SQL是什么? 什么是SQL?

SQL is a standard computer language for accessing and manipulating databases.yAvTechFeel
SQL是一种标准的计算机语言，它可以用来访问和操作数据库

What is SQL?yAvTechFeel
什么是SQL？

• SQL stands for Structured Query LanguageyAvTechFeel
  SQL代表结构化查询语言
• SQL allows you to access a databaseyAvTechFeel
  SQL能让你访问到数据库
• SQL is an ANSI standard computer languageyAvTechFeel
  SQL为ANSI标准计算机语言
• SQL can execute queries against a databaseyAvTechFeel
  SQL可以针对数据库执行查询
• SQL can retrieve data from a databaseyAvTechFeel
  SQL可以回复来自数据库的信息
• SQL can insert new records in a databaseyAvTechFeel
  SQL能在数据库中插入一条新的记录
• SQL can delete records from a databaseyAvTechFeel
  SQL能够删除数据库中的记录
• SQL can update records in a databaseyAvTechFeel
  SQL能够更新数据库里的信息
• SQL is easy to learnyAvTechFeel
  SQL并不难学

SQL is a Standard - BUT....yAvTechFeel
SQL虽为标准 - 但是...

SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems. SQL statements are used to retrieve and update data in a database. SQL works with database programs like MS Access, DB2, Informix, MS SQL Server, Oracle, Sybase, etc.yAvTechFeel
SQL是ANSI体系中标准的数据库访问以及处理语言。SQL可以使用在像MS Access,DB2,Informix,MS SQL Server,Oracle,Sybase,等等一些数据库上。

Unfortunately, there are many different versions of the SQL language, but to be in compliance with the ANSI standard, they must support the same major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT, WHERE, and others).yAvTechFeel
但它依然有多种版本，不过他们在关键字的使用上还是遵循了ANSI所规定的（比如SELECT,UPDATE,DELETE,INSERT,WHERE以及其他的一些）

Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!yAvTechFeel
注意：大多数数据库程序都带有属于他们自身特色的扩展属性。

SQL Database TablesyAvTechFeel
SQL数据表

A database most often contains one or more tables. Each table is identified by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with data.yAvTechFeel
一份数据库通常会含有一张或是多张表。每张表都有自己的名称以做区分（比如："客户"或是"定单"）。数据库表内可以记录相关的数据。

Below is an example of a table called "Persons":yAvTechFeel
下面就是一个名为"Persons"的数据库表格：

The table above contains three records (one for each person) and four columns (LastName, FirstName, Address, and City).

SQL QueriesyAvTechFeel
SQL查询

With SQL, we can query a database and have a result set returned.yAvTechFeel
利用SQL我们可以通过查询语句来对已有数据进行筛选并找到我们所需要的准确结果

A query like this:yAvTechFeel
这就是一句查询语句

SELECT LastName FROM Persons

Gives a result set like this:yAvTechFeel
参照上面的数据表，返回的结果就因该是：

Note: Some database systems require a semicolon at the end of the SQL statement. We don't use the semicolon in our tutorials.yAvTechFeel
注意:一些数据库系统需要在每条SQL语句结束后带上分号。在我们的教程中将不使用到分号。

SQL Data Manipulation Language (DML)yAvTechFeel
SQL 数据处理语言(DML)

SQL (Structured Query Language) is a syntax for executing queries. But the SQL language also includes a syntax to update, insert, and delete records.yAvTechFeel
SQL是一种针对执行查询的语法。但SQL里还包括对记录进行更新，插入以及删除的语法。

These query and update commands together form the Data Manipulation Language (DML) part of SQL:yAvTechFeel
这些查询以及更新命令构成了SQL(DML)的一部分:

• SELECT - extracts data from a database tableyAvTechFeel
  从数据表中抽出数据
• UPDATE - updates data in a database tableyAvTechFeel
  更新数据库表中的数据
• DELETE - deletes data from a database tableyAvTechFeel
  删除数据库表中的数据
• INSERT INTO - inserts new data into a database tableyAvTechFeel
  插入一条新记录

SQL Data Definition Language (DDL)yAvTechFeel
SQL 数据定义语言(DDL)

The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted. We can also define indexes (keys), specify links between tables, and impose constraints between database tables.