mysql 單表查詢
阿新 • • 發佈:2017-12-18
concat_ws 之前 max 其它 white lin sql_mod class arch
一 、單表查詢的語法
SELECT 字段1,字段2... FROM 表名
WHERE 條件
GROUP BY field
HAVING 篩選
ORDER BY field
LIMIT 限制條數
二、關鍵字的執行優先級
重點中的重點:關鍵字的執行優先級 1、from 先找表 2、where 拿著約束條件做篩選 3、group by 將取出的一條條記錄進行分組group by,如果沒有group by,則整體作為一組4、having 將分組結果進行過濾 5、select 執行select 6、distinct 去重 7、order by 將結果排序 8、limit 限制顯示條數
三、簡單查詢
company.employee 員工id id int 姓名 emp_name varchar 性別 sex enum 年齡 age int 入職日期 hire_date date 崗位 post varchar 職位描述 post_comment varchar 薪水 salary double 辦公室 office int 部門編號 depart_id int準備表#創建表 create table employee( 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 ); #查看表結構 mysql> desc employee; +--------------+-----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +--------------+-----------------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(20) | NO | | NULL | | | sex | enum(‘male‘,‘female‘) | NO | | male | | | age | int(3) unsigned | NO | | 28 | | | hire_date | date | NO | | NULL | | | post | varchar(50) | YES | | NULL | | | post_comment | varchar(100) | YES | | NULL | | | salary | double(15,2) | YES | | NULL | | | office | int(11) | YES | | NULL | | | depart_id | int(11) | YES | | NULL | | +--------------+-----------------------+------+-----+---------+----------------+ #插入記錄 #三個部門:教學,銷售,運營 insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values (‘egon‘,‘male‘,18,‘20170301‘,‘老男孩駐沙河辦事處外交大使‘,7300.33,401,1), #以下是教學部 (‘alex‘,‘male‘,78,‘20150302‘,‘teacher‘,1000000.31,401,1), (‘wupeiqi‘,‘male‘,81,‘20130305‘,‘teacher‘,8300,401,1), (‘yuanhao‘,‘male‘,73,‘20140701‘,‘teacher‘,3500,401,1), (‘liwenzhou‘,‘male‘,28,‘20121101‘,‘teacher‘,2100,401,1), (‘jingliyang‘,‘female‘,18,‘20110211‘,‘teacher‘,9000,401,1), (‘jinxin‘,‘male‘,18,‘19000301‘,‘teacher‘,30000,401,1), (‘成龍‘,‘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
1、定義顯示格式
concat()函數用於連接字符串 用法:數據庫遷移(拷貝)CONCAT() 函數用於連接字符串 SELECT CONCAT(‘姓名: ‘,name,‘ 年薪: ‘, salary*12)
AS Annual_salary FROM employee; CONCAT_WS() 第一個參數為分隔符 SELECT CONCAT_WS(‘:‘,name,salary*12) AS Annual_salary FROM employee;
高級版
select concat(‘naem:‘,name) as name,concat(‘annual_salary:‘,salary*12)
as annual_salary from emp2;
2、
普通版:
select id,concat(name,‘:‘,sex,‘:‘,salary) as info from emp2;
# 進化版 select id,concat_ws(‘:‘,name,sex,salary) as info from emp2;
2、distinct 去重
as顯示效果的區別:
3、where約束
where字句中可以使用: 1. 比較運算符:> < >= <= <> != 2. between 80 and 100 值在80到100之間,包括80和100 3. in(80,90,100) 值是80或90或100 4. like ‘egon%‘ pattern可以是%或_, %表示任意多字符 _表示任意一個字符 5. 邏輯運算符:在多個條件直接可以使用邏輯運算符 and or not
#1:單條件查詢 SELECT name FROM employee WHERE post=‘sale‘; #2:多條件查詢 SELECT name,salary FROM employee WHERE post=‘teacher‘ AND salary>10000; #3:關鍵字BETWEEN AND SELECT name,salary FROM employee WHERE salary BETWEEN 10000 AND 20000; SELECT name,salary FROM employee WHERE salary NOT BETWEEN 10000 AND 20000; #4:關鍵字IS NULL(判斷某個字段是否為NULL不能用等號,需要用IS) SELECT name,post_comment FROM employee WHERE post_comment IS NULL; SELECT name,post_comment FROM employee WHERE post_comment IS NOT NULL; SELECT