mysql五-1:单表查询
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mysql五-1:单表查询
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一 介紹
本節內容:
查詢語法
關鍵字的執行優先級
簡單查詢
單條件查詢:WHERE
分組查詢:GROUP BY
HAVING
查詢排序:ORDER BY
限制查詢的記錄數:LIMIT
使用聚合函數查詢
使用正則表達式查詢
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#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) ;二 查詢語法
SELECT 字段1,字段2... FROM 表名WHERE 條件GROUP BY fieldHAVING 篩選ORDER BY fieldLIMIT 限制條數?
三 關鍵字的執行優先級(重點)
重點中的重點：關鍵字的執行優先級 from where group by having select distinct order by limit1.找到表:from
2.拿著where指定的約束條件，去文件/表中取出一條條記錄
3.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.按照select后的字段得到一張新的虛擬表，如果有聚合函數，則將組內數據進行聚合
5.將4的結果過濾：having,如果有聚合函數也是先執行聚合再having過濾
6.查出結果：select
7.去重
8.將結果按條件排序：order by
9.限制結果的顯示條數
詳細見：http://www.cnblogs.com/linhaifeng/articles/7372774.html
四 簡單查詢
#簡單查詢 SELECT id,name,sex,age,hire_date,post,post_comment,salary,office,depart_id FROM employee;SELECT * FROM employee;SELECT name,salary FROM employee;#避免重復DISTINCT SELECT DISTINCT post FROM employee; #通過四則運算查詢SELECT name, salary*12 FROM employee;SELECT name, salary*12 AS Annual_salary FROM employee;SELECT name, salary*12 Annual_salary FROM employee;#定義顯示格式 CONCAT() 函數用于連接字符串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;小練習：
1 查出所有員工的名字，薪資,格式為<名字:egon> <薪資:3000> 2 查出所有的崗位（去掉重復） 3 查出所有員工名字，以及他們的年薪,年薪的字段名為annual_year select concat('<名字:',name,'> ','<薪資:',salary,'>') from employee; select distinct depart_id from employee; select name,salary*12 annual_salary from employee; View Code五 WHERE約束
強調：where是一種約束條件，mysql會拿著where指定的條件去表中取數據，而having則是在取出數據后進行過濾
where字句中可以使用：
1. 比較運算符：> < >= <= <> !=
2. between 80 and 100 值在10到20之間
3. in(80,90,100) 值是10或20或30
4. like 'egon%'
? ? pattern可以是%或_，
? ? %表示任意多字符
? ? _表示一個字符?
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
六 分組查詢:GROUP BY
大前提：可以按照任意字段分組,但分完組后，只能查看分組的那個字段，要想取的組內的其他字段信息，需要借助函數
單獨使用GROUP BY關鍵字分組SELECT post FROM employee GROUP BY post;注意：我們按照post字段分組，那么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，并查看組內成員名 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，并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 #!!!MySQL 5.7默認ONLY_FULL_GROUP_BY語義介紹#參考鏈接：http://www.ywnds.com/?p=8184 #分組查詢的常見問題： mysql> select id,count from tt group by id; ERROR 1055 (42000): Expression #2 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'test.tt.count' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=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#去掉ONLY_FULL_GROUP_BY模式，如下操作： 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';#！！！注意 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中的值要么是來自于聚集函數的結果，要么是來自于group by list中的表達式的值。 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 #題1：分組 mysql> select post,group_concat(name) from employee group by post; +-----------------------------------------+---------------------------------------------------------+ | post | group_concat(name) | +-----------------------------------------+---------------------------------------------------------+ | operation | 張野,程咬金,程咬銀,程咬銅,程咬鐵 | | sale | 歪歪,丫丫,丁丁,星星,格格 | | teacher | alex,wupeiqi,yuanhao,liwenzhou,jingliyang,jinxin,成龍 | | 老男孩駐沙河辦事處外交大使 | egon | +-----------------------------------------+---------------------------------------------------------+#題目2： mysql> select