15-5 單表查詢

時間 2019-11-29

標籤查詢简体版

原文原文鏈接

 一 查詢語法介紹（單表查詢）
1 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
              

2 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從上到下
from
where
group by
having
select
distinct
order by
limit
說明：
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
例子：

表結構：
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,
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 |
| 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), #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)
;

View Code

三種簡單的查詢：

　　#簡單查尋：
　　select * from employee;
 select name,salary from employee;
 #避免重複
 mysql> select distinct post from employee;
 #經過四則運算查詢
 SELECT name, salary*12 AS Annual_salary FROM employee; #查詢到的薪資乘以12，as是重命名
 +------------+---------------+
| name       | Annual_salary |
+------------+---------------+
| egon       |      87603.96 |
| alex       |   12000003.72 |
| wupeiqi    |      99600.00 |
| yuanhao    |      42000.00 |
| liwenzhou  |      25200.00 |
| jingliyang |     108000.00 |
| jinxin     |     360000.00 |
| 成龍       |     120000.00 |
| 歪歪       |      36001.56 |
| 丫丫       |      24004.20 |
| 丁丁       |      12004.44 |
| 星星       |      36003.48 |
| 格格       |      48003.96 |
| 張野       |     120001.56 |
| 程咬金     |     240000.00 |
| 程咬銀     |     228000.00 |
| 程咬銅     |     216000.00 |
| 程咬鐵     |     204000.00 |
+------------+---------------+



#查詢的時候哈能夠定義顯示格式
   CONCAT() 函數用於鏈接字符串
   
例子：

mysql> SELECT CONCAT('姓名: ',name,'  年薪: ', salary*12)  AS Annual_salary
    ->    FROM employee;
+---------------------------------------+
| Annual_salary                         |
+---------------------------------------+
| 姓名: egon  年薪: 87603.96            |
| 姓名: alex  年薪: 12000003.72         |
| 姓名: wupeiqi  年薪: 99600.00         |
| 姓名: yuanhao  年薪: 42000.00         |
| 姓名: liwenzhou  年薪: 25200.00       |
| 姓名: jingliyang  年薪: 108000.00     |
| 姓名: jinxin  年薪: 360000.00         |
| 姓名: 成龍  年薪: 120000.00           |
| 姓名: 歪歪  年薪: 36001.56            |
| 姓名: 丫丫  年薪: 24004.20            |
| 姓名: 丁丁  年薪: 12004.44            |
| 姓名: 星星  年薪: 36003.48            |
| 姓名: 格格  年薪: 48003.96            |
| 姓名: 張野  年薪: 120001.56           |
| 姓名: 程咬金  年薪: 240000.00         |
| 姓名: 程咬銀  年薪: 228000.00         |
| 姓名: 程咬銅  年薪: 216000.00         |
| 姓名: 程咬鐵  年薪: 204000.00         |
+---------------------------------------+
18 rows in set (0.32 sec)



二 where條件約束
where字句中可使用：

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：在多個條件直接可使用邏輯運算符 and or not

#1:單條件查詢
    SELECT name FROM employee
        WHERE post='sale';
        
#2:多條件查詢

SELECT name,salary FROM employee
        WHERE post='teacher' AND salary>10000;



#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;


    
#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 name,post_comment FROM employee 
        WHERE post_comment IS NULL;

    SELECT name,post_comment FROM employee 
        WHERE post_comment IS NOT NULL;
        
    SELECT 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 name,salary FROM employee 
        WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
    
    SELECT name,salary FROM employee 
        WHERE salary IN (3000,3500,4000,9000) ;

    SELECT 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 name LIKE 'eg%';

    通配符’_’#雙下劃線一個_表明一個字符，兩個表明兩個字符
例子：

  mysql> select * from emp where name like '__';
+----+--------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| id | name   | sex    | age | hire_date  | post      | post_comment | salary   | office | depart_id |
+----+--------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
|  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 |
| 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 |
+----+--------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
7 rows in set (0.00 sec)


         
         
三 分組查詢:GROUP BY
#一、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

#二、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

若是想分組，則必需要設置全局的sql的模式爲ONLY_FULL_GROUP_BY

 1 mysql> set global sql_mode='ONLY_FULL_GROUP_BY';
 2 
 3 #查看MySQL 5.7默認的sql_mode以下：
 4 mysql> select @@global.sql_mode;
 5 +--------------------+
 6 | @@global.sql_mode  |
 7 +--------------------+
 8 | ONLY_FULL_GROUP_BY |
 9 +--------------------+
10 row in set (0.00 sec)
11 
12 mysql> exit;#設置成功後，必定要退出，而後從新登陸方可生效



而後查詢：使用分組必須和聚合函數一塊兒使用

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 |
+-----------------------------------------+-----------+
4 rows in set (0.34 sec)
GROUP BY關鍵字和GROUP_CONCAT()函數一塊兒使用

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



四 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

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;
   
五 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中可使用分組的字段，沒法直接取到其餘字段,可使用聚合函數
例子：

mysql> select post,avg(salary) from emp group by post having avg(salary)> 10000;
+-----------+---------------+
| post      | avg(salary)   |
+-----------+---------------+
| operation |  16800.026000 |
| teacher   | 151842.901429 |
+-----------+---------------+
2 rows in set (0.27 sec)


mysql> select post,avg(salary) from emp group by post having salary > 10000;
ERROR 1054 (42S22): Unknown column 'salary' in 'having clause'

分組後沒法直接取到salary字段


  
例子3：

mysql> select post,group_concat(name) from emp group by post having avg(salary) > 10000;
+-----------+---------------------------------------------------------+
| post      | group_concat(name)                                      |
+-----------+---------------------------------------------------------+
| operation | 張野,程咬金,程咬銀,程咬銅,程咬鐵                        |
| teacher   | alex,wupeiqi,yuanhao,liwenzhou,jingliyang,jinxin,成龍   |
+-----------+---------------------------------------------------------+
2 rows in set (0.00 sec)

六 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;      七 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條八 使用正則表達式查詢SELECT * FROM employee WHERE name REGEXP '^ale';SELECT * FROM employee WHERE name REGEXP 'on$';SELECT * FROM employee WHERE name REGEXP 'm{2}';小結：對字符串匹配的方式WHERE name = 'egon';WHERE name LIKE 'yua%';WHERE name REGEXP 'on$';例子：查看全部員工中名字是jin開頭，n或者g結果的員工信息select * from employee where name regexp '^jin.*[gn]$';

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