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首页SQL入门指南：实战教程助初学者掌握数据处理

SQL入门指南：实战教程助初学者掌握数据处理

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"《入门SQL：实践导向指南》是一本由Thomas Nield编写的实用教程，专为初学者设计，帮助商业和IT专业人士掌握在快速膨胀的数据时代如何有效地利用SQL访问和处理数据。作者通过书中的实际练习，让读者无需依赖数据库服务器环境就能在家练习新学的SQL技能。本书内容涵盖了SQL的基础知识，包括探索关系型数据库（如轻量级和集中式模型）、SQLite和SQLiteStudio的使用，以及如何通过关键SQL语句（如SELECT、WHERE、GROUP BY和ORDER BY）查询和转换数据。学习者将学会如何连接表以获取业务数据的完整视图，根据规范化原则构建自己的表格和中央化数据库，并掌握如何插入（INSERT）、删除（DELETE）和更新（UPDATE）记录来管理数据。章节安排详实，从为何学习SQL开始，然后逐步深入到数据库概念、SQLite的创建、SQL查询的各个部分，再到数据库设计和数据管理的最佳实践。最后，还有一份附录，列出了一系列操作符和函数，以及补充的主题，供读者进一步深化理解。《入门SQL：实践导向指南》不仅教授理论知识，更强调实践操作，确保读者能够迅速上手并将其应用到实际工作场景中。适合希望通过学习SQL提升数据分析能力的企业和个人，无论你是数据库新手还是希望巩固现有技能的专业人士，这本书都是一个理想的起点。版权信息显示该书于2016年首次出版，由O'Reilly Media发行，旨在为教育、商业或销售推广提供资源，同时也提供了在线版本供选择。"

CHAPTER 1

Why Learn SQL?

What Is SQL and Why Is It Marketable?

It is an obvious statement that the business landscape is shifting rapidly. A lot of this

is enabled by technology and the explosion of business data. Companies are investing

vast amounts of capital to gather and warehouse data. But what many business lead‐

ers and managers currently struggle with is how to make sense of this data and use it.

This is where SQL, which stands for Structured Query Language, comes in. It provides

a means to access and manipulate this data in meaningful ways and provide business

insights not possible before.

Businesses are gathering data at exponential rates, and there is an equally growing

need for people who know how to analyze and manage it. Stack Overflow, the most

active programming community in the world, performed a comprehensive survey on

its members in 2015. Apple coding was the most in-demand technology and had an

average salary nearing six figures. But SQL came in in fifth place, with a salary that

was not far behind. In recent years, data has suddenly become ubiquitous—yet few

people know how to access it meaningfully, which has put SQL talent in high

demand.

Who Is SQL For?

One misperception about SQL is that it is an IT skill and therefore only applicable to

technology (not business) professionals. In the world as it exists today, this is hardly

the truth. Businesspeople, managers, IT professionals, and engineers can all reap ben‐

efits from learning SQL to better position their careers. SQL can open many career

paths because it enables individuals to know their businesses better through the data

that is driving them. On the business side, interest in SQL can lead to roles that are

analytical, managerial, strategic, and research- or project-based. On the IT front, it

can lead to roles in database design, database administration, systems engineering, IT

project management, and even software development.

2 | Chapter 1: Why Learn SQL?

CHAPTER 2

Databases

What Is a Database?

In the broadest definition, a database is anything that collects and organizes data. A

spreadsheet holding customer bookings is a database, and so is a plain-text file con‐

taining flight schedule data. Plain-text data itself can be stored in a variety of formats,

including XML and CSV.

Professionally, however, when one refers to a “database” they likely are referring to a

relational database management system (RDBMS). This term may sound technical

and intimidating, but an RDBMS is simply a type of database that holds one or more

tables that may have relationships to each other.

Exploring Relational Databases

A table should be a familiar concept. It has columns and rows to store data, much like

a spreadsheet. These tables can have relationships to each other, such as an ORDER

table that refers to a CUSTOMER table for customer information.

For example, suppose we have an ORDER table with a field called CUSTOMER_ID

(Figure 2-1).

Figure 2-1. An ORDER table with a CUSTOMER_ID

We can reasonably expect there to be another table, maybe called CUSTOMER

(Figure 2-2), which holds the customer information for each CUSTOMER_ID.

Figure 2-2. A CUSTOMER table

When we go through the ORDER table, we can use the CUSTOMER_ID to look up the cus‐

tomer information in the CUSTOMER table. This is the fundamental idea behind a “rela‐

tional database,” where tables may have fields that point to information in other

tables. This concept may sound familiar if you’ve used VLOOKUP in Excel to retrieve

information in one sheet from another sheet in a workbook.

Why Separate Tables?

But why are these tables separated and designed this way? The motivation is normal‐

ization, which is separating the different types of data into their own tables rather

than putting them in one table. If we had all information in a single table, it would be

redundant, bloated, and very difficult to maintain. Imagine if we stored customer

information in the ORDER table. Figure 2-3 shows what it would look like.

Figure 2-3. A table that is not normalized

Notice that for the Re-Barre Construction orders someone had to populate the cus‐

tomer information three times for all three orders (the name, region, street address,

city, state, and zip). This is very redundant, takes up unnecessary storage space, and is

difficult to maintain. Imagine if a customer had an address change and you had to

update all the orders to reflect that. This is why it is better to separate CUSTOMERS and

ORDERS into two separate tables. If you need to change a customer’s address, you only

need to change one record in the CUSTOMER table (Figure 2-4).

4 | Chapter 2: Databases

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