Assuming I work on the PHP-based DBMS, I believe the main parts of the projects are (for version 1):

* Framework to handle database and table objects

* Framework to handle table structures
+ Make a UNIQUE PRIMARY KEY essential
+ Support AUTOINCREMENT field

* SQL interpreter engine
+ Support simple SELECT statements
+ Support SELECT statement with WHERE clause
+ Support AND and OR operators
+ Support EQUALITY, NON-EQAULITY and LIKE operators
+ Support simple INSERT statement
+ Support simple DELETE statement
+ Support WHERE clause in DELETE statement
+ Support simple UPDATE statement
+ Support WHERE clause in UPDATE statement

* SQL framework to validate SQL statements
+ Malformed statements - syntax errors
+ Invalid statments - reference to non-existent fields

* Framework support for concurrency issues
+ Have some sort of concurrency handling

This could be quite work-intense. Hopefully I manage my time well enough, so that I don't detest this project (by working too hard on it) and stop working on it and I still have free time to do other "completely relaxing" things. But the prospect of creating something like this fills with me excitement. :)

I look forward to this.