Question

SELECT distinct text FROM oc_product_attribute as pa
LEFT JOIN oc_product_to_category as ptc ON  pa.product_id = ptc.product_id
LEFT JOIN oc_product as p ON pa.product_id = ptc.product_id
WHERE p.status = 1 AND ptc.category_id = 61 
GROUP BY pa.text LIMIT 10

虽然我使用LIMIT，但我的数据库挂起以进行上述查询。有什么方法可以优化它吗？

Answer 1

在此行中，您应使用别名p而不是ptc

LEFT JOIN oc_product as p ON pa.product_id = ptc.product_id

应该是

LEFT JOIN oc_product as p ON pa.product_id = p.product_id

Answer 2

我认为此查询中不需要DISTINCT和GROUP BY，因为您无论如何都要使用where子句中的两个表。另外，因为您只是选择文本列。您可以考虑将查询重写为：

SELECT 
    pa.text 
FROM 
    oc_product_attribute as pa
WHERE EXISTS
(
    SELECT
        NULL
    FROM
        oc_product_to_category as ptc 
        JOIN oc_product as p 
            ON p.product_id = ptc.product_id
    WHERE  
        pa.product_id = ptc.product_id
        AND p.status = 1 
        AND ptc.category_id = 61
)
LIMIT 10

优化sql多个左连接和where子句

2 个答案: