从数据库表中随机选择实体？

Question

How would I randomly select rows from a database table? I'm using JPA and would like to use the Criteria API if possible. I'm aware there is an SQL equivalent, something like:

    SELECT TOP 5 Id, Name FROM mNames
    ORDER BY NEWID()

But how would I do this with JPQL and the Criteria API? Possibly, with a NativeQuery? Is there a better way?

Answer 1

If all you need is a single random row, then you can do something like this:

//random is instance of java's Random class, and numberOfRows is total number of rows in the table
long rowIndex = random.nextLong()%numberOfRows;

TypedQuery typedQuery = ...;
typedQuery.setFirstResult(rowIndex);
typedQuery.setMaxResults(1);

I didn't test the code but you should get the idea.

Answer 2

如果您的实体的主键不是一个任意数字，而是某个功能，那么您最好选择随机创建一个有效密钥，然后查询特定的行，这是快速的操作，而不是仅仅执行可能的大量行过滤掉所有的东西。