Question

我有这个问题：

SELECT customer.CustomerID, customer.Firstname, customer.LastName, orders.DateOrdered AS LastOrdered, DATEDIFF( CURDATE( ) , MAX( orders.DateOrdered ) ) AS DaysSinseOrdered
FROM customer
LEFT JOIN orders ON customer.CustomerID = orders.CustomerID
where orders.OrdersID IS NULL 
GROUP BY customer.CustomerID, customer.FirstName, customer.LastName
UNION
SELECT customer.CustomerID, customer.Firstname, customer.LastName, orders.DateOrdered AS LastOrdered, DATEDIFF( CURDATE( ) , MAX( orders.DateOrdered ) ) AS DaysSinseOrdered
FROM customer
JOIN orders ON customer.CustomerID = orders.CustomerID
GROUP BY customer.CustomerID, customer.FirstName, customer.LastName
ORDER BY DaysSinseOrdered DESC 
LIMIT 0 , 30;

显示我需要的信息，但是我可以在没有联合的情况下执行此操作。我希望它首先显示NULL值，然后显示应用了ORDER BY的第二个查询中的值。现在它将NULL值设为0，并命令所有结果购买ORDER BY;但是我希望它在较大的数字之前位于顶部，就像在这个背景下，它是自上次订购以来的日子，如果你从来没有订购过，那应该是我书中的顶部。

非常感谢任何帮助-Tom

Answer 1

您实际上不需要UNION来首先排序空值。您只需先将查询更改为ORDER BY：

SELECT customer.CustomerID, customer.Firstname, customer.LastName, orders.DateOrdered AS LastOrdered, DATEDIFF( CURDATE( ) , MAX( orders.DateOrdered ) ) AS DaysSinseOrdered
FROM customer
JOIN orders ON customer.CustomerID = orders.CustomerID
GROUP BY customer.CustomerID, customer.FirstName, customer.LastName
ORDER BY orders.OrdersID IS NULL DESC, DaysSinseOrdered DESC
LIMIT 0, 30

因此，此查询将首先排序空值，然后排序非空值。然后，它会将DaysSinseOrdered DESC作为二级订单 -

使用UNION时，ORDER BY仅影响1个查询

1 个答案: