SELECT DISTINCT task_translation.*, (select content from "question_translation" where "question_translation"."question_id" = "task_translation"."question_id" and "question_translation".language_id = "task_translation".source_language_id) as source, (select content from "question_translation" where "question_translation"."question_id" = "task_translation"."question_id" and "question_translation".language_id = "task_translation".target_language_id) as target FROM "task_translation" WHERE ("task_translation"."status" = 3) AND ("task_translation"."editor_user_id" = 3) and source=1 ORDER BY "source" DESC LIMIT 20
SELECT DISTINCT task_translation.*, (select content from "question_translation" where "question_translation"."question_id" = "task_translation"."question_id" and "question_translation".language_id = "task_translation".source_language_id) as source, (select content from "question_translation" where "question_translation"."question_id" = "task_translation"."question_id" and "question_translation".language_id = "task_translation".target_language_id) as target FROM "task_translation" WHERE ("task_translation"."status" = 3) AND ("task_translation"."editor_user_id" = 3) and source=1 ORDER BY "source" DESC LIMIT 20
Потому что нельзя использовать алиасы из селекта в where