限制跨表的結果總數

Question

我想獲取最近發生的 100 個事件。 事件分散在多個表中。 這是一個例子：

SELECT * FROM log_items_purchased ORDER BY timestamp DESC LIMIT 100
UNION
SELECT * FROM log_items_fulfilled ORDER BY timestamp DESC LIMIT 100
UNION
SELECT * FROM log_items_shipped ORDER BY timestamp DESC LIMIT 100

這將返回最多 300 條記錄。 然后，我將獲取結果集，按時間戳排序，並獲取前 100 條記錄。 如何在單個 SQL 查詢中執行此操作，其中 SQL 將僅返回結果集中的 100 條記錄。

我意識到可以通過從每個查詢中刪除 LIMIT 100 來執行此操作，然后進行添加 LIMIT 100 的外部查詢，但是這些表確實很大，而且效率很低。

Answer 1

將其放在子查詢中，然后在主查詢中使用LIMIT 100 。

SELECT *
FROM (
    SELECT * FROM log_items_purchased ORDER BY timestamp DESC LIMIT 100
    UNION
    SELECT * FROM log_items_fulfilled ORDER BY timestamp DESC LIMIT 100
    UNION
    SELECT * FROM log_items_shipped ORDER BY timestamp DESC LIMIT 100
) AS x
ORDER BY timestamp DESC
LIMIT 100

Answer 2

如果您想在 SQL 中執行此操作，請使用子查詢：

SELECT e.*
FROM ((SELECT * FROM log_items_purchased ORDER BY timestamp DESC LIMIT 100
      ) UNION ALL
      (SELECT * FROM log_items_fulfilled ORDER BY timestamp DESC LIMIT 100
      ) UNION ALL
      (SELECT * FROM log_items_shipped ORDER BY timestamp DESC LIMIT 100
      )
     ) e
ORDER BY timestamp DESC
LIMIT 100;

注意：不要使用UNION 。 刪除重復項會產生開銷。

Answer 3

我認為這會奏效

(SELECT * FROM log_items_purchased ORDER BY timestamp DESC LIMIT 100)
UNION
(SELECT * FROM log_items_fulfilled ORDER BY timestamp DESC LIMIT 100)
UNION
(SELECT * FROM log_items_shipped ORDER BY timestamp DESC LIMIT 100)
ORDER BY timestamp DESC LIMIT 100