您可以使用CTE：
WITH cte AS ( INSERT INTO "user"(timestamp, user_id, member_id) values ($1, $2, $3) ON ConFLICT (user_id, member_id) DO NOTHING RETURNING user_id)SELECt NULL AS resultWHERe EXISTS (SELECT 1 FROM cte) -- successUNIOn ALLSELECt id FROM "user" WHERe user_id = $2 AND NOT EXISTS (SELECt 1 FROM cte); -- conflict
DBFiddle演示
