In this new posting from Xaprb's blog today, there's a look at four different types of database abstraction layers, each with their own unique strengths.

Quite a few people have chimed in on a recent discussion about PHP, MySQL, database abstraction layers, and performance. I think enough viewpoints have been covered that I don't need to comment, but one question I don't see answered is "what are the qualities of a good SQL abstraction layer?" I think it's a very interesting — and complicated — question. As it turns out, the term has several meanings, and I think it's important to understand them.

The four types he lists are:

• Libraries that provide access to a database
• Libraries that present a common interface to different server software
• Libraries that write portable SQL
• Object-relational mapping software

Each has their own description and example packages that show the strength. Following these notes, he shares some opinions on each, mentioning his likes and dislikes about the state of support for them.