带2个子查询的SELECT返回重复结果 - 如何修复？

Question

我有这个代码，它结合了订单详细信息，并为您提供订单商品的产品ID和变体ID。

SELECT (
    SELECT meta_value as product_id
    FROM ctc_woocommerce_order_itemmeta
    LEFT JOIN ctc_woocommerce_order_items ON ctc_woocommerce_order_itemmeta.order_item_id = ctc_woocommerce_order_items.order_item_id
    WHERE meta_key = '_product_id'
    AND order_id = 3989
) as product_id,
(
    SELECT meta_value as variation_id
    FROM ctc_woocommerce_order_itemmeta
    LEFT JOIN ctc_woocommerce_order_items ON ctc_woocommerce_order_itemmeta.order_item_id = ctc_woocommerce_order_items.order_item_id
    WHERE meta_key = '_variation_id'
    AND order_id = 3989
) as variation_id

结果如下：

----------------------------------
| product_id | variation_id      |
----------------------------------
| 3921       | 3922              |
----------------------------------

如果订单中只有1个项目，如果有多个项目（记录），如果我将第一个SELECT更改为SELECT * FROM，结果是：

----------------------------------
| product_id | variation_id      |
----------------------------------
| 17420       | 17422            |
| 17420       | 16963            |
| 17420       | 0                |
| 16961       | 17422            |
| 16961       | 16963            |
| 16961       | 0                |
| 33601       | 17422            |
| 33601       | 16963            |
| 33601       | 0                |
----------------------------------

我期待结果是：

----------------------------------
| product_id | variation_id      |
----------------------------------
| 17420       | 17422            |
----------------------------------
| 16961       | 16963            |
----------------------------------
| 33601       | 0                |
----------------------------------

我认为这与每个子查询的结果有关，但无法达到上述预期结果。怎么做？

Answer 1

不知何故，我想你想要这样的事情：

SELECT MAX(CASE WHEN oim.meta_key = '_product_id' THEN meta_value END) as product_id,
       MAX(CASE WHEN oim.meta_key = '_variation_id' THEN meta_value END) as variation_id
FROM ctc_woocommerce_order_itemmeta oim LEFT JOIN 
     ctc_woocommerce_order_items oi
     ON oim.order_item_id = oi.order_item_id
WHERE oi.order_id = 3989 AND oim.meta_key IN ('_product_id', '_variation_id')
GROUP BY oim.order_item_id