SQL provides a mechanism for nesting subqueries. A subquery is a select-from-where expression that is nested within another query. 继续阅读“Relational Databases ~ Introduction to SQL IV”
Relational Databases ~ Introduction to SQL III
The SQL operations union, intersect, and except operate on relations and correspond to the mathematical set-theory operations ∪, ∩, —. We shall now construct queries involving the union, intersect, and except operations over two sets. 继续阅读“Relational Databases ~ Introduction to SQL III”
Relational Databases ~ Introduction to SQL II
The basic structure of an SQL query consists of three clauses: select, from, and where. The query takes as its input the relations listed in the from clause, operates on them as specified in the where and select clause, and then produces a relation as the result. 继续阅读“Relational Databases ~ Introduction to SQL II”
Elementary Data Structures ~ Linked Lists II
Linked lists bring us into a world of computing that is markedly different from that of arrays and structures. With arrays and structures, we save an item in memory and later refer to it by name (or by index) in much the same manner as we might put a piece of information in a file drawer or an address book; with linked lists, the manner in which we save information makes it more difficult to access but easier to rearrange. Working with data that are organized in linked lists is called list processing. 继续阅读“Elementary Data Structures ~ Linked Lists II”
信号完整性小记 ~ 反射
1、反射问题概述
1.1 反射的产生
只要信号遇到瞬时阻抗突变，反射就会发生。反射可能发生在线的末端，或者是互联拓扑结构改变的地方，如拐角、过孔、T 型结构、连接件和封装处。为了保持信号的完整性，互联设计时要尽可能保持互联阻抗的恒定。 继续阅读“信号完整性小记 ~ 反射”
晶体三极管驱动继电器电路及其抗干扰的一些建议
1、连接方法
用晶体三极管驱动继电器时，建议连接集电极使用。另外，继电器为 ON 时外加额定电压、继电器为 OFF 时使电压为零是一种避免故障的使用方法。并且，在低电压线路（5V 以下）中，请选择考虑到晶体三极管电压降落的电源电压、继电器品种。（在 5V 线路中，建议使用线圈额定电压为 4.5 V型） 继续阅读“晶体三极管驱动继电器电路及其抗干扰的一些建议”
Principles of Algorithm Analysis II
Armed with the tools outlined in Principles of Algorithm Analysis I, we now consider the analysis of sequential search and binary search, two basic algorithms for determining whether or not any of a sequence of objects appears among a set of previously stored objects. 继续阅读“Principles of Algorithm Analysis II”
信号完整性小记 ~ 传输线
传输线是一种场的简化概念，关注的是互连的阻抗、时延和信号的波形。传输线的两个重要特征是特征阻抗和时延，说的都是传输线对信号的作用。 继续阅读“信号完整性小记 ~ 传输线”
Relational Databases ~ Introduction to SQL I
There are a number of database query languages in use, either commercially or experimentally. In this series we study the most widely used query language, SQL. Although we refer to the SQL language as a “query language”, it can do much more than just query a database. It can define the structure of the data, modify data in the database, and specify security constraints.
It is not our intention to provide a complete users’ guide for SQL. Rather, we present SQL’s fundamental constructs and concepts. Individual implementations of SQL may differ in details, or may support only a subset of the full language. 继续阅读“Relational Databases ~ Introduction to SQL I”