單表查詢

時間 2019-12-14

標籤查詢简体版

原文原文鏈接

語法 : mysql

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)


重點中重點 : 關鍵字的執行優先級 from
where
group by
having
select
distinct
order by
limit


1, 找到表: from 2, 拿着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, 若是沒有group by, 則總體做爲一組 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，表的字段和數據類型sql

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,設置字段的約束條件
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
);

# 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    |                |
| salart               | double(15,2)                  | YES  |         | NULL    |                |
| office              | int(11)                           | YES  |         | NULL    |                |
| depart_id        | int(11)                           | YES  |         | NULL    |                |
+--------------+-----------------------+------+-----+---------+----------------+
10 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), #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),
('xiaomage','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)
;

建立員工表，並插入記錄

(1) where 約束

where 句子中可使用
1.比較運算符：>、<、>=、<=、<>、!=

2. between 80 and 100  :  表示  值在80 到100 之間

3. in(80,90,100) 值是80或90或100

4. like 'heshunoattern'  : pattern  能夠是%或者_ , %小時任意多字符 , _表示一個字符

5. 邏輯運算符 : 在多個條件直接可使用邏輯運算符  and  or  not

驗證結果 : ide

#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''是空字符串，不是null
 SELECT name,post_comment FROM employee WHERE post_comment='';
 ps：
        執行
        update employee set post_comment='' where id=2;
        再用上條查看，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 |
+------------+---------+
2 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 |
+-----------+------------+
16 rows in set (0.00 sec)

#6:關鍵字LIKE模糊查詢
通配符’%’
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 |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
2 rows in set (0.00 sec)


通配符'_'

mysql> SELECT  age FROM employee WHERE name LIKE 'ale_';
+-----+
| age |
+-----+
|  78 |
+-----+
1 row in set (0.00 sec)

練習：
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9000-1000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

#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以後, 即分組是基於where 以後獲得的記錄而進行的

# 2, 分組指的是 : 將全部記錄按照某個相同字段進行歸類 , 好比針對員工信息表的職位分組 , 或者按照性別進行分組等

#3 , 爲什麼要分組呢 ? 
    取每一個部門的最高工資
    取每一個部門的員工數
    取每一個部門的女人數

小竅門 :    "每"  這個字後面的字段 , 就是咱們分組的依據 


# 4, 大前提 : 
    能夠 按照任意字段分組 , 可是分組完畢後 , 好比 group by post , 只能查看post字段 , 若是想查看組內信息 , 須要藉助於聚合函數

當執行如下sql語句的時候，是以post字段查詢了組中的第一條數據，沒有任何意義，由於咱們如今想查出當前組的多條記錄。函數

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 |
+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
4 rows in set (0.00 sec)

#因爲沒有設置ONLY_FULL_GROUP_BY,因而也能夠有結果，默認都是組內的第一條記錄，但其實這是沒有意義的
若是想分組，則必需要設置全局的sql的模式爲ONLY_FULL_GROUP_BY
mysql> set global sql_mode='ONLY_FULL_GROUP_BY';
Query OK, 0 rows affected (0.00 sec)

#查看MySQL 5.7默認的sql_mode以下：
mysql> select @@global.sql_mode;
+--------------------+
| @@global.sql_mode  |
+--------------------+
| ONLY_FULL_GROUP_BY |
+--------------------+
1 row in set (0.00 sec)

mysql> exit;#設置成功後，必定要退出，而後從新登陸方可生效
Bye

繼續驗證經過group by分組以後，只能查看當前字段,若是想查看組內信息，須要藉助於聚合函數post

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              |
+-----------------------------------------+
4 rows in set (0.00 sec)

（3）聚合函數

max()  求最大值

min()   求最小值

avg()   求平均數

sum()   求和

count()   求總數和


#  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 
#  每一個部門有多少個員工 
select post,count(id) from employee group by post;

# 每一個部門的最高薪水
select post,max(salary) from employee group by post;

# 每一個部門的最低薪水
select post,min(salary) from employee group by post;

# 每一個部門的平均薪水
select post,avg(salary) from employee group by post;

# 每一個部門的全部薪水
select post,sum(salary) from employee group by post;

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

小練習

(4) 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 中可使用分租的字段 , 沒法直接取到其餘字段 , 可使用聚合函數

驗證：spa

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 |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
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()函數，將全部的name值鏈接
mysql> select post,group_concat(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 'post' in 'field list'

小練習：3d

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

小練習答案：code

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 |
+-----------------------------------------+-----------------------------------------------------------+-----------+
4 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 |
+-----------------------------------------+--------------------+-----------+
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 |
+-----------+---------------+
2 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 |
+-----------+--------------+
1 row in set (0.00 sec)

小練習答案

（5）order by 查詢排序blog

按單列排序 
    select * from employee order by age;
    select * from employee order by age asc;
    select * from emplotee order by age desc;

按多列排序:先按照age升序排序，若是年紀相同，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 |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
18 rows in set (0.01 sec)

驗證多排序

小練習：排序

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

# 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 |
+-----------+---------------+
2 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 |
+-----------+---------------+
2 rows in set (0.00 sec)

小練習答案

（5）limit 限制查詢的記錄數:

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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每一个你不满意的现在，都有一个你没有努力的曾经。