day39:MySQL:查询操作之单表查询&多表查询&子查询
目录part1:单表查询 1.where条件的使用 2.group 子句 分组分类 3.having 数据在分类分组之后,进行二次数据过滤 4.order by 排序,按照什么字段进行排序 5.limit 限制查询条数 (数据分页) 6.regexp (了解)可以使用正则表达式查询数据 (不推荐,效率不高) part2:多表查询 1.内连接 inner join 2.外连接 part3:子查询 part1:单表查询SQL查询语句的完整语法:?select .. from .. where .. group by .. having? .. order by .. limit .. 1.where条件的使用功能: 对表中的数据进行过滤筛选 1.判断的符号 = > >= < <= != <> 不等于 2.拼接关键字 and or not 3.查询范围区间 between between 小值 and 大值 [小值,大值] 查询两者之间的这个范围所有数据 4.查询某个值在具体某个范围里 in in(1,2,3,4) 5.模糊查询 like "%" "_" 通配符 like "%a" 匹配以a结尾的任意长度的字符串 like "a%" 匹配以a开头的任意长度的字符串 like "%a%" 匹配含有a字母的任意长度的字符串 like "_a" 个数一共是2个字符,必须以a结尾,前面字符随意 like "a__" 个数一共是3个字符,必须以a开头,后面字符随意 创建employee并插入数据 #创建表 create table employee( id int not null unique auto_increment,emp_name varchar(20) null,sex enum('male',female') not null default 大部分是男的 age int(3) unsigned not null default 28,hire_date date ),post_comment varchar(100一个部门一个屋子 depart_id int ); 三个部门：教学，销售，运营 insert into employee(emp_name,sex,age,hire_date,post,salary,office,depart_id) values (egon20170301老男孩驻沙河办事处外交大使以下是教学部 (alex20150302teacherwupeiqi20130305yuanhao20140701liwenzhou20121101jingliyang20110211jinxin19000301成龙20101111歪歪20150311sale以下是销售部门 (丫丫20101101丁丁20110312星星20160513格格20170127张野20160311operation以下是运营部门 (程咬金19970312程咬银20130311程咬铜20150411程咬铁20140512) ; (1) 单条件的查询 查询部门是sale 的所有员工姓名 select emp_name from employee where post = ""; (2) 多条件的查询 部门是teacher,收入大于10000的所有数据 select * from employee where post=" and salary > 10000 (3) between .. and .. 收入在1万~2万之间的所有员工姓名和收入 select emp_name,salary from employee where salary between 10000 and 20000; 收入不在1万~2万之间的所有员工姓名和收入 select emp_name,1)">from employee where salary not between 10000 (4) null 关键字 在查询时候,需要使用is进行判断,不能用= select * from employee where post_comment = null; select * from employee where post_comment = ''; select * from employee where post_comment isis null; (5) in 在 ... 之中 查询收入是 3000,4000,5000,8300所有原型的姓名和收入 select emp_name,1)">in (3000,8300) (推荐) select emp_name,1)">from employee where salary = 3000 or salary=4000 or salary=5000 or salary=8300 not .. in .. select emp_name,1)">not ); (6) 模糊查询 like "%" "_" (1) 匹配员工姓名 以on结尾的. "%" 通配符 select emp_name from employee where emp_name like %on (2) "_"通配符 限定字符长度使用_ select emp_name a_e_ (7) concat (as 起别名) select concat(姓名:",emp_name,1)">工资:from employee; concat_ws(拼接的符号,参数1,参数2,参数3 .. .. ) select concat_ws( : 计算每个人的年薪 可以在mysql使用四则运算(+ - * / ) select concat_ws(from employee; employee表详情如下所示
2.group 子句 分组分类group by 字段 对当前字段进行分类,by后面接什么字段,select 就搜什么字段 """group by 字段 对当前字段进行分类,select 就搜什么字段""" select sex employee group by sex; select post employee group by post; group_concat 按照分类的形式进行字段的拼接 select group_concat(emp_name) employee group by post; 聚合函数 count 统计总数 *所有 select count(*) employee; max 统计最大值 select max(salary) min 统计最小值 select min(salary) avg 统计平均值 select avg(salary) sum 统计总和 select sum(salary) employee; 1. 查询部门名以及各部门的平均薪资 聚合函数 + 分组 配合使用 select post,avg(salary) employee group by post 2. 查询部门名以及各部门的最高薪资 select post,max(salary) employee group by post 3. 查询部门名以及各部门的最低薪资 select post,min(salary) 4. 查询公司内男员工和女员工的个数 select sex,count(*) employee group by sex 5. 查询部门名以及部门包含的所有员工名字 select post,group_concat(emp_name) employee group by post 可以group by 两个字段,搜索2个字段 select post,emp_name from employee group by post,emp_name 3.having 数据在分类分组之后,进行二次数据过滤,一般是配合group by 使用,分组之后过滤找平均薪资大于10000以上的所有部门 select post,1)">from employee group by post having avg(salary) > 10000 1.查询各岗位内包含的员工个数小于2的岗位名、岗位内包含员工名字、个数 select post,group_concat(emp_name),1)">from employee group by post having count(*) < 2 ; 2.查询各岗位平均薪资小于10000的岗位名、平均工资 select post,1)">from employee group by post having avg(salary) < 10000 3.查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资 select post,1)">from employee group by post having avg(salary) > 10000 and avg(salary) < 20000; 4.order by 排序,按照什么字段进行排序asc 升序: 从小到大 (默认) desc 降序: 从大到小 select * from employee order by age (默认是asc升序) select * from employee order by age desc (默认是desc 倒序) 1. 查询所有员工信息，先按照age升序排序，如果age相同则按照hire_date降序排序 select * employee order by age,hire_date desc; 2. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资升序排列 select post,1)">from employee group by post having avg(salary) > 10000 order by avg(salary) 3. