TDbCommand represents an SQL statement to execute against a database.
It is usually created by calling {@link TDbConnection::createCommand}.
The SQL statement to be executed may be set via {@link setText Text}.
To execute a non-query SQL (such as insert, delete, update), call
{@link execute}. To execute an SQL statement that returns result data set
(such as select), use {@link query} or its convenient versions {@link queryRow}
and {@link queryScalar}.
If an SQL statement returns results (such as a SELECT SQL), the results
can be accessed via the returned {@link TDbDataReader}.
TDbCommand supports SQL statment preparation and parameter binding.
Call {@link bindParameter} to bind a PHP variable to a parameter in SQL.
Call {@link bindValue} to bind a value to an SQL parameter.
When binding a parameter, the SQL statement is automatically prepared.
You may also call {@link prepare} to explicitly prepare an SQL statement.