post,count(id) from employee group by post; +-----------------------------------------+-----------+ | post | count(id) | +-----------------------------------------+-----------+ | operation | 5 | | sale | 5 | | teacher | 7 | | 老男孩駐沙河辦事處外交大使 | 1 | +-----------------------------------------+-----------+#題目3： mysql> select sex,count(id) from employee group by sex; +--------+-----------+ | sex | count(id) | +--------+-----------+ | male | 10 | | female | 8 | +--------+-----------+#題目4： mysql> select post,avg(salary) from employee group by post; +-----------------------------------------+---------------+ | post | avg(salary) | +-----------------------------------------+---------------+ | operation | 16800.026000 | | sale | 2600.294000 | | teacher | 151842.901429 | | 老男孩駐沙河辦事處外交大使 | 7300.330000 | +-----------------------------------------+---------------+#題目5 mysql> select post,max(salary) from employee group by post; +-----------------------------------------+-------------+ | post | max(salary) | +-----------------------------------------+-------------+ | operation | 20000.00 | | sale | 4000.33 | | teacher | 1000000.31 | | 老男孩駐沙河辦事處外交大使 | 7300.33 | +-----------------------------------------+-------------+#題目6 mysql> select post,min(salary) from employee group by post; +-----------------------------------------+-------------+ | post | min(salary) | +-----------------------------------------+-------------+ | operation | 10000.13 | | sale | 1000.37 | | teacher | 2100.00 | | 老男孩駐沙河辦事處外交大使 | 7300.33 | +-----------------------------------------+-------------+#題目七 mysql> select sex,avg(salary) from employee group by sex; +--------+---------------+ | sex | avg(salary) | +--------+---------------+ | male | 110920.077000 | | female | 7250.183750 | +--------+---------------+ View Code
七 使用聚合函數查詢
先from找到表
再用where的條件約束去表中取出記錄
然后進行分組group by，沒有分組則默認一組
然后進行聚合
最后select出結果
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;八 HAVING過濾
HAVING與WHERE在語法上是一樣的
select * from employee where salary > 10000; select * from employee having salary > 10000;HAVING與WHERE不一樣的地方在于!!!!!!
#！！！執行優先級從高到低：where > group by > 聚合函數 > having #1. Where 是一個約束聲明，使用Where約束來自數據庫的數據，Where是在結果返回之前起作用的（先找到表，按照where的約束條件，從表(文件)中取出數據），Where中不能使用聚合函數。#2. Having是一個過濾聲明，是在查詢返回結果集以后對查詢結果進行的過濾操作（先找到表，按照where的約束條件，從表(文件)中取出數據，然后group by分組，如果沒有group by則所有記錄整體為一組，然后執行聚合函數，然后使用having對聚合的結果進行過濾），在Having中可以使用聚合函數。#3. having可以放到group by之后，而where只能放到group by之前#4. 在查詢過程中聚合語句(sum,min,max,avg,count)要比having子句優先執行。而where子句在查詢過程中執行優先級高于聚合語句。驗證不同之處
#驗證之前再次強調：執行優先級從高到低：where > group by > 聚合函數 > having select count(id) from employee where salary > 10000; #正確，分析：where先執行，后執行聚合count(id),然后select出結果 select count(id) from employee having salary > 10000;#錯誤，分析：先執行聚合count(id)，后執行having過濾，無法對id進行salary>10000的過濾#以上兩條sql的順序是 1:找到表employee--->用where過濾---->沒有分組則默認一組執行聚合count(id)--->select執行查看組內id數目 2:找到表employee--->沒有分組則默認一組執行聚合count(id)---->having 基于上一步聚合的結果（此時只有count(id)字段了）進行salary>10000的過濾，很明顯，根本無法獲取到salary字段其他需要注意的問題
select post,group_concat(name) from employee group by post having salary > 10000;#錯誤，分組后無法直接取到salary字段 select post,group_concat(name) from employee group by post having avg(salary) > 10000;?
