SQL Cartesian选择框

Question

I have a PHP form that uses several select boxes that are populated by lookup tables. In an effort to reduce the number of queries per page, I'm trying to find a way to create a cartesian product that would output the results like:

-----------------------------------------------------------------
|        City       |   State    |    SchoolDistrict            |
-----------------------------------------------------------------
| Adamsville        |     AL     |   Adamsville  Public Schools |
| Brownsville       |     AK     |   Baker-Cow USD              |
| Cowtown           |     AR     |   NULL                       |
| NULL              |     AZ     |   NULL                       |
| NULL              |     CA     |   NULL                       |
-----------------------------------------------------------------

Then I could just loop through each field until it found null (or 0-length string).

When I tried:

SELECT a.City, b.State, c.SchoolDistrict FROM Cities a, State b, SchoolDistricts c

it returns:

-----------------------------------------------------------------
|        City       |   State    |    SchoolDistrict            |
-----------------------------------------------------------------
| Adamsville        |     AL     |   Adamsville  Public Schools |
| Brownsville       |     AK     |   Baker-Cow USD              |
| Cowtown           |     AR     |   Baker-Cow USD              |
| Cowtown           |     AZ     |   Baker-Cow USD              |
| Cowtown           |     CA     |   Baker-Cow USD              |
-----------------------------------------------------------------

Any ideas?

Answer 1

One solution that I have come up with is

SELECT a.City, "City" type from Cities a 
UNION ALL
SELECT b.State, "State" from States b 
UNION ALL
SELECT c.SchoolDistrict, "SchoolDistrict" from SchoolDistricts c 

This would be one query and you can check the value of the second column and do necessary operations.