MySQL單表查詢
目錄mysql
1、單表查詢的語法及關鍵字執行的優先級
1.1 單表查詢語法
SELECT DISTINCT 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
1.2 關鍵字執行的優先級
- from：找到表
- where：拿着where指定的約束條件，去文件/表中取出一條條記錄
- group by：將取出的一條條記錄進行分組group by，若是沒有group by，則總體做爲一組
- select：執行select
- distinct：去重
- having：將分組的結果進行having過濾
- order by：將結果按條件排序：order by
- limit：限制結果的顯示條數
2、簡單查詢
2.1 建表和數據準備
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
# 建立表
create table employee(
id int not null unique auto_increment,
emp_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 | |
| 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(emp_name,sex,age,hire_date,post,salary,office,depart_id) values
('nick','male',18,'20170301','老男孩駐上海虹橋最帥',7300.33,401,1), # 如下是教學部
('jason','male',78,'20150302','teacher',1000000.31,401,1),
('sean','male',81,'20130305','teacher',8300,401,1),
('tank','male',73,'20140701','teacher',3500,401,1),
('oscar','male',28,'20121101','teacher',2100,401,1),
('mac','female',18,'20110211','teacher',9000,401,1),
('rocky','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), # 如下是銷售部門
('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), # 如下是運營部門
('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：若是在windows系統中，插入中文字符，select的結果爲空白，能夠將全部字符編碼統一設置成gbk
# 簡單查詢
SELECT id,emp_name,sex,age,hire_date,post,post_comment,salary,office,depart_id
FROM employee;
SELECT * FROM employee;
SELECT emp_name,salary FROM employee;
# 避免重複DISTINCT
SELECT DISTINCT post FROM employee;
# 經過四則運算查詢
SELECT emp_name, salary*12 FROM employee;
SELECT emp_name, salary*12 AS Annual_salary FROM employee;
SELECT emp_name, salary*12 Annual_salary FROM employee;
# 定義顯示格式
CONCAT() 函數用於鏈接字符串
SELECT CONCAT('姓名: ',emp_name,' 年薪: ', salary*12) AS Annual_salary
FROM employee;
CONCAT_WS() 第一個參數爲分隔符
SELECT CONCAT_WS(':',emp_name,salary*12) AS Annual_salary
FROM employee;
結合CASE語句：
SELECT
(
CASE
WHEN emp_name = 'mac' THEN
emp_name
WHEN emp_name = 'jason' THEN
CONCAT(emp_name,'_BIGSB')
ELSE
concat(emp_name, 'SB')
END
) as new_name
FROM
employee;
2.2 練習
- 查出全部員工的名字，薪資，格式爲
<名字:nick> <薪資:3000> - 查出全部的崗位（去掉重複）
- 查出全部員工名字，以及他們的年薪，年薪的字段名爲annual_year
select concat('<名字:',emp_name,'> ','<薪資:',salary,'>') from employee;
select distinct depart_id from employee;
select emp_name,salary*12 annual_salary from employee;
3、約束條件（where）
where子句中能夠使用：正則表達式
- 比較運算符：> < >= <= <> !=
- between 80 and 100 值在80到100之間
- in(80,90,100) 值是80或90或100
- like 'n%'
- 通配符能夠是%或_，
- %表示任意多字符
- _表示一個字符
- 通配符能夠是%或_，
- 邏輯運算符：在多個條件直接能夠使用邏輯運算符 and or not
1. 單條件查詢
SELECT emp_name FROM employee
WHERE post='sale';
2. 多條件查詢
SELECT emp_name,salary FROM employee
WHERE post='teacher' AND salary>10000;
3. 關鍵字BETWEEN AND
SELECT emp_name,salary FROM employee
WHERE salary BETWEEN 10000 AND 20000;
SELECT emp_name,salary FROM employee
WHERE salary NOT BETWEEN 10000 AND 20000;
4. 關鍵字IS NULL(判斷某個字段是否爲NULL不能用等號，須要用IS)
SELECT emp_name,post_comment FROM employee
WHERE post_comment IS NULL;
SELECT emp_name,post_comment FROM employee
WHERE post_comment IS NOT NULL;
SELECT emp_name,post_comment FROM employee
WHERE post_comment=''; 注意''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 emp_name,salary FROM employee
WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
SELECT emp_name,salary FROM employee
WHERE salary IN (3000,3500,4000,9000) ;
