A.site in(select B.site from FARE AS B where money<2000); select B.site,money,date from FARE AS B where site in(select A.site from REGION AS A where region = 'north');
