Oracle数据库入门指南：SQL与PL/SQL简化教程（第2版）

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《Oracle数据库系统：SQL与PL/SQL简化指南（第二版）》是一本专为IT教育者和管理员设计的权威教材，针对当前快速变化的IT行业，它提供了一种清晰且实用的方法来教授数据库开发技能。本书由Alex Ephrem博士撰写，对于那些在计算机科学领域工作或学习的人来说，具有重要的参考价值。该书的核心内容涵盖了基础的数据库系统概念，如操作系统原理、文件或数据库结构、计算机体系架构以及通用编程理论。作者强调，尽管这些基础知识至关重要，但在选择一个用于教学和培养实践技能的应用或开发平台时，教育者面临挑战。他们需要找到既能生动展现理论知识，又能让学生获得实际操作经验、功能强大且具有市场竞争力的工具。 Oracle数据库被选为本书的主要焦点，因为它在业界广受欢迎，且具有强大的功能。SQL（Structured Query Language）是本书的核心部分，它是用于管理关系型数据库的标准语言，包括数据检索、更新、插入和删除等操作。学生通过学习SQL，可以掌握如何高效地查询和处理数据，这对于任何从事数据分析或业务智能的人员来说都是必不可少的技能。 PL/SQL（Procedural Language/Structured Query Language）则是Oracle数据库中的过程化编程语言，它扩展了SQL的功能，允许编写复杂的存储过程和函数。通过学习PL/SQL，学生们不仅能够深化对SQL的理解，还能掌握如何设计和实现数据库应用程序的逻辑，提升系统的性能和可维护性。此外，作者特别提到，这本书旨在帮助教育者应对行业变革，确保学生毕业后能适应不断发展的技术环境。书中提供了丰富的实例和实战项目，使学生能够在实践中应用所学知识，从而更好地为未来的职业生涯做好准备。无论是对初学者还是经验丰富的开发人员，这本《Oracle数据库系统：SQL与PL/SQL简化指南》都是一本极具价值的学习资源。

Chap. 1 Database Concepts: A Relational Approach 8

INTEGRITY RULES

In any database managed by an RDBMS, it is very important that the data in the un-

derlying tables be Consistent. If consistency is compromised, the data are not

usable. This need led the pioneers of database field Io formulate two integrity rules:

1. Entity integrity: No column in a primary key may be null. The primary

key provides the means of uniquely identifying a row or an entity. A null

value means a value that is not known, not entered, not defined, or not ap-

plicable. A zero or a space is not considered to be a null value. If the pri-

mary key value is a null Value in a row, we do not have enough

information about the row to uniquely identify it. The RDBMS software

strictly follows the entity integrity rule and does not allow users to enter a

row without a unique value in the primary key column.

2. Referential integrity: A foreign key Value may be a null value or it must

exist as a Value of a primary key in the referenced table.

Referential integrity is not fully supported by all commercially available sys-

tems, but Oracle supports it religiously! Oracle does not allow you to declare a for-

eign key if it does not exist as a primary key in another table. It allows you to leave

the foreign key column value as a null. If a user enters a value in the foreign key

column, Oracle cross-references the referenced primary key column in the other

table to confirm the existence of such a value.

It is not a good practice to use null values in any non-primary key columns,

because this results in extra overhead on the system’s part in search operations. The

programmers or query users have to add extra measures to include or exclude rows

with null values. In certain cases, it is not possible to avoid null values. For

example, an employee does not have middle initial, an employee is hired but does

not have an assigned department, or a student’s major is undefined. In Oracle, a

default value can be assigned to a column, and a user does not have to enter a value

for that column.

THEORETICAL RELATIONAL LANGUAGES

E. F. Codd suggested two theoretical relational languages to use with the relational

model:

1. Relational algebra, a procedural language.

2. Relational calculus, a nonprocedural language.

Third-generation high-level compiler languages can be used to manipulate

data in a table, but they can only work with one row at a time. In contrast, the

relational languages can work on the entire table or on a group of rows. The

multiple-row

Theoretical Relational Languages 9

manipulation does not even need a looping structure! The relational languages provide

more power with a very little coding. Codd proposed these languages to embed them in

other host languages for more processing capability and more sophisticated application

development. In the database systems available today, nonprocedural Structured Query

Language (SQL) is used as a data-manipulation sublanguage. The theoretical languages

have provided the basis for SQL.

Relational Algebra

Relational algebra is a procedural language, because the user accomplishes desired results

by using a set of operations in a sequence. It uses set operations on tables to produce new

resulting tables. These resulting tables are then used for subsequent sequential operations.

In Oracle, all operation names are not actually used as programming terms, and most of

these operations do not create a new resulting table, as shown in the following examples

using relational algebra.

The nine operations used by relational algebra are:

1. Union.

2. Intersection.

3. Difference.

4. Projection.

5. Selection.

6. Product.

7. Assignment.

8. Join.

9. Division.

Union. The union of two tables results in retrieval of all rows that are in one or both

tables. The duplicate rows are eliminated from the resulting table. The resulting table does

not contain two rows with identical data values. There is a basic requirement to perform a

union operation on two tables:

• Both tables must have the same degree.

• The domains of the corresponding columns in two tables must be same.

Such tables are said to be union compatible. In mathematical set theory, a union can be

performed on any two sets, but in relational algebra, a union can be performed only on

union-compatible tables.

Suppose we want to see all the projects from years 2002 and 2003.We obtain it by

performing a union (U) on the PROJ2002 and PROJ2003 tables as given in Figure 1-2.

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