SELECT emp_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 emp_name LIKE 'ni%';
通配符’_’
SELECT * FROM employee
WHERE emp_name LIKE 'ja__';
3.1 練習
- 查看崗位是teacher的員工姓名、年齡
- 查看崗位是teacher且年齡大於30歲的員工姓名、年齡
- 查看崗位是teacher且薪資在9000-1000範圍內的員工姓名、年齡、薪資
- 查看崗位描述不爲NULL的員工信息
- 查看崗位是teacher且薪資是10000或9000或30000的員工姓名、年齡、薪資
- 查看崗位是teacher且薪資不是10000或9000或30000的員工姓名、年齡、薪資
- 查看崗位是teacher且名字是jin開頭的員工姓名、年薪
select emp_name,age from employee where post = 'teacher'; select emp_name,age from employee where post='teacher' and age > 30; select emp_name,age,salary from employee where post='teacher' and salary between 9000 and 10000; select * from employee where post_comment is not null; select emp_name,age,salary from employee where post='teacher' and salary in (10000,9000,30000); select emp_name,age,salary from employee where post='teacher' and salary not in (10000,9000,30000); select emp_name,salary*12 from employee where post='teacher' and emp_name like 'mac%';
4、分組（group by）
單獨使用GROUP BY關鍵字分組
SELECT post FROM employee GROUP BY post;
注意：咱們按照post字段分組，那麼select查詢的字段只能是post，想要獲取組內的其餘相關信息，須要藉助函數
GROUP BY關鍵字和GROUP_CONCAT()函數一塊兒使用
SELECT post,GROUP_CONCAT(emp_name) FROM employee GROUP BY post; # 按照崗位分組，並查看組內成員名
SELECT post,GROUP_CONCAT(emp_name) as emp_members FROM employee GROUP BY post;
GROUP BY與聚合函數一塊兒使用
select post,count(id) as count from employee group by post; # 按照崗位分組，並查看每一個組有多少人
注意：若是咱們用unique的字段做爲分組的依據，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，該字段一般用來做爲分組的依據。sql
5、聚合函數
強調：聚合函數聚合的是組的內容，如果沒有分組，則默認一組。windows
示例：函數
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;
5.1 練習
- 查詢崗位名以及崗位包含的全部員工名字
- 查詢崗位名以及各崗位內包含的員工個數
- 查詢公司內男員工和女員工的個數
- 查詢崗位名以及各崗位的平均薪資
- 查詢崗位名以及各崗位的最高薪資
- 查詢崗位名以及各崗位的最低薪資
- 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
題目1 mysql> select post,group_concat(emp_name) from employee group by post; +-----------------------------------------+---------------------------------------------------------+ | post | group_concat(emp_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 | jason,sean,tank,oscar,mac,rocky,成龍 | | 老男孩駐上海虹橋最帥 | nick | +-----------------------------------------+---------------------------------------------------------+ 題目2 mysql> select post,count(id) from employee group by post; +-----------------------------------------+-----------+ | post | count(id) | +-----------------------------------------+-----------+ | operation | 5 | | sale | 5 | | teacher | 7 | | 老男孩駐上海虹橋最帥 | 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 | | 老男孩駐上海虹橋最帥 | 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 | | 老男孩駐上海虹橋最帥 | 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 | | 老男孩駐上海虹橋最帥 | 7300.33 | +-----------------------------------------+-------------+ 題目7 mysql> select sex,avg(salary) from employee group by sex; +--------+---------------+ | sex | avg(salary) | +--------+---------------+ | male | 110920.077000 | | female | 7250.183750 | +--------+---------------+
6、過濾（having）
6.1 where和having的區別
執行優先級從高到低：where > group by > havingpost
- Where 發生在分組group by以前，於是Where中能夠有任意字段，可是絕對不能使用聚合函數。
- Having發生在分組group by以後，於是Having中能夠使用分組的字段，沒法直接取到其餘字段,能夠使用聚合函數
6.1.1 驗證
