<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _checkTable(cls, field):
 """Split a field from _sqlFields into table, column. Registers the table in cls._tables, and returns a fully qualified table.column (default table: cls._sqlTable) """

        # Get table part
        try:
            (table, field) = field.split('.')
        except ValueError:
            table = cls._sqlTable
        # clean away white space
        table = table.strip()
        field = field.strip()
        # register table
        cls._tables[table] = None
        # and return in proper shape
        return table + '.' + field