單表查詢
詞法：
一、單表查詢的語法 SELECT 欄位1,欄位2... FROM 表名 WHERE 條件 GROUP BY field HAVING 篩選 ORDER BY field LIMIT 限制條數 二、關鍵字的執行優先順序（重點） 重點中的重點：關鍵字的執行優先順序 from where group by having select distinct order by limit 1.找到表:from 2.拿著where指定的約束條件，去檔案/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.將分組的結果進行having過濾 5.執行select 6.去重 7.將結果按條件排序：order by 8.限制結果的顯示條數
建立公司員工表，表的欄位和資料型別
company.employee 員工id id int 姓名 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
#建立表,設定欄位的約束條件 create table employee( id int primary key 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 depart_id int ); # 檢視錶結構 mysql> desc employee; +--------------+-----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +--------------+-----------------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | emp_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 | | | salart | double(15,2) | YES | | NULL | | | office | int(11) | YES | | NULL | | | depart_id | int(11) | YES | | NULL | | +--------------+-----------------------+------+-----+---------+----------------+ rows in set (0.08 sec) #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), ('xiaomage','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) ; 建立員工表，並插入記錄
1.where約束
where子句中可以使用 1.比較運算子：>、<、>=、<=、<>、!= 2.between 80 and 100 ：值在80到100之間 3.in(80,90,100)值是10或20或30 4.like 'xiaomagepattern': pattern可以是%或者_。%小時任意多字元，_表示一個字元 5.邏輯運算子：在多個條件直接可以使用邏輯運算子 and or not
驗證結果：
#1 :單條件查詢 mysql> select id,emp_name from employee where id > 5; +----+------------+ | id | emp_name | +----+------------+ | 6 | jingliyang | | 7 | jinxin | | 8 | xiaomage | | 9 | 歪歪 | | 10 | 丫丫 | | 11 | 丁丁 | | 12 | 星星 | | 13 | 格格 | | 14 | 張野 | | 15 | 程咬金 | | 16 | 程咬銀 | | 17 | 程咬銅 | | 18 | 程咬鐵 | #2 多條件查詢 mysql> select emp_name from employee where post='teacher' and salary>10000; +----------+ | emp_name | +----------+ | alex | | jinxin | +----------+ #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; #注意''是空字串，不是null SELECT name,post_comment FROM employee WHERE post_comment=''; ps： 執行 update employee set post_comment='' where id=2; 再用上條檢視，就會有結果了 #5:關鍵字IN集合查詢 mysql> SELECT name,salary FROM employee WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ; +------------+---------+ | name | salary | +------------+---------+ | yuanhao | 3500.00 | | jingliyang | 9000.00 | +------------+---------+ rows in set (0.00 sec) mysql> SELECT name,salary FROM employee WHERE salary IN (3000,3500,4000,9000) ; +------------+---------+ | name | salary | +------------+---------+ | yuanhao | 3500.00 | | jingliyang | 9000.00 | +------------+---------+ mysql> SELECT name,salary FROM employee WHERE salary NOT IN (3000,3500,4000,9000) ; +-----------+------------+ | name | salary | +-----------+------------+ | egon | 7300.33 | | alex | 1000000.31 | | wupeiqi | 8300.00 | | liwenzhou | 2100.00 | | jinxin | 30000.00 | | xiaomage | 10000.00 | | 歪歪 | 3000.13 | | 丫丫 | 2000.35 | | 丁丁 | 1000.37 | | 星星 | 3000.29 | | 格格 | 4000.33 | | 張野 | 10000.13 | | 程咬金 | 20000.00 | | 程咬銀 | 19000.00 | | 程咬銅 | 18000.00 | | 程咬鐵 | 17000.00 | +-----------+------------+ rows in set (0.00 sec) #6:關鍵字LIKE模糊查詢 萬用字元’%’ mysql> SELECT * FROM employee WHERE name LIKE 'jin%'; +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+ | 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 | +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+ rows in set (0.00 sec) 萬用字元'_' mysql> SELECT age FROM employee WHERE name LIKE 'ale_'; +-----+ | age | +-----+ | 78 | +-----+ row in set (0.00 sec) 練習： 1. 檢視崗位是teacher的員工姓名、年齡 2. 檢視崗位是teacher且年齡大於30歲的員工姓名、年齡 3. 