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Safari | Python Developer's Handbook -> Using SQL





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Using SQL

SQL stands for Structured Query Language. It was developed in the mid-1970s by IBM Research to serve as an English interface query language to the System R relational database prototype.

SQL consists of a list of powerful and flexible commands that are used to manipulate information collected in tables, by operating and controlling sets of records at a time.

  • SQL is an interactive query language for ad hoc database queries.

  • SQL is a database programming language.

  • SQL is a data definition and data administration language.

  • SQL is the language of networked database servers.

  • SQL helps protect the data in a multi-user networked environment.

Nowadays, SQL servers are the dominant model for creating client/server applications. The most important tendency among database servers of any size is the revelation of SQL as the choice for the manipulation, definition, and control of data.

SQL has been an ISO standard for a long time. It is a powerful language for databases that adhered to the relational model.

The relational model clearly separates the physical aspects of data from their logical implementation. It frees you from being concerned with the details of how data is stored and makes the access to data purely logical.

By using SQL statements, you just need to specify the tables, columns, and row qualifiers to get to any data item.

SQL Mini-Tutorial

The idea behind this mini-tutorial is to teach you how to change and query the database. Of course, this book does not cover everything. It should give you a brief understanding of the concepts and basic usage of SQL statements. If it becomes necessary to delve deeper in this topic, the last heading of this section contains a list of Web sites that have some beneficial and complete SQL tutorials.

Selecting the Information

In a relational database, data is stored in tables. In our example, we have the USERS Table. ID, NAME, EMAIL, and AGE are the columns of this table.

Table8.2. USERS
ID NAME EMAIL AGE
1 Andre alessa@bebemania.com.br 25
2 Renata rtaveira@bebemania.com.br 30
3 Cleber clessa@bebemania.com.br 45
4 Beth beth@alugueaqui.com.br 40

Now, say that you want to know the EMAIL and the AGE of each user. You have to use the SELECT statement as follows:

							
SELECT EMAIL, AGE
FROM USERS

						

The following list is the result of your query:

							
EMAIL                       AGE
alessa@bebemania.com.br    25
rtaveira@bebemania.com.br  30
clessa@bebemania.com.br    45
beth@alugueaqui.com.br     40

						

Let me explain to you what you have done: you asked to see all the rows from the USERS table, filtering only the EMAIL and AGE columns. Note that column names and table names do not have spacesthey must be entered as just one word. The general syntax for a SELECT statement (when selecting all the rows from a table) is

							
SELECT Column1Name, Column2Name, 
FROM TableName
					
						

Note

This basic syntax doesn't filter which rows are selected or do anything else interesting.



You can use the asterisk symbol in order to retrieve all the columns from a table without typing every column name:

							
SELECT * FROM TableName;
					
						
Relational Operators

Six important relational operators exist in SQL, and after introducing them, we'll see how they're used:

= Equal
<> Not Equal
< Less Than
> Greater Than
<= Less Than or Equal To
>= Greater Than or Equal To

The WHERE clause of a SELECT statement specifies which rows of a table must be selected. For example, let's determine which users are 25 years old.

							
SELECT NAME
FROM USERS
WHERE AGE = 25;

						

The resultset is as follows:

							
NAME
Andre
					
						
Joins

Good database design suggests that each table in a database must contain data of only one single entity. Detailed information can be acquired by joining tables according to their primary and foreign keys. For example, we will create Table 8.3.

Table8.3. NATIONALITY ... ... (Not finished, please download and read the complete file)
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