即使“keywords”值为空，如何更改此查询以使其返回满足“minPrice”条件的记录？
我不知道这是否是 Google App Engine for Java 特有的问题，但如果设置为 keywords 参数的值是空字符串，则查询不会返回任何内容，即使如果设置了 minPrice。
即使 keywords 值为 null，如何更改此查询以使其返回满足 minPrice 条件的记录？理想情况下，我会以某种方式对这两个条件使用相同的查询，而无需基于空字符串条件创建单独的查询。
Query qry = entityManager.createQuery("SELECT p FROM Test p
WHERE keywords = :keywords and price >= :minPrice");
qry.setParameter("keywords", keywords);
qry.setParameter("minPrice", Integer.parseInt(minPrice));
I don't know if this is a problem that is specific to Google App Engine for Java, but if the value set as the keywords parameter is a null String, then nothing is returned from the query, even if a minPrice is set.
How do I change this query to make it return records that meet the minPrice condition even if the keywords value is null? Ideally I would somehow use the same query for both conditions without creating separate queries based on a null String condition.
Query qry = entityManager.createQuery("SELECT p FROM Test p
WHERE keywords = :keywords and price >= :minPrice");
qry.setParameter("keywords", keywords);
qry.setParameter("minPrice", Integer.parseInt(minPrice));
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这就是 GAE 数据存储的工作方式（顺便说一句，大多数关系数据库也以这种方式工作！）：空值不等于任何内容，因此您的
keywords = :keywords部分对于包含 nullkeywords的记录，查询为 false - 因为该部分为 false，所以and当然也是如此。您需要两个查询，一个用于
keywords = :keywords，另一个用于“is null”检查，并使用它们的两个不相交的结果集（Python GAE 在应用程序级别模拟“IN”运算符代码，我相信 Java GAE 不会，但由于在这种情况下集合是不相交的，所以无论如何它都没有什么神秘或困难;-)。编辑：它是 Python 中的模拟
IN（此处可用），不是OR； Java 中应用程序级模拟的IN等效项实际上是contains。It's the way the GAE datastore works (most relational databases work that way too, btw!): nulls are not equal to anything, so the
keywords = :keywordspart of your query is false on records with nullkeywords-- since that part is false, so is theand, of course.You'll need two queries, one for
keywords = :keywordsand one for the "is null" check, and use their two disjoint result sets (Python GAE simulates an "IN" operator in app-level code, which I believe Java GAE doesn't, but since the sets are disjoint in this case there's really no mystery or difficulty to it anyway;-).Edit: it's a simulated
IN(which would be usable here) in Python, notOR; the Java equivalent of that app-level-simulatedINis actuallycontains.