MYSQL 查询未返回正确的行集

Question

我有两个表:

table_subcategories - sub_categories_id, sub_category_name, sub_category_image_path
table_subcategory_categories - subcategory_id, category_id

我想从 table_subcategories 中获取所有子类别，这些子类别在具有给定 Category_id 的“table_subcategory_categories”中不存在映射条目

我正在使用此查询，但结果不正确。

my $sql_query = "SELECT  TSC.sub_categories_id, TSC.sub_category_name, TSC.sub_category_image_path  from table_subcategories as TSC 
INNER JOIN
table_subcategory_categories as TSCC
ON
TSC.sub_categories_id = TSCC.subcategory_id
WHERE
TSCC.category_id != $cId ";

有人可以帮我解决一下这个问题吗？

Answer 1

使用左连接并检查是否没有匹配项:

SELECT TSC.* 
from table_subcategories TSC LEFT JOIN
     table_subcategory_categories  TSCC
     ON TSC.sub_categories_id = TSCC.subcategory_id AND
        TSCC.Category_Id = $cId
WHERE TSCC.category_id IS NULL;