如何比较 3 个或更多表的行数并返回 BOOLEAN 值？

Question

我可以得到一个 BOOLEAN 结果来说明两个表是否具有与此问题和答案相同的行数： Compare row count of two tables in a single query and return boolean

SELECT 
       CASE WHEN (select count(*) from table1)=(select count(*) from table2)
       THEN 1
       ELSE 0
END AS RowCountResult

我已经用 3 张桌子试过了：

SELECT 
    CASE WHEN (SELECT count(*) from personal)=(SELECT count(*) from exam)=(SELECT COUNT(*) from contact)    
THEN 1 
ELSE 0 
END AS RowCountResult 

但返回 0，而 3 个表实际上具有相同的行数，所以它应该返回 1。任何帮助表示赞赏！

Answer 1

像这样的表达式：

a = b = c

被评估（从左到右）为：

(a = b) = c

并且由于(a = b)是一个 boolean 表达式，它被评估为0或1 ，所以你最终得到1 = c或0 = c的评估（在你的情况下最可能是）8 总是false的。

您应该分别比较a到b和b到c并使用运算符AND获得最终结果，例如：

a = b AND b = c

所以，你的代码应该是：

SELECT 
  (SELECT count(*) from personal) = (SELECT count(*) from exam)
  AND
  (SELECT count(*) from exam) = (SELECT COUNT(*) from contact) AS RowCountResult

或者更好的子查询：

SELECT count1 = count2 AND count2 = count3 AS RowCountResult 
FROM (
  SELECT (SELECT count(*) from personal) count1,
         (SELECT count(*) from exam) count2,
         (SELECT COUNT(*) from contact) count3    
) t 

您实际上并不需要CASE表达式。

Answer 2

那是因为您将比较结果与第三行数进行比较，即 A=B（将给出真或假）与 C（在这种情况下是一个数字）。您应该得到您所期望的，例如（ A=B)=(B=C)，或者，在你的情况下

SELECT 
    CASE WHEN 
    ((SELECT COUNT(*) FROM Personal) = (SELECT COUNT(*) FROM Exam))
    = ((SELECT COUNT(*) FROM Contact) = (SELECT COUNT(*) FROM Exam))    
    THEN 1 
    ELSE 0 
    END 
AS RowCountResult