Hi there !
Many thanks to kev1n and Alohajoe5 for patience in this discussion - it helped me very much with coding my dropdown column filters ! Thank you !
But I came to a point, where I'm not able to continue. I'm trying to construct three static dropdown filters for columns which are getting data from joined tables. Very likely I have incorrect constructed my SQL statement, because I can see dropdown lists with values, but the browse form returns blank set of records

I made many attempts with the SQL statement, but no success
Can U help me pls. ?
My situation looks like this:
Javascript variables:
if (nuFormType() == 'browse') {
var data1 = ["", "Mia", "Pelc Jan", "Bartl Zdeněk"]; // static values to be added to the dropdown
addBrowseTitleDropDown(1, data1); // add dropdown to column 2 (index 1)
var data4 = ["", "beletria", "historický", "zdravie"]; // static values to be added to the dropdown
addBrowseTitleDropDown(4, data4); // add dropdown to column 5 (index 4)
var data5 = ["", "Tlač", "E-kniha", "Audio-kniha"]; // static values to be added to the dropdown
addBrowseTitleDropDown(5, data5); // add dropdown to column 6 (index 5)
}
...
SQL statement:
SELECT
tbl_knihy.*,
tbl_autori.autor AS autor,
tbl_kniky.rok,
tbl_formaty.format,
tbl_jazyky.jazyk,
tbl_kategorie.kategoria AS kategoria,
tbl_media.medium AS medium,
tbl_serie.seria
FROM
tbl_knihy
JOIN tbl_autori ON tbl_knihy.tbl_autori_id = tbl_autori.aut_id
JOIN tbl_formaty ON tbl_knihy.tbl_formaty_id = tbl_formaty.for_id
JOIN tbl_jazyky ON tbl_knihy.tbl_jazyky_id = tbl_jazyky.jaz_id
JOIN tbl_kategorie ON tbl_knihy.tbl_kategorie_id = tbl_kategorie.kat_id
JOIN tbl_media ON tbl_knihy.tbl_media_id = tbl_media.med_id
JOIN tbl_serie ON tbl_knihy.tbl_serie_id = tbl_serie.ser_id
WHERE
((autor = '#nuBrowseTitle1_dropdown#' AND LOCATE('#', '#nuBrowseTitle1_dropdown#') <> 1 )
OR '#nuBrowseTitle1_dropdown#' = '' OR LOCATE('#', '#nuBrowseTitle1_dropdown#') = 1)
AND
((kategoria = '#nuBrowseTitle4_dropdown#' AND LOCATE('#', '#nuBrowseTitle4_dropdown#') <> 1 )
OR '#nuBrowseTitle4_dropdown#' = '' OR LOCATE('#', '#nuBrowseTitle4_dropdown#') = 1)
AND
((medium = '#nuBrowseTitle5_dropdown#' AND LOCATE('#', '#nuBrowseTitle5_dropdown#') <> 1 )
OR '#nuBrowseTitle5_dropdown#' = '' OR LOCATE('#', '#nuBrowseTitle5_dropdown#') = 1)
ORDER BY
tbl_knihy.nazov ASC