Accessing Data with Android Cursors (Part 1)

Creating and connecting to a Database

Create a new project in Eclipse called TestingData. Import android.database.sqllite.SQLiteDatabase in this project.

package com.kais.testingdata;

import android.app.Activity;

import android.database.sqlite.SQLiteDatabase;

import android.os.Bundle;

import android.view.Menu;

public class MainActivity extends Activity {

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

//here we create a database called TestingData.db

SQLiteDatabase db;

db = openOrCreateDatabase("TestingData.db", SQLiteDatabase.CREATE_IF_NECESSARY, null);

}

Creating Tables

Tables are created by executing statements on the database. The queries should be executed with the execSQL statement.Use  the setVersion(), setLocale() and setLockingEnabled() methods to set these properties. These will be demonstrated in the creating tables example.

Just put these lines of codes below the creation of database.

final String CREATE_TABLE_COUNTRIES = "CREATE TABLE tbl_countries("+ "id INTEGER                                                                                     PRIMARY KEY AUTOINCREMENT,"+"country_name                                                                           TEXT);";

final String CREATE_TABLE_STATES = "CREATE_TABLE tbl_states("+" id INTEGER PRIMARY                                                                   KEY AUTOINCREMENT,"+" state_name TEXT "+"                                                                                 country_id INTEGER KEY NULL CONSTRAINT "+"                                                                               country_id INTEGER NOT NULL CONSRAINT "+"                                                                                  country_id REFRENCES tbl1_countries(id) "+" ON DELETE                                                                      CASCADE);";

db.execSQL(CREATE_TABLE_COUNTRIES);

db.execSQL(CREATE_TABLE_STATES);

final String CREATE_TRIGGER_STATES = 

        "CREATE TRIGGER fk_insert_state BEFORE "

        + "INSERT on tbl_states"

        + "FOR EACH ROW "

        + "BEGIN "

                                + "SELECT RAISE(ROLLBACK, 'insert on table

                               "+" tbl_states violates foreign key constraint 

                               "+" fk_insert_state') WHERE (SELECT id FROM "+" tbl_countries WHERE id =                                         NEW.country_id) IS NULL; "+" END;";

db.execSQL(CREATE_TRIGGER_STATES);

Additionally here we manually have to create triggers to handle the foreign key relationships between the table. In  a production application there would also need to be foreign key triggers to handle row updates and deletes. The foreign key triggers are executed with execSQL just like the table creation.

Inserting Records

Android comes with a series of classes that simplify database usage. Use a ContentValues instance to create a series of table field to data matchings that will be passed into an insert() method. Android has created similar methods for updating and deleting records. 

ContentValues values = new ContentValues();
        values.put("country_name", "US");
        long countryId = db.insert("tbl_countries", null, values);
        ContentValues stateValues = new ContentValues();
        stateValues.put("state_name", "Texas");
        stateValues.put("country_id", Long.toString(countryId));
        try {
            db.insertOrThrow("tbl_states", null, stateValues);
        } catch (Exception e) {
            //catch code
        }

Append this code to the previous example. First create a ContentValues object to store the data to insert and use the put method to load the data. Then use the insert() method to perform the insert query into SQLite. The insert() function expects three parameters, the table name, null, and the ContentValues pairs. Also a long is returned by the insert() function. This long will hold the primary key of the inserted row.