mysql> select @@sql_mode; +--------------------+ | @@sql_mode | +--------------------+ | ONLY_FULL_GROUP_BY | +--------------------+ row in set (0.00 sec) mysql> select * from emp where salary > 100000; +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ | id | emp_name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ | 2 | jason | male | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 | +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ row in set (0.00 sec) mysql> select post,group_concat(emp_name) from emp group by post having salary > 10000;#錯誤，分組後沒法直接取到salary字段 ERROR 1054 (42S22): Unknown column 'salary' in 'having clause' mysql> select post,group_concat(emp_name) from emp group by post having avg(salary) > 10000; +-----------+-------------------------------------------------------+ | post | group_concat(emp_name) | +-----------+-------------------------------------------------------+ | operation | 程咬鐵,程咬銅,程咬銀,程咬金,張野 | | teacher | 成龍,rocky,mac,oscar,tank,sean,jason | +-----------+-------------------------------------------------------+ rows in set (0.00 sec)
6.2 練習
- 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
- 查詢各崗位平均薪資大於10000的崗位名、平均工資
- 查詢各崗位平均薪資大於10000且小於20000的崗位名、平均工資
題目1 mysql> select post,group_concat(emp_name),count(id) from employee group by post having count(id) < 2; +-----------------------------------------+--------------------+-----------+ | post | group_concat(emp_name) | count(id) | +-----------------------------------------+--------------------+-----------+ | 老男孩駐上海虹橋最帥 | nick | 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 | +-----------+--------------+
7、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，若是年紀相同，則按照薪資排序
SELECT * from employee
ORDER BY age,
salary DESC;
7.1 練習
- 查詢全部員工信息，先按照age升序排序，若是age相同則按照hire_date降序排序
- 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
- 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 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 | +-----------+---------------+
8、限制查詢的記錄數（limit）
示例：編碼
SELECT * FROM employee ORDER BY salary DESC
LIMIT 3; #默認初始位置爲0
SELECT * FROM employee ORDER BY salary DESC
LIMIT 0,5; #從第0開始，即先出第一條，而後包含這一條在內日後查5條
SELECT * FROM employee ORDER BY salary DESC
LIMIT 5,5; #從第5開始，即先出第6條，而後包含這一條在內日後查5條
8.1 練習
- 分頁顯示，每頁5條
mysql> select * from employee limit 0,5; +----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ | id | emp_name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+ | 1 | nick | male | 18 | 2017-03-01 | 老男孩駐上海虹橋最帥 | NULL | 7300.33 | 401 | 1 | | 2 | jason | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 | | 3 | sean | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 | | 4 | tank | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 | | 5 | oscar | 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 | emp_name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+ | 6 | mac | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 | | 7 | rocky | 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 | emp_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)
9、使用正則表達式查詢
SELECT * FROM employee WHERE emp_name REGEXP '^jas';
SELECT * FROM employee WHERE emp_name REGEXP 'on$';
SELECT * FROM employee WHERE emp_name REGEXP 'm{2}';
小結：對字符串匹配的方式
WHERE emp_name = 'nick';
WHERE emp_name LIKE 'sea%';
WHERE emp_name REGEXP 'on$';
9.1 練習
- 查看全部員工中名字是mac開頭，n或者g結果的員工信息
select * from employee where emp_name regexp '^mac.*[gn]$';
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