结构化查询语言SQL是一种(20)语言，其主要功能有(21)，它的典型句型为：

  SELECT A1，A2，…，An

 FROM r1，r2，…，rm

 WHERE F

其中A1，A2，…，An，r1，r2，…，rm和F分别是(22)。

 关系模式WTS(Wno，Tno，Sno)中，Wno，Wno Wno分别代表工人编号，工种编码以及月工资。若要查询工种编码以“A”开头的全体工人的平均月工资，可用SQL语言写为：

 SELECT(23)

 FBOM WTS

 WHERE(24)

(81)

The main function of a processor is to(68)and carry out instructions which usually consist of operation and(69). The term to measure the speed of the processor is called a Megahertz(MHz), which means million(70)per second.

(80)

(78)

The successful spread of a virus depends on how long it can()unnoticed, before its presence is made known by the activation of side-effects.

(76)

Anti- virus software protects your computer from viruses by(75)your computer's memory and disk devices.

(70)is not a peripheral component or device. The basic input device on a small computer is a(71).

A(74)is a device that enables the computer to handle sounds.

The transmission media used t0 build a LAN is(73).