MySQL數據庫學習【第七篇】單表查詢
阿新 • • 發佈：2018-02-20
not null for 比較運算符 創建 字符串 直接 過濾 field gpo
先創建表
#創建表 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 ); #查看表結構 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) ;
#一點小知識復習
1.註意：
select * from t1 where 條件 group by 分組字段
1.分組只能查詢分組字段，要想查看其余的利用聚合函數
2.聚合函數的分類：count,min,max,avg,group_concat,sum等。
3.模糊匹配：用like關鍵字。
select * from t1 where name like ‘%eg%‘; #%表示任意字符
select * from t1 where name like ‘d__l‘; #一個下劃線表示一個字符，兩個下劃線就表示兩個字符
4.拷貝表 ：create table t2 select * from t1;
create table t2 select * from t1 where 1=2 ;
一、查詢語法
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、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;
#避免重復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() 函數用於連接字符串
SELECT CONCAT(‘姓名: ‘,name,‘ 年薪: ‘, salary*12) AS Annual_salary
FROM employee;
CONCAT_WS() 第一個參數為分隔符
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（去掉重復）
select distinct depart_id from employee;
3 查出所有員工名字，以及他們的年薪,年薪的字段名為年薪
select name,salary*12 年薪 from employee;
三、where約束
where字句中可以使用：
1. 比較運算符：> < >= <= <> !=
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_，
%表示任意多字符
_表示一個字符
like ‘e__n‘ :
5. 邏輯運算符：在多個條件直接可以使用邏輯運算符 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=‘‘; 註意‘‘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’_’
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！！！
#！！！執行優先級從高到低：where > group by > 聚合函數 > having >order by
1.where和having的區別
1. Where 是一個約束聲明，使用Where約束來自數據庫的數據，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(文件)中取出數據），Where中不能使用聚合函數
2.Having是一個過濾聲明，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(文件)中取出數據，然後group by分組，
如果沒有group by則所有記錄整體為一組，然後執行聚合函數，然後使用having對聚合的結果進行過濾），
在Having中可以使用聚合函數。
3.where的優先級比having的優先級高
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.查看員工的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---->沒有分組則默認一組執行聚合count(id)--->select執行查看組內id數目
2:找到表employee--->沒有分組則默認一組執行聚合count(id)---->having 基於上一步聚合的結果（此時只有count(id)字段了）
進行id>15的過濾，很明顯，根本無法獲取到id字段
#having 舉例
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; #查看所有id>7的員工信息
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：可以按照任意字段分組,但分完組後，只能查看分組的那個字段，要想取的組內的其他字段信息，需要借助函數
單獨使用GROUP BY關鍵字分組
select post from employee group by post;
註意：我們按照post字段分組，那麽select查詢的字段只能是post，想要獲取組內的其他相關信息，需要借助函數
GROUP BY關鍵字和group_concat()函數一起使用
select post,group_concat(name) from employee group by post;#按照崗位分組，並查看組內成員名
select post,group_concat(name) as emp_members FROM employee group by post;
GROUP BY與聚合函數一起使用
select post,count(id) as count from employee group by post;#按照崗位分組，並查看每個組有多少人
強調：
分組：一般相同的多的話就可以分成一組（一定是有重復的字段）
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、關鍵字的執行優先級(重點)
重點中的重點：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.拿著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，如果沒有group by，則整體作為一組
4.如果有聚合函數，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，默認就是升序 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. 查詢所有員工信息，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、使用聚合函數查詢
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，沒有分組則默認一組
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（使用正則表達式查詢）
1.select * from employee where name regexp ‘^ale‘; #匹配以ale開頭的員工信息
2.select * from employee where name regexp ‘on$‘; #匹配以on結尾的員工信息
3.select * from employee where name regexp ‘n{1,2}‘; #匹配name裏面包含1到2個n的員工信息
小結：對字符串匹配的方式
where name = ‘egon‘;
where name like ‘yua%‘;
where name regexp ‘on$‘;
小練習：
查看所有員工中名字是jin開頭，n或者g結果的員工信息 select * from employee where name regexp ‘^jin.*[ng]$‘;
十、限制查詢的記錄數：LIMIT
=========limit：限制打印幾條========= 1.select * from employee limit 3;#打印前三條 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; #查看後三條
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;
MySQL數據庫學習【第七篇】單表查詢