1. 程式人生 > >七、單表查詢

七、單表查詢

 

先建立表

#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','teacher',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), ('成龍','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) ;
1 1.注意:                                                                                        
2     select * from t1 where 條件 group by 分組欄位                                                  
3     1.分組只能查詢分組欄位,要想檢視其餘的利用聚合函式                                                               
4     2.聚合函式的分類:count,min,max,avg,group_concat,sum等。                                           
5     3.模糊匹配:用like關鍵字。                                                                         
6       select * from t1 where name like '%eg%'; #%表示任意字元                                      
7       select * from t1 where name like 'd__l'; #一個下劃線表示一個字元,兩個下劃線就表示兩個字元                     
8     4.拷貝表 :create table t2 select * from t1;                                                 
9                create table t2 select * from t1 where 1=2 ;                                  
一點小知識複習

 

一、查詢語法

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、簡單查詢

#簡單查詢
    SELECT id,name,sex,age,hire_date,post,post_comment,salary,office,depart_id 
    FROM employee;

    SELECT * FROM employee;

    SELECT name,salary FROM employee;

#避免重複DISTINCT
    SELECT DISTINCT post FROM employee;    

#通過四則運算查詢
    SELECT name, salary*12 FROM employee;
    SELECT name, salary*12 AS Annual_salary FROM employee;
    SELECT name, salary*12 Annual_salary FROM employee;

#定義顯示格式
   CONCAT() 函式用於連線字串
   SELECT CONCAT('姓名: ',name,'  年薪: ', salary*12)  AS Annual_salary 
   FROM employee;
   
   CONCAT_WS() 第一個引數為分隔符
   SELECT CONCAT_WS(':',name,salary*12)  AS Annual_salary 
   FROM employee;

小練習:

1 查出所有員工的名字,薪資,格式為
    <名字:egon>    <薪資:3000>
select concat('<名字:',name,'> '  ,'<薪資:',salary,'>' ) from employee;
2 查出所有的崗位(去掉重複)
select distinct depart_id from employee;
3 查出所有員工名字,以及他們的年薪,年薪的欄位名為年薪
select name,salary*12 年薪 from employee;

 

三、where約束

where字句中可以使用:

1. 比較運算子:> < >= <= <> !=
2. between 80 and 100 值在10到20之間
3. in(80,90,100) 值是80或90或100
4. like 'eg%'
    可以是%或_,
    %表示任意多字元
    _表示一個字元

 like 'e__n' :
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 

#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(判斷某個欄位是否為NULL不能用等號,需要用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''是空字串,不是null
    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
    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
    萬用字元’%’
    SELECT * FROM employee 
            WHERE name LIKE 'eg%';

    萬用字元’_’
    SELECT * FROM employee 
            WHERE name LIKE 'al__';

四、having過濾

having和where語法上是一樣的。

select * from employee where id>15;    
select * from employee having id>15;   

 

但是having和where不一樣的地方在於以下幾點!!!

#!!!執行優先順序從高到低:where > group by > 聚合函式 > having >order by
1.where和having的區別                                                                                
     1. Where 是一個約束宣告,使用Where約束來自資料庫的資料,Where是在結果返回之前起作用的                                        
     (先找到表,按照where的約束條件,從表(檔案)中取出資料),Where中不能使用聚合函式                                              
     2.Having是一個過濾宣告,是在查詢返回結果集以後對查詢結果進行的過濾操作                                                     
     (先找到表,按照where的約束條件,從表(檔案)中取出資料,然後group by分組,                                                
      如果沒有group by則所有記錄整體為一組,然後執行聚合函式,然後使用having對聚合的結果進行過濾),                                     
      在Having中可以使用聚合函式。                                                                          
     3.where的優先順序比having的優先順序高                                                                     
     4.having可以放到group by之後,而where只能放到group by 之前。                                               

驗證不同之處:

1.檢視員工的id>15的有多少個
select count(id) from employee where id>15;#正確,分析:where先執行,後執行聚合count(id),
                                            然後select出結果
select count(id) from employee having id>15; #報錯,分析:先執行聚合count(id),後執行having過濾,
                                            #無法對id進行id>15的過濾
#以上兩條sql的順序是
1:找到表employee--->用where過濾---->沒有分組則預設一組執行聚合count(id)--->select執行檢視組內id數目
2:找到表employee--->沒有分組則預設一組執行聚合count(id)---->having 基於上一步聚合的結果(此時只有count(id)欄位了)
進行id>15的過濾,很明顯,根本無法獲取到id欄位
1 ------having-----------
2 select depart_id,count(id) from employee group by depart_id;
3 select depart_id,count(id) from employee group by depart_id having depart_id = 3;
4 select depart_id,count(id) from employee group by depart_id having count(id)>7;
5 select max(salary) 最大工資 from employee where id>2 group by depart_id having count(id)>3;
6 select * from employee where id>7; #檢視所有id>7的員工資訊
having 舉例

