MYSQL 第六課 分組查詢 連線查詢
分組查詢
/*
語法：
select 查詢列表
from 表
【where 篩選條件】
group by 分組的欄位（分成若干組）
【order by 排序的欄位】;
特點：
1、和分組函式一同查詢的欄位必須是group by後出現的欄位
2、篩選分為兩類：分組前篩選和分組後篩選
針對的表 位置 連線的關鍵字
分組前篩選 原始表 group by前 where
分組後篩選 group by後的結果集 group by後 having
問題1：分組函式做篩選能不能放在where後面
答：不能
問題2：where——group by——having
一般來講，能用分組前篩選的，儘量使用分組前篩選，提高效率
3、分組可以按單個欄位也可以按多個欄位（多個欄位間用逗號隔開，沒有順序要求）
4、可以搭配著排序使用
*/
#引入：查詢每個部門的員工個數
SELECT COUNT(*) FROM employees WHERE department_id=90;
#1.簡單的分組
#案例1：查詢每個工種的員工平均工資
SELECT AVG(salary),job_id FROM employees GROUP BY job_id;
#案例2：查詢每個位置的部門個數
SELECT COUNT(*),location_id FROM departmentsGROUP BY location_id;
#2、可以實現分組前的篩選
#案例1：查詢郵箱中包含a字元的 每個部門的最高工資
SELECT MAX(salary),department_id,email FROM employees WHERE email LIKE '%a%' GROUP BY department_id;
#案例2：查詢有獎金的每個領導手下員工的平均工資
SELECT AVG(salary),manager_id FROM employees WHERE commission_pct IS NOT NULL GROUP BY manager_id;
#3、分組後篩選
#案例：查詢哪個部門的員工個數>5
#①查詢每個部門的員工個數
SELECT COUNT(*),department_id FROM employees GROUP BY department_id;
#② 篩選剛才①結果
SELECT COUNT(*),department_id FROM employees GROUP BY department_id HAVING COUNT(*)>5;
#案例2：每個工種有獎金的員工的最高工資>10000的工種編號和最高工資
SELECT job_id,MAX(salary) FROM employees WHERE commission_pct IS NOT NULL GROUP BY job_id HAVING MAX(salary)>10000;
#案例3：領導編號>102的每個領導手下的最低工資大於5000的領導編號和最低工資
# 1 查詢每個領導手下的最低工資
# 2 領導編號>102
# 3 最低工資大於5000
SELECT manager_id,MIN(salary) FROM employees WHERE manager_id>102 GROUP BY manager_id HAVING MIN(salary)>5000;
#4.新增排序
#案例：每個工種有獎金的員工的最高工資>6000的工種編號和最高工資,按最高工資升序
SELECT job_id,MAX(salary) m FROM employees WHERE commission_pct IS NOT NULL GROUP BY job_id HAVING m>6000 ORDER BY m ;
#5.按多個欄位分組
#案例：查詢每個工種每個部門的最低工資,並按最低工資降序
SELECT MIN(salary),job_id,department_id FROM employees GROUP BY department_id,job_id ORDER BY MIN(salary) DESC;
#6 按表示式或函式分組
#案例 按員工姓名的長度分組，查詢每一組的員工個數，篩選員工個數大於5 的有哪些 SELECT COUNT(*),LENGTH(last_name) FROM employees GROUP BY LENGTH(last_name) HAVING COUNT(*)>5;
例題
#1.查詢各job_id的員工工資的最大值，最小值，平均值，總和，並按job_id升序
SELECT MAX(salary),MIN(salary),AVG(salary),SUM(salary),job_id FROM employees GROUP BY job_id ORDER BY job_id;
#2.查詢員工最高工資和最低工資的差距（DIFFERENCE）
SELECT MAX(salary)-MIN(salary) DIFFRENCE FROM employees;
#3.查詢各個管理者手下員工的最低工資，其中最低工資不能低於6000，沒有管理者的員工不計算在內
SELECT MIN(salary),manager_id FROM employees WHERE manager_id IS NOT NULL GROUP BY manager_id HAVING MIN(salary)>=6000;
#4.查詢所有部門的編號，員工數量和工資平均值,並按平均工資降序
SELECT department_id,COUNT(*),AVG(salary) a FROM employees GROUP BY department_id ORDER BY a DESC;
#5.選擇具有各個job_id的員工人數
SELECT COUNT(*) 個數,job_id FROM employees GROUP BY job_id;
連線查詢
/*
含義：又稱多表查詢，當查詢的欄位來自於多個表時，就會用到連線查詢
笛卡爾乘積現象：表1 有m行，表2有n行，結果=m*n行
發生原因：沒有有效的連線條件
如何避免：新增有效的連線條件
分類：按年代分類：
sql92標準:僅僅支援內連線
sql99標準【推薦】：支援內連線+外連線（左外和右外）+交叉連線
按功能分類：
內連線：
等值連線
非等值連線
自連線
外連線：
左外連線
右外連線
全外連線
交叉連線
*/
SELECT * FROM beauty;
SELECT * FROM boys;
SELECT NAME,boyName FROM boys,beauty
WHERE beauty.boyfriend_id= boys.id;
#一、sql92標準#
1、等值連線
/*
① 多表等值連線的結果為多表的交集部分
②n表連線，至少需要n-1個連線條件
③ 多表的順序沒有要求
④一般需要為表起別名
⑤可以搭配前面介紹的所有子句使用，比如排序、分組、篩選
*/
#1、基礎
#案例1：查詢女神名和對應的男神名
SELECT NAME,boyName FROM boys,beauty WHERE beauty.boyfriend_id= boys.id;
#案例2：查詢員工名和對應的部門名
SELECT last_name,department_name FROM employees,departments WHERE employees.`department_id`=departments.`department_id`;
#2、為表起別名
/*
①提高語句的簡潔度
②區分多個重名的欄位
注意：如果為表起了別名，則查詢的欄位就不能使用原來的表名去限定
*/
#查詢員工名、工種號、工種名
SELECT e.last_name,e.job_id,j.job_title FROM employees e,jobs j WHERE e.`job_id`=j.`job_id`;
