来源：力扣（LeetCode）
1.编写一个 SQL 查询来实现分数排名。如果两个分数相同，则两个分数排名（Rank）相同。请注意，平分后的下一个名次应该是下一个连续的整数值。换句话说，名次之间不应该有“间隔”。例如，根据给定的 Scores 表，你的查询应该返回（按分数从高到低排列）：
select a.Score as score,if(a.Score,0,(select count(b.Score)+1 from Scores b where b.score >a.Score)) as Rank from Scores a order by Score Desc;
2.编写一个 SQL 查询，查找所有至少连续出现三次的数字
select disctinct l1.num as ConsecutiveNums from logs l1, logs l2, logs l3 where l1.ld=l2.ld-1 and l2.id=l3.id-1 and l1.num=l2.num and l2.num-l3.num
3.编写一个 SQL 查询，查找 Person 表中所有重复的电子邮箱。
select distinct a.Email from Person a ,Person b where a.Email=b.Email and a.id !=b.id
4.某网站包含两个表，Customers 表和 Orders 表。编写一个 SQL 查询，找出所有从不订购任何东西的客户。
select customers.name as 'Customers'
from customers
where customers.id not in ( select customerid from orders )
5
.
SELECT
Department.name AS 'Department',
Employee.name AS 'Employee',
Salary
FROM
Employee
JOIN
Department ON Employee.DepartmentId = Department.Id
WHERE
(Employee.DepartmentId , Salary) IN
( SELECT
DepartmentId, MAX(Salary)
FROM
Employee
GROUP BY DepartmentId
）
6.