它从外部查询中获取 hs_id 的值.

It is taking the value of hs_id from the outer query.

在其 select 列表中没有从选定表中投影任何列的查询是完全有效的.

It is perfectly valid to have a query that doesn't project any columns from the selected table in its select list.

例如

select 10 from HotelSupplier where id = 142

将返回一个结果集，其中包含与 where 子句匹配的行数以及所有行的值 10.

would return a result set with as many rows as matched the where clause and the value 10 for all rows.

不合格的列引用从最近的范围向外解析，因此这只是被视为相关的子查询.

Unqualified column references are resolved from the closest scope outwards so this just gets treated as a correlated sub query.

只要 HotelSupplier 至少有一行 id = 142(并且所以子查询至少返回一行)

The result of this query will be to delete all rows from Photo where hs_id is not null as long as HotelSupplier has at least one row where id = 142 (and so the subquery returns at least one row)

如果你考虑一下这样做的效果可能会更清楚一点

It might be a bit clearer if you consider what the effect of this is

delete from Photo  where Photo.hs_id  in (select Photo.hs_id)

这当然等价于

delete from Photo where Photo.hs_id = Photo.hs_id

顺便说一句，这是我个人看到的 Microsoft Connect 上错误报告的最常见的错误".Erland Sommarskog 将其包含在他的 wishlist 中，用于 SET STRICT_CHECKS ON

By the way this is far and away the most common "bug" that I personally have seen erroneously reported on Microsoft Connect. Erland Sommarskog includes it in his wishlist for SET STRICT_CHECKS ON