Question

I have one group table with a recursive relation, so each record has a parent_id. Given a group, I need to get all the student (each belong to a group) names in all its subgroups, but ordered by student name.

Do you know if there is any “easy” way to do it? If I have to do multiple queries, then I should order the results of the different Cursors, but Cursor has no orderBy().

Any ideas? Thank you so much!

Answer 1

As SQLite does not support recursive queries I implemented the select with two steps:

First, I have a method called getRecursiveDiningGroupIdsAsString() that retreives all the group ids recursively whose parent id is the one you pass by parameter. The result is a String in the form of: “(2, 3, 4)” so you can later use it in an IN clause. The method looks like:

public String getRecursiveDiningGroupIdsAsString(int depth, long diningGroupId) {
    Cursor childDiningGroups = mDatabase.query(
            "group",
            new String[] {"_id"},
            "parent_id = "+diningGroupId,
            null, null, null, null
    );
    String recursiveDiningGroupIds = "";
    while (childDiningGroups.moveToNext()) {
        long childDiningGroupId = childDiningGroups.getLong(childDiningGroups.getColumnIndex("_id"));
        recursiveDiningGroupIds += getRecursiveDiningGroupIdsAsString(depth+1, childDiningGroupId);
    }
    recursiveDiningGroupIds += diningGroupId;
    if (depth > 0) {
        recursiveDiningGroupIds += ", ";
    } else {
        recursiveDiningGroupIds = "("+recursiveDiningGroupIds+")";
    }
    return recursiveDiningGroupIds;
}

Once I have the group ids I need, I just do a simple query using the ids returned by the previous method and that is it!

Hope it helps!