Inserting columns in Sqlite database using android studio - android-sqlite

Question

Inserting columns in Sqlite database using android studio and on how to set some as autoincrement and primarykeys

Answer 1

If you set the column type as INTEGER PRIMARY KEY and if you do not specify a value when inserting a row, then a value (64bit signed integer) will be assigned an unused integer, usually one greater then the largest.
You very likely do not need to use the AUTOINCREMENT keyword (this assigns an integer that is unique within the database, rather than at the table level and thus has overheads in determining that integer).
Frequently _id is used, so will will often see
db.execSQL("CREATE TABLE tablename (_id INTEGER PRIMARY KEY, column_name column_type, ...more column_name / column_types as required...);");
As an example, the following code uses a subclass of SQLiteOpenHelper (not needed but frequently used), which requires an onCreate method (which is called when the database is created e.g. the very first time the helper is used) and an onUpgrade method (required if the version number is incremented/increased).
This code will create, if need be, a database called mydb (filename mydb.sqlite for Operating Systems that support extension greater than 3 characters).
Note if need be is just once (with some rare exceptions) unless the database is deleted. i.e. onCreate is not called every time an instance of the helper is constructed, it is only called when the database file itself doesn't exist (again rare exceptions apply).
It will then, if creating the database, create a table in the database named testfloat. The table will consists of 2 columns namely, _id and myfloat.
The _id column type is INTEGER PRIMARY KEY, if when inserting a row and no value is specified for the column then it will be a unique incrementing integer (1 at first, then 2 ......).
The myfloat column is of type FLOAT (perhaps checkout Datatypes In SQLite Version 3 for SQLite's flexibility in regards to Datatypes), if a value isn't given when inserting a row then a value of 0.0 will be given.
public class MyDBHelper extends SQLiteOpenHelper {
public static final String DBNname = "mydb";
public static final int DBVersion = 1;
public static final String TESTFLOATTABLE = "testfloat";
public static final String STDIDCOL = "_id INTEGER PRIMARYKEY";
public static final String MYFLOATCOL = "myfloat";
public static final String MYFLOATTYPE = " FLOAT DEFAULT 0.0";
public MyDBHelper(Context context) {
super(context,DBNname,null,DBVersion);
}
#Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("CREATE TABLE " +
TESTFLOATTABLE +
"(" +
STDIDCOL +
"," +
MYFLOATCOL +
MYFLOATTYPE +
")");
}
#Override
public void onUpgrade(SQLiteDatabase db, int oldeversion, int newversion) {
}
public long insertRow(double myfloatvalue) {
long rv;
SQLiteDatabase db = getWritableDatabase();
ContentValues cv = new ContentValues();
cv.put(MYFLOATCOL,myfloatvalue);
rv = db.insert(TESTFLOATTABLE,null,cv);
return rv;
}
public Cursor getAllMyFloats() {
Cursor rv;
SQLiteDatabase db = getReadableDatabase();
rv = db.query(TESTFLOATTABLE,null,null,null,null,null,null);
return rv;
}
}
The code above, has an empty method for onUpgrade. Additionally there are two methods insertRow (to insert a row) and getAllMyFloats to return a Cursor, with in this case, all rows containing all columns.
In you invoking activity you could do something along the lines of :-
MyDBHelper mydbhelper = new MyDBHelper(this);
mydbhelper.insertRow(1.3);
mydbhelper.insertRow(1);
mydbhelper.insertRow(5.674389123459834);
The first line gets a MyDBHelper instance, which when first run will create the database and in doing so call the onCreate method, thus creating the tables.
The next three lines invoke the insertRow method (note if the app is rerun 3 additional rows will be added........ i.e. the code is for demonstration), which will cause 3 rows to be added, the first will have 1 in the _id column, the next 2 etc.
The following code (which follows on from the code above) demonstrates obtaining and interrogating a Cursor:-
Cursor getfloats = mydbhelper.getAllMyFloats();
Log.d("TESTFLOAT","Rows returned from getALlFloats = " + getfloats.getCount());
while (getfloats.moveToNext()) {
Log.d("TESTFLOAT","Via getString = " + getfloats.getString(getfloats.getColumnIndex(mydbhelper.MYFLOATCOL)));
Log.d("TESTFLOAT","Via getFloat = " + Float.toHexString(
getfloats.getFloat(
getfloats.getColumnIndex(
mydbhelper.MYFLOATCOL
)
)
));
Log.d("TESTFLOAT","Via getDouble = " +Double.toString(
getfloats.getDouble(
getfloats.getColumnIndex(
mydbhelper.MYFLOATCOL
)
)));
}
getfloats.close();
The first line invokes the getAllMyFLoats that returns a cursor.
The next line wtires a log message that details how many rows are in the resultant cursor.
The while clause transverses the cursor if there are any rows in the cursor. For each row it gets the value of the myfloat column, using some of the cursor.get????? methods (to demonstrate how getting the value is affected by each). Note instead of getfloats.get????(getfloats.getColumnIndex(column)), getfloats.get????(1) would work.