檢視崗位是teacher且薪資在9000-1000範圍內的員工姓名、年齡、薪資 4. 檢視崗位描述不為NULL的員工資訊 5. 檢視崗位是teacher且薪資是10000或9000或30000的員工姓名、年齡、薪資 6. 檢視崗位是teacher且薪資不是10000或9000或30000的員工姓名、年齡、薪資 7. 檢視崗位是teacher且名字是jin開頭的員工姓名、年薪 #對應的sql語句 select name,age from employee where post = 'teacher'; select name,age from employee where post='teacher' and age > 30; select name,age,salary from employee where post='teacher' and salary between 9000 and 10000; select * from employee where post_comment is not null; select name,age,salary from employee where post='teacher' and salary in (10000,9000,30000); select name,age,salary from employee where post='teacher' and salary not in (10000,9000,30000); select name,salary*12 from employee where post='teacher' and name like 'jin%'; where約束
2.group by分組查詢
#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欄位進行歸類，比如針對員工資訊表的職位分組，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’這個字後面的欄位，就是我們分組的依據 #4、大前提： 可以按照任意欄位分組，但是分組完畢後，比如group by post，只能檢視post欄位，如果想檢視組內資訊，需要藉助於聚合函式
當執行以下sql語句的時候，是以post欄位查詢了組中的第一條資料，沒有任何意義，因為我們現在想查出當前組的多條記錄。
mysql> select * from employee group by post; +----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ | 14 | 張野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 | | 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 | | 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 | | 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 | +----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ rows in set (0.00 sec) #由於沒有設定ONLY_FULL_GROUP_BY,於是也可以有結果，預設都是組內的第一條記錄，但其實這是沒有意義的 如果想分組，則必須要設定全域性的sql的模式為ONLY_FULL_GROUP_BY mysql> set global sql_mode='ONLY_FULL_GROUP_BY'; Query OK, 0 rows affected (0.00 sec) #檢視MySQL 5.7預設的sql_mode如下： mysql> select @@global.sql_mode; +--------------------+ | @@global.sql_mode | +--------------------+ | ONLY_FULL_GROUP_BY | +--------------------+ row in set (0.00 sec) mysql> exit;#設定成功後，一定要退出，然後重新登入方可生效 Bye
繼續驗證通過group by分組之後，只能檢視當前欄位,如果想檢視組內資訊，需要藉助於聚合函式
mysql> select * from emp group by post;# 報錯 ERROR 1054 (42S22): Unknown column 'post' in 'group statement' mysql> select post from employee group by post; +-----------------------------------------+ | post | +-----------------------------------------+ | operation | | sale | | teacher | | 老男孩駐沙河辦事處外交大使 | +-----------------------------------------+ rows in set (0.00 sec)
3.聚合函式
max()求最大值 min()求最小值 avg()求平均值 sum() 求和 count() 求總個數 #強調：聚合函式聚合的是組的內容，若是沒有分組，則預設一組 # 每個部門有多少個員工 select post,count(id) from employee group by post; # 每個部門的最高薪水 select post,max(salary) from employee group by post; # 每個部門的最低薪水 select post,min(salary) from employee group by post; # 每個部門的平均薪水 select post,avg(salary) from employee group by post; # 每個部門的所有薪水 select post,sum(age) from employee group by post;
4.having過濾
having 與where的不同：
執行的優先順序從高到低依次是where > group by >havimg
where發生在group by 之前，where中沒有任何欄位，到但是絕對不能使用聚合函式
having 發生的group by之後，having可以使用分組欄位,無法直接取到其他的欄位,可以使用聚合函式
驗證:
驗證： mysql> select * from employee where salary>1000000; +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ | 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 | +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ row in set (0.00 sec) mysql> select * from employee having salary>1000000; ERROR 1463 (42000): Non-grouping field 'salary' is used in HAVING clause # 必須使用group by才能使用group_concat()函式，將所有的name值連線 mysql> select post,group_concat(name) from emp group by post having salary > 10000; ##錯誤，分組後無法直接取到salary欄位 ERROR 1054 (42S22): Unknown column 'post' in 'field list'
練習:
