There is also a more advanced Google Gears database query tool here at: http://www.xcdsql.org/GIQT/ based on a YUI interface. That example demonstrates how to use the Google Gears workerpool to execute Google Gears Database queries and send chunks of the result in JSON format to the YUI data table while the remainder of the query is being executed in the workerpool. (Currently does not work on IE - let me know if you have fix ;-).
Enter an SQL statement in the textarea below and hit the "Execute" button.
Use the "List All" button to get a list of available tables.
Use Explain to show the query plan for a SELECT statement.