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  (...)

我如何计算从上面返回的结果数量？

Answer 1

使用COUNT()子查询功能

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