單表查詢

單表查詢的語法及關鍵字執行的優先順序

單表查詢語法

SELECT DISTINCT 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

關鍵字執行的優先順序

from
where
group by
select
distinct
having
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.***執行select（****去重）*******

5.將分組的結果進行having過濾

6.將結果按條件排序：order by

7.限制結果的顯示條數

簡單查詢

#employee 表
    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',
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,
depart_id int 
);

#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), #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)
;

#ps：如果在windows系統中，插入中文字元，select的結果為空白，可以將所有字元編碼統一設定成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;    

#通過四則運算查詢
    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() 函式用於連線字串
   SELECT CONCAT('姓名: ',emp_name,'  年薪: ', salary*12)  AS Annual_salary 
   FROM employee;
   
   CONCAT_WS() 第一個引數為分隔符
   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;
 

小練習：

1 查出所有員工的名字，薪資,格式為
    <名字:egon>    <薪資:3000>
2 查出所有的崗位（去掉重複）
3 查出所有員工名字，以及他們的年薪,年薪的欄位名為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;

where約束

where字句中可以使用：

\1. 比較運算子：> < >= <= <> !=
\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%'
萬用字元可以是%或_，
%表示任意多字元
_表示一個字元
\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 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(判斷某個欄位是否為NULL不能用等號，需要用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 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
    萬用字元’%’
    SELECT * FROM employee 
            WHERE emp_name LIKE 'eg%';

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

小練習：

1. 檢視崗位是teacher的員工姓名、年齡
2. 檢視崗位是teacher且年齡大於30歲的員工姓名、年齡
3. 檢視崗位是teacher且薪資在9000-10000範圍內的員工姓名、年齡、薪資
4. 檢視崗位描述不為NULL的員工資訊
5. 檢視崗位是teacher且薪資是10000或9000或30000的員工姓名、年齡、薪資
6. 檢視崗位是teacher且薪資不是10000或9000或30000的員工姓名、年齡、薪資
7. 檢視崗位是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 'jin%';

group by

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(emp_name) FROM employee GROUP BY post;#按照崗位分組，並檢視組內成員名
    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：

如果我們用unique的欄位作為分組的依據，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欄位值相同，該欄位通常用來作為分組的依據

聚合函式

#強調：聚合函式聚合的是組的內容，若是沒有分組，則預設一組

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;

小練習：

1. 查詢崗位名以及崗位包含的所有員工名字
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
#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 |
+--------+---------------+

HAVING過濾

#！！！執行優先順序從高到低：where > group by > having 
#1. Where 發生在分組group by之前，因而Where中可以有任意欄位，但是絕對不能使用聚合函式。
#2. Having發生在分組group by之後，因而Having中可以使用分組的欄位，無法直接取到其他欄位,可以使用聚合函式
# 適合去篩選符合條件的某一組資料,而不是某一行資料

mysql> select post,group_concat(emp_name) from emp group by post having salary > 10000;#錯誤，分組後無法直接取到salary欄位
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 |
+-----------+-------------------------------------------------------+

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

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

ORDER BY 查詢排序

排序 order by
預設是升序 asc
降序 desc
order by age ,salary desc
優先根據age從小到大排,在age相同的情況下,再根據薪資從大到小排

小練習：

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 * from employee
order by age asc, 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
order by avg(salary) asc;
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
order by avg(salary) desc;

LIMIT 限制查詢的記錄數

limit m,n ===（limit n offset m）
從m+1項開始,取n項
如果不寫m,m預設為0

    SELECT * FROM employee ORDER BY salary DESC
        LIMIT 0,5; #從第1開始，即先查詢出第一條，然後包含這一條在內往後查5條

    SELECT * FROM employee ORDER BY salary DESC
        LIMIT 5,5; #從第6開始，即先查詢出第6條，然後包含這一條在內往後查5條

使用正則表示式查詢

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

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 emp_name regexp '^jin.*[gn]$';