7.1 Introduction

As we've said before, this is not a book about SQL per se, but is a book about using SQL and Java together. Why, then, should you read this chapter a chapter that covers SQL's user-defined types with very little attention to Java? In fact, one aspect of using SQL and Java together, which you'll encounter in Chapter 8, allows application developers to use Java classes within SQL database systems and, under some circumstances, to use them just as though they were SQL's own UDTs! This facility has proven to be both powerful and popular among the users of certain products, but understanding its use and appeal depends in part on an understanding of the SQL approach to user-defined types. In addition, the new version of JDBC (JDBC 2.0), which we'll discuss in Chapter 9, provides the ability to map SQL user-defined types to Java classes. Understanding SQL's UDTs will therefore enable you to take better advantage of JDBC.

In this chapter, we'll tell you about user-defined types in general and about SQL's approach to them. We're not going to go into excruciating detail about SQL's UDT capabilities (there are other books that cover this subject comprehensively or at least there will be shortly after publication of the book you have in your hands right now). Instead, we'll give a solid overview of the facility and the supporting technology in SQL. We expect that this will be enough to help you understand the relationship between SQL's type system and Java's,...