Function map_sql_event_data 

pub fn map_sql_event_data<T, U, Mat, F>(
    source: Source<SqlEvent<T>, Mat>,
    f: F,
) -> Source<SqlEvent<U>, Mat>
where
    T: Send + 'static,
    U: Send + 'static,
    Mat: Send + 'static,
    F: Fn(T) -> StreamResult<U> + Send + Sync + 'static,

Applies a function to the data branch of a SQL event stream.