小練習：
1. 查詢各崗位內包含的員工個數小于2的崗位名、崗位內包含員工名字、個數 3. 查詢各崗位平均薪資大于10000的崗位名、平均工資 4. 查詢各崗位平均薪資大于10000且小于20000的崗位名、平均工資 #題1： mysql> select post,group_concat(name),count(id) from employee group by post having count(id) < 2; +-----------------------------------------+--------------------+-----------+ | post | group_concat(name) | count(id) | +-----------------------------------------+--------------------+-----------+ | 老男孩駐沙河辦事處外交大使 | egon | 1 | +-----------------------------------------+--------------------+-----------+#題目2： mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000; +-----------+---------------+ | post | avg(salary) | +-----------+---------------+ | operation | 16800.026000 | | teacher | 151842.901429 | +-----------+---------------+#題目3： mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) <20000; +-----------+--------------+ | post | avg(salary) | +-----------+--------------+ | operation | 16800.026000 | +-----------+--------------+九 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 employeeORDER BY age,salary DESC;小練習：
1. 查詢所有員工信息，先按照age升序排序，如果age相同則按照hire_date降序排序 2. 查詢各崗位平均薪資大于10000的崗位名、平均工資,結果按平均薪資升序排列 3. 查詢各崗位平均薪資大于10000的崗位名、平均工資,結果按平均薪資降序排列?
#題目1 mysql> select * from employee ORDER BY age asc,hire_date desc;#題目2 mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) asc; +-----------+---------------+ | post | avg(salary) | +-----------+---------------+ | operation | 16800.026000 | | teacher | 151842.901429 | +-----------+---------------+#題目3 mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) desc; +-----------+---------------+ | post | avg(salary) | +-----------+---------------+ | teacher | 151842.901429 | | operation | 16800.026000 | +-----------+---------------+ View Code十 限制查詢的記錄數: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 DESCLIMIT 0,5; #從第0開始，即先查詢出第一條，然后包含這一條在內往后查5條 SELECT * FROM employee ORDER BY salary DESCLIMIT 5,5; #從第5開始，即先查詢出第6條，然后包含這一條在內往后查5條小練習：
1. 分頁顯示，每頁5條 mysql> select * from employee limit 0,5; +----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ | 1 | egon | male | 18 | 2017-03-01 | 老男孩駐沙河辦事處外交大使 | NULL | 7300.33 | 401 | 1 | | 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 | | 3 | wupeiqi | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 | | 4 | yuanhao | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 | | 5 | liwenzhou | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 | +----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ rows in set (0.00 sec)mysql> select * from employee limit 5,5; +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+ | 6 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 | | 7 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 | | 8 | 成龍 | male | 48 | 2010-11-11 | teacher | NULL | 10000.00 | 401 | 1 | | 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 | | 10 | 丫丫 | female | 38 | 2010-11-01 | sale | NULL | 2000.35 | 402 | 2 | +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+ rows in set (0.00 sec)mysql> select * from employee limit 10,5; +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ | 11 | 丁丁 | female | 18 | 2011-03-12 | sale | NULL | 1000.37 | 402 | 2 | | 12 | 星星 | female | 18 | 2016-05-13 | sale | NULL | 3000.29 | 402 | 2 | | 13 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 | | 14 | 張野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 | | 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 | +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ rows in set (0.00 sec) View Code十一 使用正則表達式查詢
SELECT * FROM employee WHERE name REGEXP '^ale';SELECT * FROM employee WHERE name REGEXP 'on$';SELECT * FROM employee WHERE name REGEXP 'm{2}';小結：對字符串匹配的方式 WHERE name = 'egon'; WHERE name LIKE 'yua%'; WHERE name REGEXP 'on$';小練習：
查看所有員工中名字是jin開頭，n或者g結果的員工信息 select * from employee where name regexp '^jin.*[gn]$'; View Code?
轉載于:https://www.cnblogs.com/ctztake/p/7512084.html
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