Confusing about JPQL

问题

I want to JPQL likes following example:

Let Hotel object has two objects of Customer and Organization. If Customer is not null, organization will be null. And if the Customer is null , organization will not be null. So

I JPQL likes this;

Select h.customer.name.firstName, h.organization.name from Hotel h

i want to assign one of these two result only to one variable because one is always null. Thanks and Sorry for my english.

回答1:

Probably, you are looking for COALESCE() function. Try the following:

SELECT COALESCE(h.customer.name.firstName, h.organization.name) FROM Hotel h

You also can use CASE expression if you want.

More details: http://docs.jboss.org/hibernate/orm/4.2/devguide/en-US/html/ch11.html#d5e3257

回答2:

You can select both fields & then can check for null values in code, else you can try using case statement, selecting non-null value.

SELECT CASE WHEN (h.customer.name.firstName NOT NULL) THEN h.customer.name.firstName
            WHEN (h.organization.name NOT NULL) THEN h.organization.name 
            ELSE DEFAULT_VALUE 
       END 
FROM Hotel h

[I haven't tried executing it, you can try it with modifications as required]

来源：https://stackoverflow.com/questions/23774916/confusing-about-jpql