How to use the database classes in your script

Joomla provides a sopisticated database abstraction layer to simplify the usage for 3PD. This guide should help you using this layer.
1. Why should I use the Joomla database class?
Joomla is build to be able to use several different kinds of SQL-database-systems and to run in a variety of environments with different table-prefixes. In addition to these functions, the class automatically creates the database connection. Besides instantiating the object, you only need 2 lines of code to get a result from the database and that in a variety of formats. Using the Joomla database layer ensures a maximum of compatibility and flexibility for your extension.
2. Preparating the query
Database usage
// Get a database object
$db = JFactory::getDBO();
$query = "SELECT * FROM #__example_table WHERE id = 999999";
$db->setQuery($query);
First we instantiate the database object, then we prepare the query. You can use the normal SQL-syntax, the only thing you have to change is the table-prefix. To make this as flexible as possible, Joomla uses a placeholder for the prefix, the #__. In the next step, the setQuery(), this string is replaced with the correct prefix.
Now, if we don’t want to get information from the database, but insert a row into it, we need one more function. Every string-value in the SQL-syntax has to be quoted, therefore we have to hand over these values to the function $db→Quote($value).
Whatever we want to do, we have to set the query with the setQuery()-function. Although you could write the query directly as a parameter for setQuery(), its commonly done by first saving it in a variable, normally $query, and then handing this variable over. This helps writing clean, readable code.
3. Executing the Query
To execute the query, Joomla provides several functions, which differ in their return value.
Basic Query Execution
query
Query Execution Information
getAffectedRows
explain
insertid
Insert Query Execution
insertObject
4. Query Results
The database class contains many methods for working with a query’s result set.
Single Value Result
loadResult
Single Row Results
loadObject
loadRow
loadResultArray
Multi-Row Results
loadObjectList
loadRowList
loadAssocList
Misc Result Set Methods
getNumRows
5. Tips, Tricks & FAQ
We had a few people lately using sub-queries like these:
Sub-queries
SELECT * FROM #__example WHERE id IN (SELECT * FROM #__example2);
These kind of queries are only possible in MySQL 4.1 and above. Another way to achieve this, is splitting the query into two:
Splitted query
$query = "SELECT * FROM #__example2";
$database->setQuery($query);
$query = "SELECT * FROM #__example WHERE id IN (". implode(",", $database->loadArray()) .")";