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


Question

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?

1
92
7/17/2013 2:02:52 PM

Accepted Answer

You need rawQuery method.

Example:

private final String MY_QUERY = "SELECT * FROM table_a a INNER JOIN table_b b ON a.id=b.other_id WHERE b.property_id=?";

db.rawQuery(MY_QUERY, new String[]{String.valueOf(propertyId)});

Use ? bindings instead of putting values into raw sql query.

198
9/16/2014 2:01:32 PM

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.


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