1. 查詢各崗位內包含的員工個數小於2的崗位名、崗位內包含員工名字、個數 2. 查詢各崗位平均薪資大於10000的崗位名、平均工資 3. 查詢各崗位平均薪資大於10000且小於20000的崗位名、平均工資
# 題1： mysql> select post,group_concat(name),count(id) from employee group by post; +-----------------------------------------+-----------------------------------------------------------+-----------+ | post | group_concat(name) | count(id) | +-----------------------------------------+-----------------------------------------------------------+-----------+ | operation | 程咬鐵,程咬銅,程咬銀,程咬金,張野 | 5 | | sale | 格格,星星,丁丁,丫丫,歪歪 | 5 | | teacher | xiaomage,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex | 7 | | 老男孩駐沙河辦事處外交大使 | egon | 1 | +-----------------------------------------+-----------------------------------------------------------+-----------+ rows in set (0.00 sec) 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 | +-----------------------------------------+--------------------+-----------+ row in set (0.00 sec) #題2： mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000; +-----------+---------------+ | post | avg(salary) | +-----------+---------------+ | operation | 16800.026000 | | teacher | 151842.901429 | +-----------+---------------+ rows in set (0.00 sec) #題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 | +-----------+--------------+ row in set (0.00 sec)
5.order by查詢排序
按單列排序 SELECT * FROM employee ORDER BY age; SELECT * FROM employee ORDER BY age ASC; SELECT * FROM employee ORDER BY age DESC; 按多列排序:先按照age升序排序，如果年紀相同，則按照id降序 SELECT * from employee ORDER BY age ASC, id DESC;
驗證多列排序： SELECT * from employee ORDER BY age ASC,id DESC; mysql> SELECT * from employee ORDER BY age ASC,id DESC; +----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ | 18 | 程咬鐵 | female | 18 | 2014-05-12 | operation | NULL | 17000.00 | 403 | 3 | | 17 | 程咬銅 | male | 18 | 2015-04-11 | operation | NULL | 18000.00 | 403 | 3 | | 16 | 程咬銀 | female | 18 | 2013-03-11 | operation | NULL | 19000.00 | 403 | 3 | | 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 | | 12 | 星星 | female | 18 | 2016-05-13 | sale | NULL | 3000.29 | 402 | 2 | | 11 | 丁丁 | female | 18 | 2011-03-12 | sale | NULL | 1000.37 | 402 | 2 | | 7 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 | | 6 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 | | 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 | | 14 | 張野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 | | 13 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 | | 5 | liwenzhou | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 | | 10 | 丫丫 | female | 38 | 2010-11-01 | sale | NULL | 2000.35 | 402 | 2 | | 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 | | 8 | xiaomage | male | 48 | 2010-11-11 | teacher | NULL | 10000.00 | 401 | 1 | | 4 | yuanhao | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 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 | +----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ rows in set (0.01 sec) mysql> 驗證多列排序
小練習：
1. 查詢所有員工資訊，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於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於10000的崗位名、平均工資,結果按平均薪資降序排列
# 題目1 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 | +-----------+---------------+ rows in set (0.00 sec) # 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 | +-----------+---------------+ rows in set (0.00 sec) mysql> 小練習答案
6.limit限制查詢的記錄數:
select * from employee order by salary desc limit 0,5;#每五個資料一列,第一頁
select * from employee order by salary desc limit 5,5;#每五個資料一列,第二頁
select * from employee order by salary desc limit 10,5;#每五個資料一列,第三頁