mysql的单表查询 (where group by having order by limit)
1.设置数据库为严格模式:
2.数据准备
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# 创建一张部门表 create table emp( id int not null unique auto_increment, name varchar(20) not null, sex enum('male','female') not null default 'male', #大部分是男的 age int(3) unsigned not null default 28, hire_date date not null, post varchar(50), post_comment varchar(100), salary double(15,2), office int, # 一个部门一个屋子 depart_id int ); # 插入记录 # 三个部门:教学,销售,运营 insert into emp(name,sex,age,hire_date,post,salary,office,depart_id) values ('tank','male',17,'20170301','张江第一帅形象代言部门',7300.33,401,1), # 以下是教学部 ('egon','male',78,'20150302','teacher',1000000.31,401,1), ('kevin','male',81,'20130305','teacher',8300,401,1), ('jason','male',73,'20140701','teacher',3500,401,1), ('owen','male',28,'20121101','teacher',2100,401,1), ('jerry','female',18,'20110211','teacher',9000,401,1), ('大饼','male',18,'19000301','teacher',30000,401,1), ('sean','male',48,'20101111','teacher',10000,401,1), ('歪歪','female',48,'20150311','sale',3000.13,402,2),# 以下是销售部门 ('丫丫','female',38,'20101101','sale',2000.35,402,2), ('丁丁','female',18,'20110312','sale',1000.37,402,2), ('星星','female',18,'20160513','sale',3000.29,402,2), ('格格','female',28,'20170127','sale',4000.33,402,2), ('张野','male',28,'20160311','operation',10000.13,403,3), # 以下是运营部门 ('程咬金','male',18,'19970312','operation',20000,403,3), ('程咬银','female',18,'20130311','operation',19000,403,3), ('程咬铜','male',18,'20150411','operation',18000,403,3), ('程咬铁','female',18,'20140512','operation',17000,403,3); # PS:如果在windows系统中,插入中文字符,select的结果为空白,可以将所有字符编码统一设置成gbk ```
完整的查询语句:
完整的查询语句 select [distinct] {* | 字段名 | 聚合函数 | 表达式} from 表名 [where 条件 group by 字段名 having 条件 order by 字段名 limit 显示的条数]
查询顺序:
- from
- where
- group by
- having
- select
- distinct
3 . where 约束
# 1.查询id大于等于3小于等于6的数据 select * from emp where id >= 3 and id <= 6; # between: 两者之间 # and: 与 select * from emp where id between 3 and 6; select * from table where post_time > '2009-01-20' # 2.查询薪资是20000或者18000或者17000的数据 # or: 或者 select * from emp where salary=20000 or salary=18000 or salary=17000; # in: 在什么里 select * from emp where salary in (20000, 18000, 17000); # 3.查询员工姓名中包含o字母 的 员工姓名和薪资 # like: 模糊匹配 # %: 匹配0个或多个任意字符 以什么开头 以什么结尾 包含什么 # _: 匹配一个任意字符 ___匹配任意的三个字符 select name, salary from emp where name like "%o%"; # 4.查询员工姓名是由四个字符组成的员工姓名与薪资 select name, salary from emp where name like "_____"; # char_length(字段): 获取字段长度 select name, salary from emp where char_length(name) = 4; # 5.查询id小于3或者大于6的数据 select * from emp where id < 3 or id > 6; select * from emp where id not between 3 and 6; # 6.查询薪资不在20000,18000,17000范围的数据 select * from emp where salary not in (20000, 18000, 17000); # 7.查询岗位描述为空的员工名与岗位名 # 注意: 针对null不能用等号,只能用is is null select name, post from emp where post_comment = null; select name, post from emp where post_comment is null; select name,post from emp where post_comment is not null; ```
4. group by 聚合函数: max, min, sum, avg, count
concat 拼接: select concat('Name: ', name) as '名字', concat('Sal: ', salary) as '薪资' from emp;
比如: 一张员工表中有性别字段,可以根据性别分组,一组是男性,一组是女性,或者是根据部门分组,有教学部、销售部等... # 1.按部门分组 # 严格模式下只能获取分组字段post数据, 无法获取其他字段信息,就好比是进程之间数据隔离,但是可以使用聚合函数来获取 ''' 聚合函数: max, min, sum, avg, count ''' # 非严格模式下不报错 select * from emp group by post; # 报错 select id, name from emp group by post; # 报错 select post from emp group by post; # 严格模式设置 """ 设置sql_mode为only_full_group_by,意味着以后但凡分组,只能取到分组的依据, 不应该在去取组里面的单个元素的值,那样的话分组就没有意义了,因为不分组就是对单个元素信息的随意获取 """ show variables like "%mode%"; set global sql_mode="strict_trans_tables,only_full_group_by"; # 2.获取每个部门的最高工资 ----> 画图,讲述数据分组后,发生了什么事情 select post, max(salary) from emp group by post; # as: 起别名; 给获取出来的数据字段名,设置别名 select post as '部门', max(salary) as '薪资' from emp group by post; # 可简写, 但不推荐 select post '部门', max(salary) '薪资' from emp group by post; # 每个部门的最低工资 select post, min(salary) from emp group by post; # 每个部门的平均工资 select post, avg(salary) from emp group by post; # 每个部门的工资总和 select post, sum(salary) from emp group by post; # 每个部门的人数: count() 中传任意参数都没问题 select post, count(id) from emp group by post; select post, count(post) from emp group by post; # 3.查询分组之后的部门名称和每个部门下所有员工的姓名 # group_concat(name): 不仅可以获取分组后的某一个字段,并且可以对字符串进行拼接 select post, group_concat(name) from emp group by post; # 给每个部门的员工名字前 + NB_ select post, group_concat('NB_', name) from emp group by post; # 拼接部门员工名字+薪资 select post, group_concat(name, ":", salary) from emp group by post; # 4.补充concat(不分组时用)拼接字符串达到更好的显示效果 as语法使用 select concat('Name: ', name) as '名字', concat('Sal: ', salary) as '薪资' from emp; # 5.补充as语法 即可以给字段起别名也可以给表起 select emp.name as '名字', emp.salary as '薪资' from emp; # 6.查询四则运算 # 求各部门所有员工的年薪 select name, salary * 12 as annual_salary from emp; ```
5.having 语句
where 与 having的区别:
1. having与where语法一样,只不过having必须需要在group by后使用;
2. where 不能使用聚合函数,但having可以;
如 where avg(salary) >1000 是错误的 having avg(salary) > 1000
6. distinct : 去重
select distinct id, post from emp; select distinct post from emp; select distinct age from emp;
7.order by
# 1、根据薪资进行升序 select * from emp order by salary; # 默认升序 select * from emp order by salary asc; # 指定升序 select * from emp order by salary desc; # 指定降序
8. limit
# 1、从第一条开始,获取4条记录; select * from emp limit 4; # 查询前4行 # 2、limit可以有两个参数, 参数1:是限制的开始位置, 参数2:是从开始位置展示的条数; select * from emp limit 0, 4; select * from emp limit 4, 4; # 获取5-8行
9、正则
在编程中,凡是看到reg开头的,基本上都是跟正则有关 select * from emp where name regexp '^程.*(金|银|铜|铁)$';