Its not possible in prompt. You can create column in criteria or in rpd with your condition and use that column in prompt.

Make your prompt province to set a presentation variable, for example var_province (details in  http://gerardnico.com/wiki/dat/obiee/presentation_variable_initialization). Then you can use this variable in sql query from prompt district. Like on screenshot.

To debug a sql expression in prompt you can use "Create analysis by entering simple logical SQL" in dropdown New Analysis.

Can you just try the default option of "Limit values by" all prompts?

When in prompt is selected "all column values" to variable is assigned in some cases empty string (''), and in some cases (mostly in answer) null value (ifnull(@{var}, 0) = 0) regardless default value defined in variable expression.

For example prompt sets variable p_var. When you select "all column values" the expression '@{p_var}{default value}' will equals string (''):

'@{p_var}{default value}' = ''

Have you tried to debug a sql expression in prompt in answer from logical sql query?

Please, post here your logical SQL.

.. That you´re using in the prompt

Thanks