在SQL中，如何计算UNION的结果？

Question

假设我有一个查询，例如

SELECT * from frequency where (...) 
UNION 
SELECT * from frequency where  (...)

如何计算从上面返回的结果数量？

Answer 1

将COUNT()函数与子查询一起使用

SELECT COUNT(*) AS Row_Counts
FROM 
(
  SELECT * from frequency where (...) 
  UNION 
  SELECT * from frequency where  (...)
) A

Answer 2

为什么UNION？如果你结合WHERE条款，你可以这样计算：

SELECT COUNT(*)
FROM frequency 
WHERE (...) OR (...)