当changin WHERE to =而不是＆gt; =时，MySQL SUM查询不起作用

Question

为什么这个查询总结来自不同表的不同值有效：

SELECT paymentMethodId, SUM(price) AS total FROM (
    SELECT paymentMethodId, price FROM flights WHERE status >= 5 
    UNION ALL
    SELECT paymentMethodId, price FROM cruises WHERE status >= 5 
    UNION ALL
    SELECT paymentMethodId, price FROM cars WHERE status >= 5 
    UNION ALL
    SELECT paymentMethodId, price FROM hotels WHERE status >= 5 
 ) alias GROUP BY paymentMethodId

和这个（仅在WHERE条件下更改）不会：

SELECT paymentMethodId, SUM(price) AS total FROM (
    SELECT paymentMethodId, price FROM flights WHERE status = 5 
    UNION ALL
    SELECT paymentMethodId, price FROM cruises WHERE status = 5 
    UNION ALL
    SELECT paymentMethodId, price FROM cars WHERE status = 5 
    UNION ALL
    SELECT paymentMethodId, price FROM hotels WHERE status = 5 
 ) alias GROUP BY paymentMethodId

我必须指出，只有在汽车桌上才有现有行。