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It's pretty difficult to help you with a lot of this stuff as as it's very dependent on the database structure and contents.
Your preview might not contain data because you're joining incorrectly. Perhaps you need to have a left join instead of inner, or maybe you're joining on the wrong fields.
For your single table calculation for "I need to count the numbers of photographers (photographers_id) when is_trial = 1", try COUNTD(IF _trial = 1 THEN photographers_id)
thanks for your answer. The COUNTD function worked. Concerning the no shown data problem, I think it is due to the order of the joined tables and also the join type. I am still figuring out which table to start with and which join type to use because the functions are working very well and correctly when using the single table option but if I use the multiple tables option the calculations are not working correctly anymore.
Thanks for your help again.