With any conditional query you need to consider the case where there is nothing to consider; and then add AND's to the query for each additional argument. say you have a form with a checkbox's that implies you only want records from certain days(if checked, or from all if none are checked). If none are checked; you might want to write this:
SELECT *
FROM tbl
but here you have no way to deal with the situations where the user has checked the boxes. The correct way would be to write your default SELECT statment with a WHERE clause that is always true, WHERE 1=1; and then dynamically add statements to this using AND after the first WHERE 1=1. Ex:
if user wants records from MONDAY
SELECT *
FROM tbl
WHERE 1=1
AND DAY='MONDAY'
if user wants all of the records
SELECT *
FROM tbl
WHERE 1=1
if user wants records from MONDAY,TUESDAY, and FRIDAY
SELECT *
FROM tbl
WHERE 1=1
AND DAY='MONDAY'
AND DAY='TUESDAY'
AND DAY='FRIDAY'