Querying a Database

Each database query is returned as a Cursor. This lets Android manage resources more efficiently by retrieving and releasing row and column values on demand.

To execute a query on a database use the query method, passing in:

> An optional Boolean that specifies if the result set should contain only unique values.

> The name of the table to query.

> A projection, as an array of strings, that lists the columns to include in the result set.

> A "where" clause that defines the rows to be returned. You can include ? wildcards that will be replaced by the values passed in through the selection argument parameter.

> An array of selection argument strings that will replace the ?'s in the where clause.

> A "group by'' clause that defines how the resulting rows will be grouped.

> A "having" filter that defines which row groups to include if you specified a group by clause.

> A string that describes the order of the returned rows.

> An optional string that defines a limit for the number of returned rows. Listing 7-4 shows snippets for returning some, and all, of the rows in a particular table.

LISTING 7-4: Querying a database Available for download on // Return all rows for columns one and three, no duplicates Wrox.com String[] result_columns = new String[] {KEY_ID, KEY_COL1, KEY_COL3};

Cursor allRows = myDatabase.query(true, DATABASE_TABLE, result_columns, null, null, null, null, null, null);

// Return all columns for rows where column 3 equals a set value // and the rows are ordered by column 5. String where = KEY_COL3 + "=" + requiredValue; String order = KEY_COL5;

Cursor myResult = myDatabase.query(DATABASE_TABLE, null, where, null, null, null, order);

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