name,post_comment FROM employee WHERE post_comment=‘‘; 註意‘‘是空字符串,不是null ps: 執行 update employee set post_comment=‘‘ where id=2; 再用上條查看,就會有結果了 #5:關鍵字IN集合查詢 SELECT name,salary FROM employee WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ; SELECT name,salary FROM employee WHERE salary IN (3000,3500,4000,9000) ; SELECT name,salary FROM employee WHERE salary NOT IN (3000,3500,4000,9000) ; #6:關鍵字LIKE模糊查詢 通配符’%’ SELECT * FROM employee WHERE name LIKE ‘eg%‘; 通配符’_’ SELECT * FROM employee WHERE name LIKE ‘al__‘;
4、分組 group by
註意:使用分組只能查看分組的依據和聚合函數,若是查看其它信息,則默認是該組的第一條信息,這其實是沒有意義的。如下:
#1、首先明確一點:分組發生在where之後,即分組是基於where之後得到的記錄而進行的 #2、分組指的是:將所有記錄按照某個相同字段進行歸類,比如針對員工信息表的職位分組,
或者按照性別進行分組等 #3、為何要分組呢? 取每個部門的最高工資 取每個部門的員工數 取男人數和女人數 小竅門:‘每’這個字後面的字段,就是我們分組的依據 #4、大前提: 可以按照任意字段分組,但是分組完畢後,比如group by post,只能查看post字段,
如果想查看組內信息,需要借助於聚合函數
二 ONLY_FULL_GROUP_BY
#查看MySQL 5.7默認的sql_mode如下: mysql> select @@global.sql_mode; ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION #!!!註意 ONLY_FULL_GROUP_BY的語義就是確定select target list中的所有列的值都是明確語義,簡單的說來,在ONLY_FULL_GROUP_BY模式下,target list中的值要麽是來自於聚集函數的結果,
要麽是來自於group by list中的表達式的值。 #設置sql_mole如下操作(我們可以去掉ONLY_FULL_GROUP_BY模式): mysql> set global sql_mode=‘STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,
NO_ENGINE_SUBSTITUTION‘;
三 GROUP BY
# 單獨使用GROUP BY關鍵字分組 SELECT post FROM employee GROUP BY post; 註意:我們按照post字段分組,那麽select查詢的字段只能是post,想要獲取組內的其他相關信息,需要借助函數 GROUP BY關鍵字和GROUP_CONCAT()函數一起使用 SELECT post,GROUP_CONCAT(name) FROM employee GROUP BY post;
#按照崗位分組,並查看組內成員名 SELECT post,GROUP_CONCAT(name) as emp_members FROM employee GROUP BY post; GROUP BY與聚合函數一起使用 select post,count(id) as count from employee group by post;
#按照崗位分組,並查看每個組有多少人
強調:
如果我們用unique的字段作為分組的依據,則每一條記錄自成一組,這種分組沒有意義
多條記錄之間的某個字段值相同,該字段通常用來作為分組的依據
四 聚合函數
#強調:聚合函數聚合的是組的內容,若是沒有分組,則默認一組 示例: SELECT COUNT(*) FROM employee; SELECT COUNT(*) FROM employee WHERE depart_id=1; SELECT MAX(salary) FROM employee; SELECT MIN(salary) FROM employee; SELECT AVG(salary) FROM employee; SELECT SUM(salary) FROM employee; SELECT SUM(salary) FROM employee WHERE depart_id=3;
五、having 過濾
HAVING與WHERE不一樣的地方在於!!!!!!
#!!!執行優先級從高到低:where > group by > having #1. Where 發生在分組group by之前,因而Where中可以有任意字段,但是絕對不能使用聚合函數。 #2. Having發生在分組group by之後,因而Having中可以使用分組的字段,
無法直接取到其他字段,可以使用聚合函數
六、查詢排序:ORDER BY
按單列排序 SELECT * FROM employee ORDER BY salary; SELECT * FROM employee ORDER BY salary ASC; SELECT * FROM employee ORDER BY salary DESC; 按多列排序:先按照age排序,如果年紀相同,則按照薪資排序 SELECT * from employee ORDER BY age, salary DESC;
七、限制顯示數據:LIMIT
示例: SELECT * FROM employee ORDER BY salary DESC LIMIT 3; #默認初始位置為0 SELECT * FROM employee ORDER BY salary DESC LIMIT 0,5; #從第0開始,即先查詢出第一條,然後包含這一條在內往後查5條 SELECT * FROM employee ORDER BY salary DESC LIMIT 5,5; #從第5開始,即先查詢出第6條,然後包含這一條在內往後查5條
八、使用正則表達式查詢
SELECT * FROM employee WHERE name REGEXP ‘^ale‘; SELECT * FROM employee WHERE name REGEXP ‘on$‘; SELECT * FROM employee WHERE name REGEXP ‘m{2}‘; 小結:對字符串匹配的方式 WHERE name = ‘egon‘; WHERE name LIKE ‘yua%‘; WHERE name REGEXP ‘on$‘;View Code
mysql 單表查詢