relationship tables in sqlite on android

Question

Can you help me about relationship between two tables in sqlite.

I do insert,delete and update steps but I have to support relationship between two tables now. I guess
all of code steps which are done before will be changed Am I right? Have you got any link or example which explains tables relationships and any activities after relationship?

Answer 1

To establish relationship between two tables, you can use Foreign keys. A foreign key is a field in a relational table that matches a Candidate Key of another table.

For example, say we have two tables, a CUSTOMER table that includes all customer data, and an ORDER table that includes all customer orders. The intention here is that all orders must be associated with a customer that is already in the CUSTOMER table. To do this, we will place a foreign key in the ORDER table and have it relate to the primary key of the CUSTOMER table.

In SQLite Foreign Key Constraints can be added in following way ::

edit:: you can design item_order table like ::

CREATE TABLE customer(
         id INTEGER,
         firstName TEXT,
         middleName TEXT,
         lastName   TEXT,
         address TEXT,
         contactNum TEXT
);

 CREATE TABLE item(
        id INTEGER,
        name TEXT,
        description TEXT
 );

 CREATE TABLE order(
        id INTEGER,
        customerID INTEGER,
        date TEXT,
        FOREIGN KEY(customerId) REFERENCES customer(id)
 );

 CREATE TABLE item_order(
        id INTEGER,
        orderID INTEGER,
        itemId  INTEGER,
        quantity INTEGER,
        FOREIGN KEY(orderId) REFERENCES order(Id),
        FOREIGN KEY(itemId) REFERENCES item(Id)
 );

Answer 2

Good sample http://www.androidhive.info/2013/09/android-sqlite-database-with-multiple-tables/

// Table Create Statements
// Todo table create statement
private static final String CREATE_TABLE_TODO = "CREATE TABLE "
        + TABLE_TODO + "(" + KEY_ID + " INTEGER PRIMARY KEY," + KEY_TODO
        + " TEXT," + KEY_STATUS + " INTEGER," + KEY_CREATED_AT
        + " DATETIME" + ")";

// Tag table create statement
private static final String CREATE_TABLE_TAG = "CREATE TABLE " + TABLE_TAG
        + "(" + KEY_ID + " INTEGER PRIMARY KEY," + KEY_TAG_NAME + " TEXT,"
        + KEY_CREATED_AT + " DATETIME" + ")";

// todo_tag table create statement
private static final String CREATE_TABLE_TODO_TAG = "CREATE TABLE "
        + TABLE_TODO_TAG + "(" + KEY_ID + " INTEGER PRIMARY KEY,"
        + KEY_TODO_ID + " INTEGER," + KEY_TAG_ID + " INTEGER,"
        + KEY_CREATED_AT + " DATETIME" + ")";

SELECT * FROM todos td, tags tg, todo_tags tt WHERE tg.tag_name = ‘Watchlist’ AND tg.id = tt.tag_id AND td.id = tt.todo_id;

/*
 * getting all todos under single tag
 * */
public List<Todo> getAllToDosByTag(String tag_name) {
    List<Todo> todos = new ArrayList<Todo>();

    String selectQuery = "SELECT  * FROM " + TABLE_TODO + " td, "
            + TABLE_TAG + " tg, " + TABLE_TODO_TAG + " tt WHERE tg."
            + KEY_TAG_NAME + " = '" + tag_name + "'" + " AND tg." + KEY_ID
            + " = " + "tt." + KEY_TAG_ID + " AND td." + KEY_ID + " = "
            + "tt." + KEY_TODO_ID;

    Log.e(LOG, selectQuery);

    SQLiteDatabase db = this.getReadableDatabase();
    Cursor c = db.rawQuery(selectQuery, null);

    // looping through all rows and adding to list
    if (c.moveToFirst()) {
        do {
            Todo td = new Todo();
            td.setId(c.getInt((c.getColumnIndex(KEY_ID))));
            td.setNote((c.getString(c.getColumnIndex(KEY_TODO))));
            td.setCreatedAt(c.getString(c.getColumnIndex(KEY_CREATED_AT)));

            // adding to todo list
            todos.add(td);
        } while (c.moveToNext());
    }

    return todos;
}