【正文】 Because the JDBC API will be used as a base API for developing higherlevel database access tools and APIs, it also has to address the problem of conformance for anything built on it. The designation JDBC Compliant(TM) was created to set a standard level of JDBC functionality on which users can rely. In order to use this designation, a driver must support at least ANSI SQL2 Entry Level. (ANSI SQL2 refers to the standards adopted by the American National Standards Institute in 1992. Entry Level refers to a specific list of SQL capabilities.) Driver developers can ascertain that their drivers meet these standards by using the test suite available with the JDBC API. The JDBC Compliant(TM) designation indicates that a vendor39。 The escape syntax provides a standard JDBC syntax for several of the more mon areas of SQL divergence. For example, there are escapes for date literals and for stored procedure calls. For plex applications, JDBC deals with SQL conformance in a third way. It provides descriptive information about the DBMS by means of the interface DatabaseMetaData so that applications can adapt to the requirements and capabilities of each DBMS. Typical end users need not worry about metadata, but experts may want to refer to the overview in ``DatabaseMetaData.39。s robustness, multithreading, and security features. Of course, JDBC is important in allowing database access from a Java middle tier. SQL Conformance Structured Query Language (SQL) is the standard language for accessing relational databases. Unfortunately, SQL is not yet as standard as one would like. One area of difficulty with SQL conformance is that although most DBMSs use a standard form of SQL for basic functionality, they do not conform to the more recentlydefined standard SQL syntax or semantics for more advanced functionality. For example, not all databases support stored procedures or outer joins, and those that do are not consistent with each other. It is hoped that the portion of SQL that is truly standard will expand to include more and more functionality. In the meantime, however, the JDBC API must support SQL as it is. One way the JDBC API deals with this problem is to allow any query string to be passed through to an underlying DBMS driver. This means that an application is free to use as much SQL functionality as desired, but it runs the risk of receiving an error on some DBMSs. In fact, an application query need not even be SQL, or it may be a specialized derivative of SQL designed for specific DBMSs (for document or image queries, for example). A second way JDBC deals with problems of SQL conformance is to provide ODBCstyle escape clauses, which are discussed in the overview for ``Statement.39。39。s SQL statements are delivered to the database, and the results of those statements are sent back to the user. The database may be located on another machine to which the user is connected via a work. This is referred to as a client/server configuration, with the user39。39。39。 in fact, both interfaces are based on the X/Open SQL CLI (Call Level Interface). The big difference is that JDBC builds on and reinforces the style and virtues of Java, and, of course, it is easy to use. More recently, Microsoft has introduced new APIs beyond ODBC: RDO, ADO, DAO, and OLE DB. These designs move in the same direction as JDBC in many ways, for example, in being objectoriented interfaces to databases based on classes that can be implemented on ODBC. However, we did not see functionality in any of these interfaces pelling enough to make them an alternative basis to ODBC, especially with the ODBC driver market wellestablished. Mostly they represent a thin veneer on ODBC. This is not to say that JDBC does not need to evolve from the initial release described in this book。39。39。39。. More sophisticated mappings are also provided, for example, where rows of multiple tables are bined in a Java class. As interest in JDBC has grown, more developers have been working on JDBCbased tools to make building programs easier, as well. Programmers have also been writing applications that make accessing a database easier for the end user. For example, an application might present a menu of database tasks from which to choose. After a task is selected, the application presents prompts and blanks for filling in information needed to carry out the selected task. With the requested input typed in, the application then automatically invokes the necessary SQL mands. With the help of such an application, users can perform database tasks even when they have little or no knowledge of SQL syntax. JDBC versus ODBC and other APIs At this point, Microsoft39。 the r