15.4 What are the different types of joins? Please explain how they differ and why certain types are better in certain situations. Join是用来联合两个表的,每个表至少需要有一列是相同的,不同的Join类型会返回不同的结果.我们来看一个例子,有两个表,普通饮料和无卡饮料如下: -- TABLE RegularBeverages +-----------+------…

15.7 Imagine a simple database storing information for students' grades. Design what this database might look like and provide a SQL query to return a list of the honor roll students (top 10%), sorted by their grade point average. 在一个简化的数据库中我们有三个表,St…

15.5 What is denormalization? Explain the pros and cons. 逆规范化Denormalization是一种通过添加冗余数据的数据库优化技术,可以帮助我们减少关系数据库中耗时的交Join.在一般的规范化的数据库中,我们将数据存在不同的表中是为了减少冗余数据,所以我们会尝试着每条数据在数据库中只有一份. 比如说,在一个规范化的数据库中,我们有Courses表和Teachers表,每个Courses表的一项都会保存teacherID,但是没有teac…

Write a SQL query to get a list of all buildings and the number of open requests (Requests in which status equals 'Open'). -- TABLE Apartments +-------+------------+------------+ | AptID | UnitNumber | BuildingID | +-------+------------+------------+…

Write a SQL query to get a list of tenants who are renting more than one apartment. -- TABLE Apartments +-------+------------+------------+ | AptID | UnitNumber | BuildingID | +-------+------------+------------+ | 101 | A1 | 11 | | 102 | A2 | 12 | |…

15.6 Draw an entity-relationship diagram for a database with companies, people, and professionals (people who work for companies). 在公司Companies工作的人是专家Proferssionals,所以人和专家是ISA("is a")的关系,每个专家有例如等级,和工作经验和其他从人Person继承来的属性.一个专家一次只能为一个公司工作,而一个公司可以雇佣…

Building #11 is undergoing a major renovation. Implement a query to close all requests from apartments in this building. -- TABLE Apartments +-------+------------+------------+ | AptID | UnitNumber | BuildingID | +-------+------------+------------+ |…

by HIMANSHU ARORA on OCTOBER 16, 2012 http://www.thegeekstuff.com/2012/10/15-linux-split-and-join-command-examples-to-manage-large-files/ Linux split and join commands are very helpful when you are manipulating large files. This article explains how…

Chapter 1. Arrays and Strings 1.1 Unique Characters of a String 1.2 Reverse String 1.3 Permutation String 1.4 Replace Spaces 1.5 Compress String 1.6 Rotate Image 1.7 Set Matrix Zeroes 1.8 String Rotation Chapter 2. Linked Lists 2.1 Remove Duplicates…

在SQL语言中,存在着各种Join,有Left Join, Right Join, Inner Join, and Natural Join等,对于初学者来说肯定一头雾水,都是神马跟神马啊,它们之间到底有着怎样的区别和联系呢,我们先用一张图片来说明:…