PHP - 订阅符合搜索条件的新用户

I'm developing a job portal where users can subscribe to search criteria if no results are shown.

My form is based on these fields:

Field, education and experience

The only required field i field name, so education and experience is optional.

Imagine I have 2 entries in my notification table which by the way looks like this:

 id user_id field      education    experience
 33 1       Landbrug                        
 34 1       Landbrug   Yes

So what I want to do is have a script running (CRON) every half hour to do a search in the user fields table and see if there's a match somewhere, if yes, add email to mail queue (another table).

User fields table:

 field_id   user_id  field_title         field_education     field_experience   
 29         80       Landbrug            Yes                 Nogen
 30         80       IT                  No                  Nogen
 31         81                                      

I just can't figure out in my head how to do the proper SQL query, because I want to be able to find matches, even if the user only have 1 search criteria being "field" which is required, other criteria can be chosen to "all" or blank.

Any help is appreciated very much!