#3、兩個表的順序是否可以調換
#查詢員工名、工種號、工種名
SELECT last_name,department_name,commission_pct FROM employees e,departments d WHERE e.`department_id`=d.`department_id` AND e.`commission_pct` IS NOT NULL;
#4、可以加篩選
#案例：查詢有獎金的員工名、部門名
SELECT last_name,department_name,commission_pct FROM employees e,departments d WHERE e.`department_id`=d.`department_id` AND e.`commission_pct` IS NOT NULL;
#案例2：查詢城市名中第二個字元為o的部門名和城市名
SELECT department_name,city FROM departments d,locations l WHERE d.`location_id` = l.`location_id` AND city LIKE '_o%';
#5、可以加分組
#案例1：查詢每個城市的部門個數
SELECT COUNT(*) 個數,city FROM departments d,locations l WHERE d.`location_id`=l.`location_id` GROUP BY city;
#6、可以加排序
#案例：查詢每個工種的工種名和員工的個數，並且按員工個數降序
SELECT job_title,COUNT(*) FROM employees e,jobs j WHERE e.`job_id`=j.`job_id` GROUP BY job_title ORDER BY COUNT(*) DESC;
#7、可以實現三表連線？
#案例：查詢員工名、部門名和所在的城市
SELECT last_name,department_name,city FROM employees e,departments d,locations l WHERE e.`department_id`=d.`department_id` AND d.`location_id`=l.`location_id` AND city LIKE 's%' ORDER BY department_name DESC;
#2、非等值連線
#案例1：查詢員工的工資和工資級別
SELECT salary,grade_level FROM employees e,job_grades g WHERE salary BETWEEN g.`lowest_sal` AND g.`highest_sal` AND g.`grade_level`='A';
/*
select salary,employee_id from employees;
select * from job_grades;
CREATE TABLE job_grades
(grade_level VARCHAR(3),
lowest_sal int,
highest_sal int);
INSERT INTO job_grades
VALUES ('A', 1000, 2999);
INSERT INTO job_grades
VALUES ('B', 3000, 5999);
INSERT INTO job_grades
VALUES('C', 6000, 9999);
INSERT INTO job_grades
VALUES('D', 10000, 14999);
INSERT INTO job_grades
VALUES('E', 15000, 24999);
INSERT INTO job_grades
VALUES('F', 25000, 40000);
*/
#3、自連線
#案例：查詢 員工名和上級的名稱 (找了兩遍)
SELECT e.employee_id,e.last_name,m.employee_id,m.last_name FROM employees e,employees m WHERE e.`manager_id`=m.`employee_id`;
#1.顯示所有員工的姓名，部門號和部門名稱。
USE myemployees; SELECT last_name,d.department_id,department_name FROM employees e,departments d WHERE e.`department_id` = d.`department_id`;
#2.查詢90號部門員工的job_id和90號部門的location_id
SELECT job_id,location_id FROM employees e,departments d WHERE e.`department_id`=d.`department_id` AND e.`department_id`=90;
#3. 選擇所有有獎金的員工的
last_name , department_name , location_id , city
SELECT last_name , department_name , l.location_id , city FROM employees e,departments d,locations l WHERE e.department_id = d.department_id AND d.location_id=l.location_id AND e.commission_pct IS NOT NULL;
#4.選擇city在Toronto工作的員工的
last_name , job_id , department_id , department_name
SELECT last_name , job_id , d.department_id , department_name FROM employees e,departments d ,locations l WHERE e.department_id = d.department_id AND d.location_id=l.location_id AND city = 'Toronto';
#5.查詢每個工種、每個部門的部門名、工種名和最低工資
SELECT department_name,job_title,MIN(salary) 最低工資 FROM employees e,departments d,jobs j WHERE e.`department_id`=d.`department_id` AND e.`job_id`=j.`job_id` GROUP BY department_name,job_title;
#6.查詢每個國家下的部門個數大於2的國家編號
SELECT country_id,COUNT(*) 部門個數 FROM departments d,locations l WHERE d.`location_id`=l.`location_id` GROUP BY country_id HAVING 部門個數>2;
#7、選擇指定員工的姓名，員工號，以及他的管理者的姓名和員工號，結果類似於下面的格式
employees Emp# manager Mgr#
kochhar 101 king 100
SELECT e.last_name employees,e.employee_id "Emp#",m.last_name manager,m.employee_id "Mgr#" FROM employees e,employees m WHERE e.manager_id = m.employee_id AND e.last_name='kochhar';