Spring Data JPA Distinct - Return results from a single column

Question

I have some data that contains a STATE field (String/Text) that defines what state a given request is currently in (eg pending, approved denied etc.) and to get all the unique values from that column I can run the following TSQL query

SELECT DISTINCT STATE FROM CALLOUT_REQUEST

where CALLOUT_REQUEST is my table name and STATE being the field which returns something like:

STATE

• approved
• denied
• pending
• ...

However I don't understand how I would turn that into a query in my repository as it seems I need a "by" statement or some other filter mechanism which i can get the STATE based on?

What I am looking to return - as shown in the raw TSQL query above - is some kind of List or Array object which contains all the unique/distinct values in all of the STATE fields.

So in pseudo code i think i am looking for something like this:

String[] states = repository.findDisinctState();

where findDistinctState() would then return an array of sorts.

Hope that makes sense - I am very new to Java and Spring in general so I think I am missing some conceptual knowledge to utilise the above.

UPDATE:

The 'state' concept is closed so i could implement that as an enum - only problem is i dont know how to do that :) Ill look into how i can do that as i think it fits perfectly with what i am trying to achieve.

The List i get from the query provided is intended to be used to get a count of all the occurrences. I had this code before to get a total count for each of the 'states':

Map stats = new HashMap();
String[] states = {"approved", "denied", "pending", "deactivated"};
for (int i = 0; i < states.length; i++) {
    stats.put(states[i], repository.countByState(states[i]));
}

Am i correct in understanding that the states Array that i have in the above code snippet could be turned into an enum and then i dont even need the custom @Query anymore?

Answer 1

If that state concept is closed - you know its possible set of values - it should be an enum.

After that you can create queries that you invoke like:

repository.findByState(State.APPROVED)

If you can't create an enum, you need a separate method to get the distinct values, which can't be provided by JPA, because you need a list of strings and not a list of CalloutRequest s. Then you need to specify a query manually like:

@Query("SELECT DISTINCT State FROM CALLOUT_REQUEST")
List<String> findDistinctStates();

Answer 2

You can use a JPQL query for this, with the @org.springframework.data.jpa.repository.Query annotation:

@Query("select distinct state from CalloutRequest")
List<String> findDistinctStates();

Answer 3

If you don't want to use @Query then one solution is there to create an interface "StateOnlyInterface" with method named "getState()".

Then create method in your repo with name, getDistinctState(). Return type of this method to be kept as ArrayList of StateOnlyInterface.