如何從兩個表中選擇平衡的隨機樣本記錄？

Question

爲了訓練機器學習模型，我必須檢索由平衡數量的當前用戶和以前用戶組成的用戶樣本。 Tha數據庫由表all_users和former_users組成。

不平衡樣品的情況下（100條記錄），下面的查詢與所需的列返回的記錄：

SELECT t1.user_property1, t2.user_property2, t3.valid_to FROM additional_info t1 LEFT JOIN all_users t2 ON t1.user_ID = t2.user_ID LEFT JOIN former_users t3 ON t1.user_ID = t3.user_ID ORDER BY random() LIMIT 100 

爲了獲得均衡的樣品，應該有一半的記錄與用戶被存儲在表以前的用戶和一半的表all_users，而這些用戶在同一時間不在表前用戶（否則樣本不會被平衡）。

有誰知道，從表additional_info的附加屬性沿表all_users和former_users從表all_users和former_users檢索平衡隨機樣本的最方便的方法是什麼？

謝謝！你可能會考慮做

Answer 1

做了以下內容：

(SELECT t1.user_property1, t2.user_property2, t3.valid_to FROM additional_info t1 LEFT JOIN all_users t2 ON t1.user_ID = t2.user_ID INNER JOIN former_users t3 ON t1.user_ID = t3.user_ID ORDER BY random() LIMIT 50) 
UNION 
(SELECT t1.user_property1, t2.user_property2, NULL FROM additional_info t1 LEFT JOIN all_users t2 ON t1.user_ID = t2.user_ID WHERE t1.user NOT IN (SELECT user_ID FROM former_users) ORDER BY random() LIMIT 50) 

，但一直在尋找更好的解決方案。

Answer 2

的一件事是：

Query 1 - SELECTS random non-former users joined to additional_info with a LIMIT of 50 
Query 2 - SELECTS random former users joined to additional_info with a LIMIT of 50 

，然後將結果與UNION

(Query 1) UNION (Query 2) 

這會給你隨機結果爲兩個標準相結合，共100個用戶。