我可以在TIMESTAMPDIFF中使用字段作为"单位"吗？

Question

这是另一个问题的后续问题,但我认为值得提出自己的问题:

我可以使用字段作为'unit'参数TIMESTAMPDIFF()吗？

例如,这是一个片段:

SELECT foo FROM table t LEFT JOIN frequencies f USING (frequency_id)
WHERE MOD(TIMESTAMPDIFF(f.frequency_code, t.date, NOW()), t.frequency_num) = 0

运行它时会出现语法错误.现在,如果我取代f.frequency_code与实际字- MINUTE,DAY等-它的工作原理.但这些正是目前包含的词语f.frequency_code.

是否只能在该位置使用表字段？

Answer 1

TIMESTAMPDIFF() 不支持这样的动态单位.

由于您使用的是一组已知单位,因此您可以使用CASE语句来实现此目的.

从您的示例查询开始,这样的事情可能会起作用:

SELECT foo 
FROM table t 
LEFT JOIN frequencies f USING (frequency_id)
WHERE MOD(
(CASE 
  WHEN f.frequency_code = 'MICROSECOND' THEN TIMESTAMPDIFF(MICROSECOND, t.date, NOW())
  WHEN f.frequency_code = 'SECOND' THEN TIMESTAMPDIFF(SECOND, t.date, NOW())
  WHEN f.frequency_code = 'MINUTE' THEN TIMESTAMPDIFF(MINUTE, t.date, NOW())
  WHEN f.frequency_code = 'HOUR' THEN TIMESTAMPDIFF(HOUR, t.date, NOW())
  WHEN f.frequency_code = 'DAY' THEN TIMESTAMPDIFF(DAY, t.date, NOW())
  WHEN f.frequency_code = 'WEEK' THEN TIMESTAMPDIFF(WEEK, t.date, NOW())
  WHEN f.frequency_code = 'MONTH' THEN TIMESTAMPDIFF(MONTH, t.date, NOW())
  WHEN f.frequency_code = 'QUARTER' THEN TIMESTAMPDIFF(QUARTER, t.date, NOW())
  WHEN f.frequency_code = 'YEAR' THEN TIMESTAMPDIFF(YEAR, t.date, NOW())
  END)
, t.frequency_num) = 0