Database Programming with JDBC & Java, Second Edition

Advanced Datatypes

JDBC 1.x supported the SQL2 datatypes. JDBC 2.0 introduces support for more advanced datatypes, including the SQL3 “object” types and direct persistence of Java objects. Except for the BLOB and CLOB datatypes, few of these advanced datatype features are likely to be relevant to most programmers for a few years. While they are important features for bridging the gap between the object and relational paradigms, they are light years ahead of where database vendors are with relational technology and how people use relational technology today.

Blobs and Clobs

Stars of a bad horror film? No. These are the two most important datatypes introduced by JDBC 2.0. A blob is a B inary Large Object, and a clob is a C haracter Large Object. In other words, they are two datatypes designed to hold really large amounts of data. Blobs, represented by the BLOB datatype, hold large amounts of binary data. Similarly, clobs, represented by the CLOB datatype, hold large amounts of text data.

You may wonder why these two datatypes are so important when SQL2 already provides VARCHAR and VARBINARY datatypes. These two old datatypes have two important implementation problems that make them impractical for large amounts of data. First, they tend to have rather small maximum data sizes. Second, you retrieve them from the database all at once. While the first problem is more of a tactical issue (those maximum sizes are arbitrary), the second problem is more serious. Fields with sizes of 100 ...