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
select post,group_concat(name) 員工姓名,count(id) 個數 from employee group by post having count(id)<2;
2. 查詢各崗位平均薪資大於10000的崗位名、平均工資
select post,avg(salary) from employee group by post having avg(salary)>10000;
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
select post,avg(salary)  from employee group by post having  avg(salary) between 10000 and 20000;

五、分組查詢 group by

大前提:可以按照任意欄位分組,但分完組後,只能檢視分組的那個欄位,要想取的組內的其他欄位資訊,需要藉助函式

單獨使用GROUP BY關鍵字分組
    select post from employee group by post;
    注意:我們按照post欄位分組,那麼select查詢的欄位只能是post,想要獲取組內的其他相關資訊,需要藉助函式

GROUP BY關鍵字和group_concat()函式一起使用
      select post,group_concat(name) from  employee group by post;#按照崗位分組,並檢視組內成員名
      select  post,group_concat(name) as emp_members FROM employee group by post;

GROUP BY與聚合函式一起使用
    select post,count(id) as count from employee group by post;#按照崗位分組,並檢視每個組有多少人

強調:

分組:一般相同的多的話就可以分成一組(一定是有重複的欄位)
小練習:
1. 查詢崗位名以及崗位包含的所有員工名字
select post,group_concat(name) from employee group by post;
2. 查詢崗位名以及各崗位內包含的員工個數 select post,count(id) from employee group by post;
3. 查詢公司內男員工和女員工的個數 select sex,count(id) from employee group by sex;
4. 查詢崗位名以及各崗位的平均薪資 select post,max(salary) from employee group by post;
5. 查詢崗位名以及各崗位的最高薪資 select post,max(salary) from employee group by post;
6. 查詢崗位名以及各崗位的最低薪資 select post,min(salary) from employee group by post;
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 select sex,avg(salary) from employee group by sex;

六、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.如果有聚合函式,則將組進行聚合

5.將4的結果過濾:having

6.查出結果:select

7.去重

8.將6的結果按條件排序:order by

9.將7的結果限制顯示條數

七、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,如果年紀相同,則按照薪資排序
    SELECT * from employee
        ORDER BY age,
        salary DESC;


===========order by========== 1.select * from employee order by salary;#如果不指定,預設就是升序 2.select * from employee order by salary asc; 3.select * from employee order by salary desc; #先按照年齡升序,當年齡相同的太多,分不清大小時,在按照工資降序 4.select * from employee order by age asc, salary desc;

小例子:

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
select * form employee order by age,hire_date desc;
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
select post ,avg(salary) from employee group by post having avg(salary)>10000;
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
select post ,avg(salary) from employee group by post having avg(salary)>10000 desc;

八、使用聚合函式查詢

先from找到表

再用where的條件約束去表中取出記錄

然後進行分組group by,沒有分組則預設一組

然後進行聚合

最後select出結果

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) form employee WHERE depart_id=3;

九、where的補充(使用正則表示式查詢)

1.select * from employee where name regexp '^ale';  #匹配以ale開頭的員工資訊
2.select * from employee where name regexp 'on$'; #匹配以on結尾的員工資訊
3.select * from employee where name regexp 'n{1,2}'; #匹配name裡面包含1到2個n的員工資訊
小結:對字串匹配的方式
where name = 'egon';
where name like 'yua%';
where name regexp 'on$';

小練習:

檢視所有員工中名字是jin開頭,n或者g結果的員工資訊
select * from employee where name regexp '^jin.*[ng]$';

十、限制查詢的記錄數:LIMIT

=========limit:限制列印幾條=========
1.select * from employee limit 3;#列印前三條
2.像這樣表示的:指的是從哪開始,往後取幾條 (這樣的操作一般用來分頁)
select * from employee limit 0,3;
select * from employee limit 3,4;
select * from employee limit 6,3;
select * from employee limit 9,3;
3.select * from employee order by id desc limit 3; #檢視後三條

小練習

1. 分頁顯示,每頁5條
select * from employee limit 0,5;
select * from employee limit 5,5;
select * from employee limit 10,5;

