MySQL從多個表中選擇一條記錄

Question

我有400多個表，每個表都有列時間戳。 我想為這些表中的每一個拉時間戳的第一行/記錄。 我不想運行400多個查詢，最好的方法是什么？

Answer 1

您可以這樣運行UNION查詢：

SELECT Id, yourColumn, timestamp FROM Table001 ORDER BY timestamp DESC LIMIT 1
UNION ALL
SELECT Id, yourColumn, timestamp FROM Table002 ORDER BY timestamp DESC LIMIT 1
UNION ALL
SELECT Id, yourColumn, timestamp FROM Table003 ORDER BY timestamp DESC LIMIT 1
ORDER BY timestamp 

使用循環創建此語句：

SET @resultQuery = NULL;
SELECT
  GROUP_CONCAT(
    DISTINCT
    CONCAT('SELECT * FROM ', table_name, ' ORDER BY timestamp DESC LIMIT 1 ')
    SEPARATOR '\r\nUNION\r\n'
  )
INTO
  @resultQuery
FROM
  information_schema.COLUMNS
WHERE
  table_schema = '???' AND column_name LIKE '%timestamp';

SELECT @resultQuery;

Answer 2

需要從400多個表中獲得第一條記錄看起來有點糟糕的設計，但是如果它們具有相同的架構，則可以使用UNION ALL ：

SELECT * FROM foo ORDER BY timestamp ASC LIMIT 1
UNION ALL
SELECT * FROM bar ORDER BY timestamp ASC LIMIT 1
UNION ALL
SELECT * FROM baz ORDER BY timestamp ASC LIMIT 1
-- etc...