單表查詢 · Mysql問題總結

## **單表查詢** ## <blockquote class="danger">表創建</blockquote> ``` 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, #一個部門一個屋子 depart_id int ); ``` ## <blockquote class="danger">插入記錄【三個部門：教學，銷售，運營】</blockquote> insert into emp(name,sex,age,hire_date,post,salary,office,depart_id) values ('jason','male',18,'20170301','張江第一帥形象代言',7300.33,401,1), #以下是教學部 ('egon','male',78,'20150302','teacher',1000000.31,401,1), ('kevin','male',81,'20130305','teacher',8300,401,1), ('tank','male',73,'20140701','teacher',3500,401,1), ('owen','male',28,'20121101','teacher',2100,401,1), ('jerry','female',18,'20110211','teacher',9000,401,1), ('nick','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),#以下是銷售部門 ('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), ('張野','male',28,'20160311','operation',10000.13,403,3), #以下是運營部門 ('程咬金','male',18,'19970312','operation',20000,403,3), ('程咬銀','female',18,'20130311','operation',19000,403,3), ('程咬銅','male',18,'20150411','operation',18000,403,3), ('程咬鐵','female',18,'20140512','operation',17000,403,3) ; #ps：如果在windows系統中，插入中文字符，select的結果為空白，可以將所有字符編碼統一設置成gbk ## <blockquote class="danger">查詢記錄</blockquote> 1. 語法執行順序 1. 初識查詢語句 select id,name from emp where id \>= 3 and id <= 6; 先后順序 from where select 2. where約束條件 ## 1.查詢id大于等于3小于等于6的數據 select id,name from emp where id >=3 and id <= 6; select * from emp where id between 3 and 6; ## 2.查詢薪資是20000或18000或17000的數據 select * from emp where salary = 20000 or salare = 18000 or salary = 17000; select * from emp where salary in (20000,18000,17000); # 簡寫 ## 3.查詢員工姓名中包含字母o的員工姓名和薪資在剛開始接觸mysql查詢時，建議按照查詢的優先級順序拼寫出你的sql語句 """ 先是查哪張表 from emp 再是根據什么條件去查 where name like ‘%o%’ 再是對查詢出來的數據篩選展示部分 select name,salary select name,salary from emp where name like '%o%'; ## 4.查詢員工姓名是由四個字符組成的員工姓名和其薪資 select name,salary from emp where name like '____'; select name,salary from emp where char_length(name) = 4; ## 5.查詢id小于3或者大于6的數據 select * from emp where id <3 or id >6; select * from emp where id not between 3 and 6; ## 6.查詢薪資不在20000,18000,17000范圍的數據 select * from emp where salary not in (20000,18000,17000); ## 7.查詢崗位描述為空的員工姓名與崗位名針對null不能用等號，只能用is select name,post from emp where post_comment = null; # 查詢為空！ select name,post from emp where post_comment is null; select name,post from emp where post_comment is not null; 3. group by ## 數據分組應用場景：每個部門的平均薪資，男女比例等 ## 1.按部門分組 select * from emp group by post; # 分組后取出的是每一個組的第一條數據 select id,name,sex from emp group by post # 驗證 """ 設置sql_mode為only_full_group_by，意味著以后但凡分組，只能取到分組的依據，不應該在去取組里面的單個元素的值，那樣的話分組就沒有意義了，因為不分組就是對單個元素信息的隨意獲取 """ set global sql_mode="strict_trans_tables,only_full_group_by"; ## 重新鏈接客戶端 select * from emp group by post; # 報錯 select id,name,sex from emp group by post; # 報錯 select post from emp group bty post; # 獲取部門信息 ## 強調：只要分組了，就不能夠再"直接"查找到單個數據信息了，只能獲取到組名 ## 2.