力扣sql基础篇(六)

1 学生参加各科测试的次数

1.1 题目内容

1.1.1 基本题目信息

1.1.2 示例输入输出

a 输入示例

b 输出示例

1.2 示例sql语句

# 无论考没考试都要该科目这栏且无连接字段,就可以考虑笛卡尔积了
SELECT  s.student_id,s.student_name,s.subject_name,IFNULL(e.num,0) attended_exams
FROM
(SELECT *FROM StudentsCROSS JOIN Subjects
)s
LEFT JOIN
(SELECT student_id,subject_name,count(*) numFROM ExaminationsGROUP BY student_id,subject_name
)e
ON s.student_id=e.student_id AND s.subject_name=e.subject_name
ORDER BY s.student_id asc,s.subject_name asc;

1.3 运行截图

2 可以放心投资的国家

2.1 题目内容

2.1.1 基本题目信息1

2.1.2 基本题目信息2

2.1.3 示例输入输出

a 示例输入

b 示例输出

2.2 示例sql语句

# 先求出每个国家的平均通话时长,在求出全球通话时长,然后把两者进行比较即可
#substring函数从1开始,
#全球通话时长是无论是接听者还是打电话的人都需要算进去
SELECT c1.name country
FROM Country c1
INNER JOIN Person p
ON c1.country_code=SUBSTRING(p.phone_number,1,3)
LEFT JOIN
(SELECT caller_id cid,durationFROM CallsUNION AllSELECT callee_id cid,durationFROM Calls
)c2
ON p.id=c2.cid
GROUP BY c1.name
HAVING avg(c2.duration)>(SELECT avg(duration) FROM Calls);

2.3 运行截图

3 部门工资最高的员工

3.1 题目内容

3.1.1 基本题目信息

3.1.2 示例输入输出

3.2 示例sql语句

SELECT e1.department,e1.employee,e1.salary
FROM
(SELECT e.name Employee,d.name Department,e.Salary,rank() over(partition by e.departmentId ORDER BY e.salary desc) rkFROM Employee eINNER JOIN Department dON e.departmentId=d.id
)e1
WHERE e1.rk=1;

3.3 运行截图