A SQL based question in join column does not show up

When I have made a SQL based question and then trying to make a join based on the question using custom query view, for some reason only 1 column shows up as available for join, while in actual fact the key columns I included should be the right column instead, may I know whats the reason? My source data is from PostgreSQL.

Hi @goodwill
Please post “Diagnostic Info” from Admin > Troubleshooting.
What’s the type of column from the SQL and the view?