Choosing value in SQL query vs (Java) code

Question

I need to choose one of three values of an integer using the value of a column on a nullable column of a table.

There are at least two approaches: 1) use SQL to do all the work: test null values, and choose between the other values, or 2) read the value and use code -in this case Java- to choose.

Which one is "better", ie. easier to understand & more maintainable? Do you have any other metric use to decide?

As an example, I have the following code:

// If id is equal to:
//   -1, then make v = 1
//   null, then make v = 2
//   in any other case, make v = 3

// Option 1:
int v;
String query = "SELECT CASE min(Id) WHEN NULL THEN 2 WHEN -1 THEN 1 ELSE 3 END AS Id"
        + "FROM TableA WHERE SomeField IN (SELECT ...blah blah...)";
ResultSet rs = // execute query
if (rs.next()) {
    v = rs.getInt("Id");
} else {
    // TODO something went *very* wrong...
}

// Option 2:
int v;
String query = "SELECT CASE min(Id) Id"
        + "FROM TableA WHERE SomeField IN (SELECT ...blah blah...)";
ResultSet rs = // execute query
if (rs.next()) {
    final int id = rs.getInt("Id");
    if (rs.wasNull()) {
        v = 2;
    } else if (id == -1) {
        v = 1;
    } else {
        v = 3;
    }
} else {
    // TODO something went *very* wrong...
}

Answer 1

I’d say have SQL do the work. It’s fairly trivial and won’t soak up CPU time, and SQL will have to load the pertinent info in memory anyway so it’s already there for processing. Doing it on the app side, to a certain extent it seems like you have to “re-stage” the data for analysis, and (imho) the java code seems more difficult to read through and understand.

Note that there’s a minor flaw in your SQL code, you can’t use WHEN NULL that way in a case statement. You’d want something like

...case
     when min(Id) is null then 2
     when min(Id) = -1 then 1
     else 3
   end

Answer 2

I'd go for SQL if the query is not very complex (mean executes in a reasonable time). If the query is 10min, better try java. But I always would prefere sql approach if the DB can do the job for me.

(just copy of my comment)