如何删除没有产品的分类？

Question

我对下面的SQL有点卡壳。

delete  from oc_category_description where 'category_id' NOT in (SELECT category_id FROM oc_product_to_category);
delete from oc_category_path where 'category_id' NOT in(SELECT category_id from oc_product_to_category);
delete from oc_category_to_store where 'category_id' NOT in(SELECT category_id from oc_product_to_category);
delete from oc_category_to_layout where 'category_id' NOT in(SELECT category_id from oc_product_to_category);
delete from oc_category_filter where 'category_id' NOT in(SELECT category_id from oc_product_to_category);
delete from oc_coupon_category where 'category_id' NOT in(SELECT category_id from oc_product_to_category);
delete from oc_category where 'category_id' NOT in(SELECT category_id from oc_product_to_category);

它删除了所有数据，忽略了 where 部分。我到底做错了什么？

Answer 1

你需要去掉列名周围的单引号，否则它就会变成一个文字字符串，而这个表达式并不能达到你想要的效果。实际上，它检查的是文字串是否为 'category_id' 属于子查询的resulset：因为它可能不属于，所以所有的记录都会被删除。

另外，我建议使用 exists，也就是 null-安全，而 not in 不是（也就是说，如果子查询中的任何值是 null，所有的行将被删除）。)

delete from oc_category 
where not exists (
    select 1 
    from oc_product_to_category pc 
    where pc.category_id = oc_category.category_id
);

为了保证性能，可以考虑在 oc_product_to_category(category_id) (或至少是本列先出现的复合索引)。