[android] How do I join two SQLite tables in my Android application?

Background

I have an Android project that has a database with two tables: tbl_question and tbl_alternative.

To populate the views with questions and alternatives I am using cursors. There are no problems in getting the data I need until I try to join the two tables.

    Tbl_question  
    -------------
    _id  
    question  
    categoryid  

    Tbl_alternative
    ---------------
    _id 
    questionid 
    categoryid 
    alternative

I want something like the following:

SELECT tbl_question.question, tbl_alternative.alternative where 
categoryid=tbl_alternative.categoryid AND tbl_question._id = 
tbl_alternative.questionid.` 

This is my attempt:

public Cursor getAlternative(long categoryid) {
            String[] columns = new String[] { KEY_Q_ID, KEY_IMAGE, KEY_QUESTION, KEY_ALT, KEY_QID};
             String whereClause = KEY_CATEGORYID + "=" + categoryid +" AND "+ KEY_Q_ID +"="+ KEY_QID;
             Cursor cursor = mDb.query(true, DBTABLE_QUESTION + " INNER JOIN "+ DBTABLE_ALTERNATIVE, columns, whereClause, null, null, null, null, null);
             if (cursor != null) {
                  cursor.moveToFirst();
             }
             return cursor;

I find this way to form queries harder than regular SQL, but have gotten the advice to use this way since it is less error prone.

Question

How do I join two SQLite tables in my application?

In addition to @pawelzieba's answer, which definitely is correct, to join two tables, while you can use an INNER JOIN like this

SELECT * FROM expense INNER JOIN refuel
ON exp_id = expense_id
WHERE refuel_id = 1

via raw query like this -

String rawQuery = "SELECT * FROM " + RefuelTable.TABLE_NAME + " INNER JOIN " + ExpenseTable.TABLE_NAME
        + " ON " + RefuelTable.EXP_ID + " = " + ExpenseTable.ID
        + " WHERE " + RefuelTable.ID + " = " +  id;
Cursor c = db.rawQuery(
        rawQuery,
        null
);

because of SQLite's backward compatible support of the primitive way of querying, we turn that command into this -

SELECT *
FROM expense, refuel
WHERE exp_id = expense_id AND refuel_id = 1

and hence be able to take advanatage of the SQLiteDatabase.query() helper method

Cursor c = db.query(
        RefuelTable.TABLE_NAME + " , " + ExpenseTable.TABLE_NAME,
        Utils.concat(RefuelTable.PROJECTION, ExpenseTable.PROJECTION),
        RefuelTable.EXP_ID + " = " + ExpenseTable.ID + " AND " + RefuelTable.ID + " = " +  id,
        null,
        null,
        null,
        null
);

For a detailed blog post check this http://blog.championswimmer.in/2015/12/doing-a-table-join-in-android-without-using-rawquery

An alternate way is to construct a view which is then queried just like a table. In many database managers using a view can result in better performance.

CREATE VIEW xyz SELECT q.question, a.alternative  
   FROM tbl_question AS q, tbl_alternative AS a
  WHERE q.categoryid = a.categoryid 
    AND q._id = a.questionid;

This is from memory so there may be some syntactic issues. http://www.sqlite.org/lang_createview.html

I mention this approach because then you can use SQLiteQueryBuilder with the view as you implied that it was preferred.

"Ambiguous column" usually means that the same column name appears in at least two tables; the database engine can't tell which one you want. Use full table names or table aliases to remove the ambiguity.

Here's an example I happened to have in my editor. It's from someone else's problem, but should make sense anyway.

select P.* 
from product_has_image P
inner join highest_priority_images H 
        on (H.id_product = P.id_product and H.priority = p.priority)