MySQL：如何从多行中选择首选值（并回退到默认值）？

Question

我有以下查询，它返回附加到IN子句中指定的产品ID的组ID的列表。

SELECT t1.product_id, t2.group_id 
FROM table_1 t1 
LEFT JOIN table_2 t2 
ON t1.product_id = t2.product_id 
WHERE t1.product_id IN(1,2,3,4,5) 
AND t2.group_id = -1 
OR t2.group_id = 2 

这回是这样的：

+------------+----------+ 
| product_id | group_id | 
+------------+----------+ 
| 1   | -1  | 
| 1   | 2  | 
| 2   | -1  | 
| 3   | -1  | 
| 4   | -1  | 
| 5   | -1  | 
+------------+----------+ 

的GROUP_ID总是会有-1的默认值，但在某些情况下，可能有不止一个值（例如，PRODUCT_ID“ 1'具有'-1'和'2'的group_id）。我想要做的就是忽视了“-1”的值时，还存在另一种，所以我结束了这一点：

+------------+----------+ 
| product_id | group_id | 
+------------+----------+ 
| 1   | 2  | 
| 2   | -1  | 
| 3   | -1  | 
| 4   | -1  | 
| 5   | -1  | 
+------------+----------+ 

我用集团通过声明上的product_id，但我刚刚结束了group_id列中的-1值。

请有人可以给我一些关于我如何能够做到这一点的指针！

Answer 1

SELECT t1.product_id, MAX(t2.group_id) 
FROM table_1 t1 , table_2 t2 
WHERE t1.product_id IN(1,2,3,4,5) 
AND t1.product_id = t2.product_id 
AND t2.group_id = -1 
OR t2.group_id = 2 
GROUP BY (t1.product_id) 

试试这个。如果-1和2都可用，这将返回2。即，我只选择可用组ID的最大值。希望这是你所需要的是什么

Answer 2

SELECT t1.product_id, COALESCE(MAX(t2.group_id),-1) AS group_id -- if there is no group id default to -1 
FROM table_1 t1 
LEFT JOIN table_2 t2 
ON t1.product_id = t2.product_id 
WHERE t1.product_id IN(1,2,3,4,5) 
AND t2.group_id IN (-1, 2) 
GROUP BY t1.product_id 

Answer 3

你可以使用这个SQL：

SELECT t1.product_id, t2.group_id 
FROM table_1 t1 
LEFT JOIN (SELECT product_id, MAX(group_id) AS group_id 
      FROM table_2 
      WHERE group_id IN (-1,2) 
      GROUP BY product_id) t2 ON t1.product_id = t2.product_id 
WHERE t1.product_id IN (1,2,3,4,5) 

Answer 4

试试这个，

SELECT product_id, group_id= '-1' From products GROUP BY product_id HAVING COUNT(product_id) = 1 
UNION 
SELECT product_id, MAX(group_id) FROM products GROUP BY product_id HAVING COUNT(product_id) > 1