What Is SQL?
What Is SQL?
SQL (pronounced as the letters S-Q-L or as sequel) is an abbreviation for Structured Query Language. SQL is a language designed specifically for communicating with databases.
Unlike other languages (spoken languages like English, or programming languages like Java or Visual Basic), SQL is made up of very few words. This is deliberate. SQL is designed to do one thing and do it well—provide you with a simple and efficient way to read and write data from a database.
What are the advantages of SQL?
SQL is not a proprietary language used by specific database vendors. Almost every major DBMS supports SQL, so learning this one language will enable you to interact with just about every database you'll run into.
SQL is easy to learn. The statements are all made up of descriptive English words, and there aren't that many of them.
Despite its apparent simplicity, SQL is actually a very powerful language, and by cleverly using its language elements you can perform very complex and sophisticated database operations.
And with that, let's learn SQL.
SQL Extensions : Many DBMS vendors have extended their support for SQL by adding statements or instructions to the language. The purpose of these extensions is to provide additional functionality or simplified ways to perform specific operations. And while often extremely useful, these extensions tend to be very DBMS specific, and they are rarely supported by more than a single vendor.
Standard SQL is governed by the ANSI standards committee, and is thus called ANSI SQL. All major DBMSs, even those with their own extensions, support ANSI SQL. Individual implementations have their own names (PL-SQL, Transact-SQL, and so forth).
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/// <summary>
/// 过滤标记
/// </summary>
/// <param name="NoHTML">包括HTML，脚本，数据库关键字，特殊字符的源码 </param>
/// <returns>已经去除标记后的文字</returns>
&nbs ......
错误提示，有一个文件已经挂起，必须重新启动计算机。重新启动，再次安装仍然是同样的问题。
注册表键值的问题。解决方法：
只要删除\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\下的PendingFileRenameOperations键值，就可以重新安装了。
删除后安装，不必重新启动计算机。 ......
---返回表达式中指定字符的开始位置
select charindex('c','abcdefg',1)
---两个字符的值之差
select difference('bet','bit')
---字符最左侧指定数目
select left('abcdef',3)
---返回字符数
select len('abcdefg')
--转换为小字符
select lower('ABCDEFG')
--去左空格后
select ltrim(' &nbs ......
有些常见的问题在论坛中不断出现，不妨整理一下。
以下语句是在SQLServer2005上实现的，一些语句无法在SS2000上执行。
有用指数是我根据这个问题的常见程度打的分，仅供参考。实际上，当你遇到了这个问题，这个问题哪怕再少见，解决方案也是非常有用的。
1. 生成若干行记录
有用指数：★★★★★
常见的问题类型：根 ......
--聚合函数
use pubs
go
select avg(distinct price) --算平均数
from titles
where type='business'
go
use pubs
go
select max(ytd_sales) --最大数
from titles
go
use pubs
go
select min(ytd_sales) --最小� ......
