在Spring Data JPA查询中将整数转换为字符串

Question

I need to optimize a query that iterates over several objects and I wanted Spring Data to let the database handle it. I want to end up with a HashMap<String,String> that looks like

2134_9877, 9877
2134_2344, 2344
3298_9437, 9437

The SQL would be select convert(varchar,b.id)+'_'+convert(varchar,a.id)',a.id from t1 a join t2 b on a.jc = b.jc

So far, I've got Whatever-QL in the repository that looks like:

@Query("SELECT new map (a.bkey, a.akey) FROM mergeTable a WHERE a.discr= ?1")

The problem is, bkey is not unique, it is only unique when paired with akey and the monstrosity that I have to feed it to wants them combined with an underscore: 2345_2177 .

I have tried a.bkey.toString and ''+a.bkey and new String(a.bkey) and just string(a.bkey) (that last gives a new exception but still doesn't work) but Spring doesn't like any of these. I can find no questions asking this and it seems I cannot use SQLServer's convert() function as this ain't SQL.

How can I concatenate the Integers as Strings with an underscore in this @Query?

PS: Using the native query that's been debugged in SQLServer throws some weird alias exception in Hibernate so I think 'going native' is predetermined to be a dead end.

Answer 1

If I have understood it right, the 'Whatever-QL' is called JPQL, and the operator CONCAT can be used. Only the use of it, as it accepts two or more parameters depends on the JPA version you are running.

Here is the answer.

JPA concat operator

Answer 2

You could add a getter to your entity like this:

public String getCombinedKey(){
  return a.akey + "_" + a.bkey;
}

The advantage is you could handle here null's and other things if you want and it's more reusable in case you need this in another place. If you do it just in the repository you will have to copy it everytime.

Your query would then be:

@Query("SELECT new map (a.combinedKey, a.akey) FROM mergeTable a WHERE a.discr= ?1")