Jim Melton

A critique of ANSI SQL isolation levels

ANSI SQL-92 [MS, ANSI] defines Isolation Levels in terms of phenomena: Dirty Reads, Non-Repeatable Reads, and Phantoms. This paper shows that these phenomena and the ANSI SQL definitions fail to properly characterize several popular isolation levels, including the standard locking implementations of the levels covered. Ambiguity in the statement of the phenomena is investigated and a more formal statement is arrived at; in addition new phenomena that better characterize isolation types are introduced. Finally, an important multiversion isolation type, called Snapshot Isolation, is defined.

Database Language SQL

SQL, a data sublanguage used to access relational databases, is sometimes described as “English-like” because many of its statements read a bit like English. It is a non-procedural language since complex data operations are formulated by specifying their intended result rather than the method used to obtain that result. Both ANSI and ISO have published three generations of the de jure SQL standards. The syntax and semantics of SQL is examined and the conformance requirements are stated; a few components of the language are considered in greater detail and the future of the language is outlined.

SQL:2003 has been published

SQL:2003 has finally achieved final publication as an International Standard, replacing SQL:1999. SQL:2003 is popularly believed to be largely a “bugfix release” of the SQL standard — except, of course, for the SQL/XML work on which we have previously reported. However, as you will learn from this and future columns, there are many compelling new features in the 2003 edition of the SQL standard. We are pleased that three of the more active SQL proposal writers have joined forces to present several of those new features in this month’s column. Krishna Kulkarni is, among other responsibilities, the formal International Representative for the INCITS H2 Technical Committee for Database. Jan-Eike Michels is a frequent USA representative to the corresponding international group, ISO/IEC JTC1/SC32/WG3. Fred Zemke is widely acknowledged as a principle expert in many areas of the SQL standard and also a regular USA representative to WG3. In the coming months, we will provide information about even more of SQL:2003’s new features.

SQL multimedia and application packages (SQL/MM)

Regular readers of this column will have become familiar with database language SQL -- indeed, most readers are already familiar with it. We have also discussed the fact that the SQL standard is being published in multiple parts and have even discussed one of those parts in some detail[l].Another standard, based on SQL and its structured user-defined types[2], has been developed and published by the International Organization for Standardization (ISO). This standard, like SQL, is divided into multiple parts (more independent than the parts of SQL, in fact). Some parts of this other standard, known as SQL/MM, have already been published and are currently in revision, while others are still in preparation for initial publication.In this issue, we introduce SQL/MM and review each of its parts, necessarily at a high level.

SQL and management of external data

In late 2000, work was completed on yet another part of the SQL standard [1], to which we introduced our readers in an earlier edition of this column [2].Although SQL database systems manage an enormous amount of data, it certainly has no monopoly on that task. Tremendous amounts of data remain in ordinary operating system files, in network and hierarchical databases, and in other repositories. The need to query and manipulate that data alongside SQL data continues to grow. Database system vendors have developed many approaches to providing such integrated access.In this (partly guested) article, SQLs new part, Management of External Data (SQL/MED), is explored to give readers a better notion of just how applications can use standard SQL to concurrently access their SQL data and their non-SQL data.

Advancements in SQL/XML

Since we last wrote about SQL/XML in [2], the first edition of that new part of the SQL standard has been officially published as an international standard [1], commonly called SQL/XML:2003. At the time of that earlier column, SQL/XML was just entering its first official ballot, meaning that (possibly significant) changes to the text were expected in response to ballot comments submitted by the various participants in the SQL standardization process.

The WS-DAI family of specifications for web service data access and integration

This month, we are pleased to provide to our readers a column that addresses an important aspect of grid computing: data access.

Standards for databases on the grid

Relational databases supported applications in a centralized environment in the 1960s and 1970s. They progressed to a client/server environment in the 1980s. The 1990s saw application servers with a multi-tiered architecture, in most cases supported by an RDBMS. Most recently we have seen the emergence of XML, XML storage in DBMSs, navigation within an XML document via XPath, and the XQuery query language for XML. In this article, Susan provides an introduction to the Grid and describes how databases will be used in this new environment. The Global Grid Forum (GGF) is producing technical specification to enable both Relational and XML databases to be located, accessed, and replicated in this environment. They make use of a variety of existing an emerging database, file, networking, and web services standards. Susan is a Senior Technical Staff Member at IBM. She is a member of the GGF DAIS (Data Access and Integration Services) working group. Jim Melton and Andrew Eisenberg

SQL/MED: a status report

In March, 2001, we delivered a (partly) guested column covering the topic of Management of External Data [1]. The column you are reading right now reports on the on-going development of the SQL/MED standard and is authored by all but one of the authors of that earlier column. We trust that our readers will benefit from this update on an interesting and important part of SQL. Jim Melton and Andrew Eisenberg

XML schema

This month’s column deals with metadata for XML, primarily the W3C’s XML Schema Recommendation. XML Schema is often seen as highly complex, but quite powerful. We have worked with Chuck Campbell in several standards arenas, including the SQL standard and XML Query. Chuck is an invited expert to the W3C’s XML Schema WG, so we invited him to write a column outlining the features and futures of XML Schema. Jim Melton and Andrew Eisenberg