獲取每個部門的最高工資 ## 以組為單位統計組內數據>>>聚合查詢(聚集到一起合為一個結果) ## 每個部門的最高工資 select post,max(salary) from emp group by post; ## 每個部門的最低工資 select post,min(salary) from emp group by post; ## 每個部門的平均工資 select post,avg(salary) from emp group by post; ## 每個部門的工資總和 select post,sum(salary) from emp group by post; ## 每個部門的人數 select post,count(id) from emp group by post; ## 3.查詢分組之后的部門名稱和每個部門下所有學生姓名 ## group_concat(分組之后用)不僅可以用來顯示除分組外字段還有拼接字符串的作用 select post,group_concat(name) from emp group by post; select post,group_concat(name,"_SB") from emp group by post; select post,group_concat(name,"：") from emp group by post; select post,group_concat(salary) from emp group by post; ## 4.補充concat(不分組是用)拼接字符串達到更好的顯示效果 as語法使用 select name as 姓名,salary as 薪資 from emp; select concat("NAME:",name) as 姓名,concat("SAL:",salary) as 薪資 from emp; ## 補充 as 語法,即可以給字段起別名也可以給表起 select emp.id,emp.name from emp as t1; # 報錯因為表名已經被你改成了t1 select t1.id,t1.name from emp as t1; ## 查詢四則運算 ## 查詢每個人的年薪 select name,salary*12 as annual_salary from emp; select name,salary*12 annual_salary from emp; # as可以省略 4. having select 查詢字段1,查詢字段2,... from 表名 where 過濾條件 group by分組依據 ## 語法這么寫，但是執行順序卻不一樣 from where group by select 5. distinct 1. 對有重復的展示數據進行去重操作 ```select distinct post from emp;``` 6. order by 1. 默認升序排 ```select * from emp order by salary asc; ``` 2. 降序排 ```select * from emp order by salary desc;``` 3. 先按照age降序排，在年輕相同的情況下再按照薪資升序排 ```select * from emp order by age desc,salary asc;``` 4. 統計各部門年齡在10歲以上的員工平均工資，并且保留平均工資大于1000的部門，然后對平均工資進行排序 ```select post,avg(salary) from emp where age > 10 group by post having avg(salary) > 1000 order by avg(salary) ;``` 7. limit 1. 限制展示條數 ```select * from emp limit 3; ``` 2. 查詢工資最高的人的詳細信息 ```select * from emp order by salary desc limit 1; ``` 3. 分頁顯示 ```select * from emp limit 0,5;``` 8. 正則 regexp 命令：``select * from emp where name regexp '^j.\*(n|y)$'; ``` 9. 練習 ## 剛開始查詢表，一定要按照最基本的步驟，先確定是哪張表，再確定查這張表也沒有限制條件，再確定是否需要分類，最后再確定需要什么字段對應的信息 1. 查詢崗位名以及崗位包含的所有員工名字 2. 查詢崗位名以及各崗位內包含的員工個數 3. 查詢公司內男員工和女員工的個數 4. 查詢崗位名以及各崗位的平均薪資 5. 查詢崗位名以及各崗位的最高薪資 6. 查詢崗位名以及各崗位的最低薪資 7. 查詢男員工與男員工的平均薪資，女員工與女員工的平均薪資 """ 參考答案： select post,group_concat(name) from emp group by post; select post,count(id) from emp group by post; select sex,count(id) from employee group by sex; select post,avg(salary) from emp group by post; select post,max(salary) from employee group by post; select post,min(salary) from employee group by post; select sex,avg(salary) from employee group by sex; """ ## 關鍵字where group by同時出現的情況下，group by必須在where之后 ## where先對整張表進行一次篩選，如何group by再對篩選過后的表進行分組 ## 如何驗證where是在group by之前執行而不是之后利用聚合函數因為聚合函數只能在分組之后才能使用 select id,name,age from emp where max(salary) > 3000; # 報錯！ select max(salary) from emp; ## 正常運行，不分組意味著每一個人都是一組，等運行到max(salary)的時候已經經過where，group by操作了，只不過我們都沒有寫這些條件 ## 語法順序 select from where group by ## 再識執行順序 from where group by select 8、統計各部門年齡在30歲以上的員工平均工資 select post,avg(salary) from emp where age > 30 group by post; ## 對where過濾出來的虛擬表進行一個分組 ## 還不明白可以分步執行查看結構 select * from emp where age>30; ## 基于上面的虛擬表進行分組 select * from emp where age>=30 group by post;