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资降序排列 select post,1)">from employee group by post having avg(salary) > 10000 order by avg(salary) desc 5.limit 限制查询条数 (数据分页)limit m,n m代表从第几条数据进行查询,0代表第一条,n代表的查询几条 select * from employee limit 0,5 从第一条数据开始搜,搜5条数据 select * from employee limit 5,1)"> 从第六条数据开始搜,搜5条数据 只搜索一条数据 select * from employee limit 1 只搜索3条数据 select * from employee limit 3 搜索这个表中最后一条数据 select * from employee order by id desc limit 1 6.regexp (了解)可以使用正则表达式查询数据 (不推荐,效率不高)select * from employee where emp_name regexp .*on$"; .*? 这个?号 mysql 不识别 select * 程程.*金"; part2:多表查询创建employee表和department表，并插入数据 建表 create table department( id int,name varchar(20) ); create table employee( id int primary key auto_increment,1)">'插入数据 insert into department values (200,1)">技术人力资源销售运营); insert into employee(name,dep_id) values () ; dapartment表和employee表详情如图所示
1.内连接 inner join内连接 : (inner join ) -> 两表或者多表满足条件的所有数据查询出来(两表之间的共有数据) 两表查询 select 字段 from 表1 inner join 表2 on 必要的关联条件 多表查询 select 字段 from 表1 inner join 表2 on 必要的关联条件1 inner join 表3 on 必要的关联条件2 ... 基本语法 inner join on + 条件 select * from employee inner join department on employee.dep_id = department.id; 用as 起别名(推荐) select * from employee as e inner join department as d on e.dep_id = d.id; as 可以省略 select * from employee e inner join department d on e.dep_id = d.id; where 默认实现的就是内联查询的效果 select * from employee,department where employee.dep_id = department.id; select * from employee as e,department as d where e.dep_id = d.id; 2.外连接1.左连接(左联查询 left join )? 以左表为主,右表为辅,完整查询左表所有数据,右表没有的补null 2.右连接(右联查询 right join ) 以右表为主,左表为辅,1)">完整查询右表所有数据,左表没有的补null 3.全连接 union 1.左连接(左联查询 left join ) 以左表为主,完整查询左表所有数据,右表没有的补null select * from employee left join department on employee.dep_id = department.id; 2.右连接(右联查询 right join ) 以右表为主,完整查询右表所有数据,左表没有的补null select * from employee right join department on employee.dep_id = 3.全连接 select * department.id union select * from employee right join department on employee.dep_id = department.id part3:子查询子查询: 嵌套查询 1.sql语句当中又嵌套了另外一条sql语句,用()括号抱起来,表达一个整体 2.一般应用在from 字符后面(表达一张表)
2.查看技术部门员工姓名select e.name,1)"> employee as e,1)"> d.id and d.name = " (2) inner join 写法 employee as e inner join department as d on e.dep_id = d.id where 非必要条件写在where字句中 d.name = (3) 子查询 1.找计数部门对应的id select id from department where name =" 2.通过id找员工姓名 select name from employee where dep_id = 200; 3.综合拼接 select name,dep_id from employee where dep_id = (select id "); 3.查看哪个部门没员工联表差生null值,谁是null谁就没员工 department as d left join employee as e on e.dep_id = d.id where e.id null 1.查询员工都在哪些部门 (200,201 202 204) select dep_id employee group by dep_id 2.把不在这些部门的数据找出来 select id in (200,201,202,1)">) from employee group by dep_id ); 4.查询大于平均年龄的员工名与年龄如果平均年龄是25; select name,age from employee where age > 25 计算平均年龄 select avg(age) employee 综合拼接 select name,1)">from employee where age > (select avg(age) employee); 5.把大于其本部门平均年龄的员工名和姓名查出来1.先计算各部门平均年龄是多少 select dep_id,avg(age) as avg_age employee group by dep_id 2.把查询各部门的平均年龄和过去employee做联表,变成更大的表方便后期做单表查询; select * employee as t1 inner join (1号数据) as t2 on t1.dep_id = t2.dep_id 3.综合拼接 employee as t1 inner join (select dep_id,avg(age) as avg_age from employee group by dep_id) as t2 on t1.dep_id = t2.dep_id 4.做最后的条件帅选 t2.dep_id where t1.age > t2.avg_age 6.查询每个部门最新入职的那位员工 --->利用上一套数据表进行查询# 1.找每个部门最大的入职时间 select post,max(hire_date) as max_date 2.把子查询搜索出来的数据和 employee 联合成一张更大的表,做一次单表查询 select * employee as t1 inner join (1号查询出来的数据) as t2 on t1.post = t2.post where t1.hire_date = t2.max_date 综合拼接 select t1.emp_name,t1.hire_date employee as t1 inner join (select post,max(hire_date) as max_date from employee group by post) as t2 on t1.post = t2.post where t1.hire_date = t2.max_date 7.带EXISTS关键字的子查询EXISTS 关键字,表达存在 如果内层sql 能够查询到数据,返回True,外层sql执行查询语句 如果内层sql 不能够查询到数据,返回False,外层sql不执行查询语句 select * from employee where exists ( select * from employee where id = 100); 8.关于子查询的总结子查询总结: 子查询可以单独作为一个临时数据表,临时数据,临时字段 一般用在 from where select 子句后面 可以通过查询出来的临时数据和另外的表做一次联表,变成更大的表,然后做单表查询,以得到想要的结果. 9.distinct去重额外 distinct 去重[尝试操作] select distinct depart_id from employee; （编辑：李大同） 【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容! |