 

     

先建立表

#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','teacher',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),
('成龍','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)
;
1 1.注意:                                                                                        
2     select * from t1 where 條件 group by 分組欄位                                                  
3     1.分組只能查詢分組欄位,要想檢視其餘的利用聚合函式                                                               
4     2.聚合函式的分類:count,min,max,avg,group_concat,sum等。                                           
5     3.模糊匹配:用like關鍵字。                                                                         
6       select * from t1 where name like '%eg%'; #%表示任意字元                                      
7       select * from t1 where name like 'd__l'; #一個下劃線表示一個字元,兩個下劃線就表示兩個字元                     
8     4.拷貝表 :create table t2 select * from t1;                                                 
9                create table t2 select * from t1 where 1=2 ;                                  
一點小知識複習

 

一、查詢語法

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、簡單查詢

#簡單查詢
    SELECT id,name,sex,age,hire_date,post,post_comment,salary,office,depart_id 
    FROM employee;

    SELECT * FROM employee;

    SELECT name,salary FROM employee;

#避免重複DISTINCT
    SELECT DISTINCT post FROM employee;    

#通過四則運算查詢
    SELECT name, salary*12 FROM employee;
    SELECT name, salary*12 AS Annual_salary FROM employee;
    SELECT name, salary*12 Annual_salary FROM employee;

#定義顯示格式
   CONCAT() 函式用於連線字串
   SELECT CONCAT('姓名: ',name,'  年薪: ', salary*12)  AS Annual_salary 
   FROM employee;
   
   CONCAT_WS() 第一個引數為分隔符
   SELECT CONCAT_WS(':',name,salary*12)  AS Annual_salary 
   FROM employee;

小練習:

1 查出所有員工的名字,薪資,格式為
    <名字:egon>    <薪資:3000>
select concat('<名字:',name,'> '  ,'<薪資:',salary,'>' ) from employee;
2 查出所有的崗位(去掉重複)
select distinct depart_id from employee;
3 查出所有員工名字,以及他們的年薪,年薪的欄位名為年薪
select name,salary*12 年薪 from employee;

 

三、where約束

where字句中可以使用:

1. 比較運算子:> < >= <= <> !=
2. between 80 and 100 值在10到20之間
3. in(80,90,100) 值是80或90或100
4. like 'eg%'
    可以是%或_,
    %表示任意多字元
    _表示一個字元

 like 'e__n' :
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 

#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(判斷某個欄位是否為NULL不能用等號,需要用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''是空字串,不是null
    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
    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
    萬用字元’%’
    SELECT * FROM employee 
            WHERE name LIKE 'eg%';

    萬用字元’_’
    SELECT * FROM employee 
            WHERE name LIKE 'al__';

四、having過濾

having和where語法上是一樣的。

select * from employee where id>15;    
select * from employee having id>15;   

 

但是having和where不一樣的地方在於以下幾點!!!

#!!!執行優先順序從高到低:where > group by > 聚合函式 > having >order by
1.where和having的區別                                                                                
     1. Where 是一個約束宣告,使用Where約束來自資料庫的資料,Where是在結果返回之前起作用的                                        
     (先找到表,按照where的約束條件,從表(檔案)中取出資料),Where中不能使用聚合函式                                              
     2.Having是一個過濾宣告,是在查詢返回結果集以後對查詢結果進行的過濾操作                                                     
     (先找到表,按照where的約束條件,從表(檔案)中取出資料,然後group by分組,                                                
      如果沒有group by則所有記錄整體為一組,然後執行聚合函式,然後使用having對聚合的結果進行過濾),                                     
      在Having中可以使用聚合函式。                                                                          
     3.where的優先順序比having的優先順序高                                                                     
     4.having可以放到group by之後,而where只能放到group by 之前。                                               

驗證不同之處:

1.檢視員工的id>15的有多少個
select count(id) from employee where id>15;#正確,分析:where先執行,後執行聚合count(id),
                                            然後select出結果
select count(id) from employee having id>15; #報錯,分析:先執行聚合count(id),後執行having過濾,
                                            #無法對id進行id>15的過濾
#以上兩條sql的順序是
1:找到表employee--->用where過濾---->沒有分組則預設一組執行聚合count(id)--->select執行檢視組內id數目
2:找到表employee--->沒有分組則預設一組執行聚合count(id)---->having 基於上一步聚合的結果(此時只有count(id)欄位了)
進行id>15的過濾,很明顯,根本無法獲取到id欄位
1 ------having-----------
2 select depart_id,count(id) from employee group by depart_id;
3 select depart_id,count(id) from employee group by depart_id having depart_id = 3;
4 select depart_id,count(id) from employee group by depart_id having count(id)>7;
5 select max(salary) 最大工資 from employee where id>2 group by depart_id having count(id)>3;
6 select * from employee where id>7; #檢視所有id>7的員工資訊
having 舉例

