Leetcode之Database篇
早晨和陈John一起来实验室的路上听他说起leetcode上也有数据库和shell的练习。于是拿来练练手,发现数据库的题只有几道而且做得也很快AC率也蛮高,权当复习了一下数据库的基本语法了吧。
1:Employees Earning More Than Their Managers
The Employee
table holds all employees including their managers. Every employee has an Id, and there is also a column for the manager Id.
+----+-------+--------+-----------+ | Id | Name | Salary | ManagerId | +----+-------+--------+-----------+ | 1 | Joe | 70000 | 3 | | 2 | Henry | 80000 | 4 | | 3 | Sam | 60000 | NULL | | 4 | Max | 90000 | NULL | +----+-------+--------+-----------+
Given the Employee
table, write a SQL query that finds out employees who earn more than their managers. For the above table, Joe is the only employee who earns more than his manager.
+----------+ | Employee | +----------+ | Joe | +----------+
刚开始没看清楚,后来研究了一下第一张表才发现原来ManagerId是属于Id的,所以用到两张表,这道题很简单
SELECT a.NAME FROM Employee a, Employee b WHERE a.ManagerId = b.Id AND a.Salary > b.Salary;
2:Duplicate Emails
Write a SQL query to find all duplicate emails in a table named Person
.
+----+---------+ | Id | Email | +----+---------+ | 1 | [email protected] | | 2 | [email protected] | | 3 | [email protected] | +----+---------+
For example, your query should return the following for the above table:
+---------+ | Email | +---------+ | [email protected] | +---------+
Note: All emails are in lowercase.
SELECT Email FROM Person GROUP BY Email HAVING COUNT(*)>1
具体关于group by 和having 的用法参考了别人的一篇博客
http://www.cnblogs.com/gaiyang/archive/2011/04/01/2002452.html
3:Combine Two Tables
Table: Person
+-------------+---------+ | Column Name | Type | +-------------+---------+ | PersonId | int | | FirstName | varchar | | LastName | varchar | +-------------+---------+ PersonId is the primary key column for this table.
Table: Address
+-------------+---------+ | Column Name | Type | +-------------+---------+ | AddressId | int | | PersonId | int | | City | varchar | | State | varchar | +-------------+---------+ AddressId is the primary key column for this table.
Write a SQL query for a report that provides the following information for each person in the Person table, regardless if there is an address for each of those people:
FirstName, LastName, City, State
SELECT p.FirstName, p.LastName, a.City, a.State FROM Person p LEFT JOIN Address a USING (PersonId)
主要是两个表的连接,参考链接如下:
http://www.bkjia.com/Mysql/777046.html
http://www.cnblogs.com/devilmsg/archive/2009/03/24/1420543.html
4:Customers Who Never Order
Suppose that a website contains two tables, the Customers
table and the Orders
table. Write a SQL query to find all customers who never order anything.
Table: Customers
.
+----+-------+ | Id | Name | +----+-------+ | 1 | Joe | | 2 | Henry | | 3 | Sam | | 4 | Max | +----+-------+
Table: Orders
.
+----+------------+ | Id | CustomerId | +----+------------+ | 1 | 3 | | 2 | 1 | +----+------------+
Using the above tables as example, return the following:
+-----------+ | Customers | +-----------+ | Henry | | Max | +-----------+
SELECT name FROM Customers c LEFT JOIN Orders o on c.Id = o.CustomerId WHERE o.Id IS NULL
网址http://www.tuicool.com/articles/miAfii给出了三种方法,可供参考
5:Rising Temperature
Given a Weather
table, write a SQL query to find all dates‘ Ids with higher temperature compared to its previous (yesterday‘s) dates.
+---------+------------+------------------+ | Id(INT) | Date(DATE) | Temperature(INT) | +---------+------------+------------------+ | 1 | 2015-01-01 | 10 | | 2 | 2015-01-02 | 25 | | 3 | 2015-01-03 | 20 | | 4 | 2015-01-04 | 30 | +---------+------------+------------------+
For example, return the following Ids for the above Weather table:
+----+ | Id | +----+ | 2 | | 4 | +----+
首先要内联,其次要注意查询的ID是哪一张表的ID,w1.Id.
其次有计算date天数的函数,课参考http://blog.chinaunix.net/uid-26921272-id-3385920.html
SELECT w1.Id FROM Weather w1 INNER JOIN Weather w2 ON TO_DAYS(w1.Date) = TO_DAYS(w2.Date) + 1 AND w1.Temperature > w2.Temperature
6:Second Highest Salary
Write a SQL query to get the second highest salary from the Employee
table.
+----+--------+ | Id | Salary | +----+--------+ | 1 | 100 | | 2 | 200 | | 3 | 300 | +----+--------+
For example, given the above Employee table, the second highest salary is 200
. If there is no second highest salary, then the query should return null
.
SELECT Max(Salary) FROM Employee WHERE Salary < (SELECT Max(Salary) FROM Employee)
7:明天继续
郑重声明:本站内容如果来自互联网及其他传播媒体,其版权均属原媒体及文章作者所有。转载目的在于传递更多信息及用于网络分享,并不代表本站赞同其观点和对其真实性负责,也不构成任何其他建议。