MySQL單表查詢

時間  2019-12-22
標籤 mysql 查詢 欄目 MySQL 简体版
原文   原文鏈接

單表查詢的語法及關鍵字執行的優先級

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

關鍵字執行的優先級

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from
where
group by
select
distinct
having
order by
limit

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1.找到表:frommysql

2.拿着where指定的約束條件，去文件/表中取出一條條記錄正則表達式

3.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sql

4.執行select（去重）windows

5.將分組的結果進行having過濾函數

6.將結果按條件排序：order bypost

7.限制結果的顯示條數編碼

簡單查詢

img

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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
('egon','male',18,'20170301','老男孩駐沙河辦事處外交大使',7300.33,401,1), #如下是教學部
('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),#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

準備表和記錄

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#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 = 'jingliyang' THEN
               emp_name
           WHEN emp_name = 'alex' THEN
               CONCAT(emp_name,'_BIGSB')
           ELSE
               concat(emp_name, 'SB')
           END
       ) as new_name
   FROM
       employee;

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小練習：code

1 查出全部員工的名字，薪資,格式爲
    <名字:egon>    <薪資:3000>
2 查出全部的崗位（去掉重複）
3 查出全部員工名字，以及他們的年薪,年薪的字段名爲annual_year

img

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;

where約束

where字句中能夠使用：regexp

\1. 比較運算符：> < >= <= <> !=
\2. between 80 and 100 值在80到100之間
\3. in(80,90,100) 值是80或90或100
\4. like 'e%'
通配符能夠是%或_，
%表示任意多字符
_表示一個字符
\5. 邏輯運算符：在多個條件直接能夠使用邏輯運算符 and or not排序

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#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 'eg%';

    通配符’_’
    SELECT * FROM employee 
            WHERE emp_name LIKE 'al__';

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小練習：

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1. 查看崗位是teacher的員工姓名、年齡
2. 查看崗位是teacher且年齡大於30歲的員工姓名、年齡
3. 查看崗位是teacher且薪資在9000-10000範圍內的員工姓名、年齡、薪資
4. 查看崗位描述不爲NULL的員工信息
5. 查看崗位是teacher且薪資是10000或9000或30000的員工姓名、年齡、薪資
6. 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
7. 查看崗位是teacher且名字是jin開頭的員工姓名、年薪

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img

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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 'jin%';

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group by

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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

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強調：

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

聚合函數

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#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

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;

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小練習：

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1. 查詢崗位名以及崗位包含的全部員工名字
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

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img

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#題1：分組
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                                 | 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 |
+--------+---------------+

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HAVING過濾

HAVING與WHERE不同的地方在於!!!!!!

#！！！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。
#2. 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

img

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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 | alex | 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,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex |
+-----------+-------------------------------------------------------+
rows in set (0.00 sec)

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小練習：

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
3. 查詢各崗位平均薪資大於10000的崗位名、平均工資
4. 查詢各崗位平均薪資大於10000且小於20000的崗位名、平均工資

img

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#題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 |
+-----------+--------------+

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ORDER BY 查詢排序

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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;

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小練習：

1. 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
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

img

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#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 |
+-----------+---------------+

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LIMIT 限制查詢的記錄數

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示例：
    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，即先查詢出第一條，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，即先查詢出第6條，而後包含這一條在內日後查5條

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小練習：

1. 分頁顯示，每頁5條

img

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mysql> select * from  employee limit 0,5;
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | emp_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 | emp_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 | 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)

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使用正則表達式查詢

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SELECT * FROM employee WHERE emp_name REGEXP '^ale';

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 = 'egon';
WHERE emp_name LIKE 'yua%';
WHERE emp_name REGEXP 'on$';

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小練習：

查看全部員工中名字是jin開頭，n或者g結果的員工信息

img

select * from employee where emp_name regexp '^jin.*[gn]$';
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