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
select post,group_concat(name) 員工姓名,count(id) 個數 from employee group by post having count(id)<2;
2. 查詢各崗位平均薪資大於10000的崗位名、平均工資
select post,avg(salary) from employee group by post having avg(salary)>10000;
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
select post,avg(salary)  from employee group by post having  avg(salary) between 10000 and 20000;

五、分組查詢 group by

大前提:可以按照任意欄位分組,但分完組後,只能檢視分組的那個欄位,要想取的組內的其他欄位資訊,需要藉助函式

單獨使用GROUP BY關鍵字分組
    select post from employee group by post;
    注意:我們按照post欄位分組,那麼select查詢的欄位只能是post,想要獲取組內的其他相關資訊,需要藉助函式

GROUP BY關鍵字和group_concat()函式一起使用
      select post,group_concat(name) from  employee group by post;#按照崗位分組,並檢視組內成員名
      select  post,group_concat(name) as emp_members FROM employee group by post;

GROUP BY與聚合函式一起使用
    select post,count(id) as count from employee group by post;#按照崗位分組,並檢視每個組有多少人

強調:

分組:一般相同的多的話就可以分成一組(一定是有重複的欄位)
小練習:
1. 查詢崗位名以及崗位包含的所有員工名字
select post,group_concat(name) from employee group by post;
2. 查詢崗位名以及各崗位內包含的員工個數 select post,count(id) from employee group by post;
3. 查詢公司內男員工和女員工的個數 select sex,count(id) from employee group by sex;
4. 查詢崗位名以及各崗位的平均薪資 select post,max(salary) from employee group by post;
5. 查詢崗位名以及各崗位的最高薪資 select post,max(salary) from employee group by post;
6. 查詢崗位名以及各崗位的最低薪資 select post,min(salary) from employee group by post;
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 select sex,avg(salary) from employee group by sex;

六、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.如果有聚合函式,則將組進行聚合

5.將4的結果過濾:having

6.查出結果:select

7.去重

8.將6的結果按條件排序:order by

9.將7的結果限制顯示條數

七、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,如果年紀相同,則按照薪資排序
    SELECT * from employee
        ORDER BY age,
        salary DESC;


===========order by========== 1.select * from employee order by salary;#如果不指定,預設就是升序 2.select * from employee order by salary asc; 3.select * from employee order by salary desc; #先按照年齡升序,當年齡相同的太多,分不清大小時,在按照工資降序 4.select * from employee order by age asc, salary desc;

小例子:

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
select * form employee order by age,hire_date desc;
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
select post ,avg(salary) from employee group by post having avg(salary)>10000;
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
select post ,avg(salary) from employee group by post having avg(salary)>10000 desc;

八、使用聚合函式查詢

先from找到表

再用where的條件約束去表中取出記錄

然後進行分組group by,沒有分組則預設一組

然後進行聚合

最後select出結果

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) form employee WHERE depart_id=3;

九、where的補充(使用正則表示式查詢)

1.select * from employee where name regexp '^ale';  #匹配以ale開頭的員工資訊
2.select * from employee where name regexp 'on$'; #匹配以on結尾的員工資訊
3.select * from employee where name regexp 'n{1,2}'; #匹配name裡面包含1到2個n的員工資訊
小結:對字串匹配的方式
where name = 'egon';
where name like 'yua%';
where name regexp 'on$';

小練習:

檢視所有員工中名字是jin開頭,n或者g結果的員工資訊
select * from employee where name regexp '^jin.*[ng]$';

十、限制查詢的記錄數:LIMIT

=========limit:限制列印幾條=========
1.select * from employee limit 3;#列印前三條
2.像這樣表示的:指的是從哪開始,往後取幾條 (這樣的操作一般用來分頁)
select * from employee limit 0,3;
select * from employee limit 3,4;
select * from employee limit 6,3;
select * from employee limit 9,3;
3.select * from employee order by id desc limit 3; #檢視後三條

小練習

1. 分頁顯示,每頁5條
select * from employee limit 0,5;
select * from employee limit 5,5;
select * from employee limit 10,5;