WHERE expression in Lookup wizard

Hi,
Please bear with me, this is probably a very basic question, but I'm not advanced in PHP/MySQL usage!
I have created an Add and Edit page for a particular table (Table 1). One of the editable fields is for dates, which I need to populate from a separate table (Table 2).
E.g. for simplicities sake:

Problem is, I only want dates from Table 2 to populate the dropdown list for Table 1 where table2.flag = 0, but if I use "flag=0" as the WHERE expression, it doesn't seem to work and ALL dates are output..?
Any help appreciated!
Sol