查询左连接取数据问题-Mysql

Question

我用左连接连接两个表。当我用 Select 全选时，所有行都来了（query1）。但是当我使用 select 拉出一些行时，我只加载了一个数据，而不是图 1 中的所有数据都加载了。是什么原因？

正在加载所有数据查询1

查询1：

SELECT * 
FROM oc_review AS OYLAMA 
JOIN oc_product AS PRODUCT 
    ON (PRODUCT.product_id = OYLAMA.product_id)
INNER JOIN oc_product_to_category AS KATEGORI 
    ON (KATEGORI.product_id = PRODUCT.product_id) 
WHERE KATEGORI.category_id = 69

仅加载 1 个数据查询2

查询2：

SELECT
    AVG(ALL OYLAMA.rating) AS ORTALAMA,
    COUNT(OYLAMA.rating) AS TOPLAMOY 
FROM oc_review AS OYLAMA 
JOIN oc_product AS PRODUCT
    ON (PRODUCT.product_id = OYLAMA.product_id) 
INNER JOIN oc_product_to_category AS KATEGORI
    ON (KATEGORI.product_id = PRODUCT.product_id) 
WHERE KATEGORI.category_id = 69

Answer 1

一旦你引入聚合函数，如AVG() 和COUNT()，MySQL 就会创建一个单独的组，所有记录都会放在其中。另请参阅问题MySQL Aggregate Functions without GROUP BY clause 了解更多信息。

您可能想要做的是明确指定GROUP BY 子句，并将GROUP BY 之后的字段也添加到SELECT，例如：

SELECT 
   PRODUCT.product_id, 
   AVG(ALL OYLAMA.rating) AS ORTALAMA,
   COUNT(OYLAMA.rating) AS TOPLAMOY 
FROM oc_review AS OYLAMA 
JOIN oc_product AS PRODUCT 
   ON(PRODUCT.product_id = OYLAMA.product_id) 
INNER JOIN oc_product_to_category AS KATEGORI 
   ON (KATEGORI.product_id = PRODUCT.product_id) 
WHERE KATEGORI.category_id = 69
GROUP BY PRODUCT.product_id