Ok I am very new to postgres so bear with me. I have been using the string_agg function in postgres to get my CSV. I want to know how to use my CSV in a query. I did this in SQL server by using a DECLARE but as I have read there is not a way to DECLARE globally in postgres. Any help, documentation, examples would be greatly appreciated.
Ok, so based on your question and your comment. PostgreSQL has ARRAY type which fits nicely in SQL statements, so you can make an array from your CSV-string and use it in queries like:
SELECT * FROM table WHERE record_id = ANY(string_to_array( string_agg , ','));
Actually there is no need to use CSV at all. If you have such option change to use array_agg in the first place (instead of string_agg).
Function string_to_array(text, delimiter) splits string into array with delimiter passed as an argument. Expression record_id = ANY( array ) returns TRUE when "any" of the array elements is equal to record_id and FALSE otherwise.