七、單表查詢
阿新 • • 發佈：2018-10-31
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;