def drop_column(table_name, column_name, conn, schema=None):
    """Drops a column of a table.

    Parameters
    ----------
    table_name : str
        table name
    column_name : str
        column name
    conn : sqlalchemy.engine.base.Engine
        an sqlalchemy connection engine created by function `create_engine()`
    schema : str or None
        schema name
    """
    column_name = column_name.replace('%', '%%')
    if schema is None:
        query_str = 'ALTER TABLE "{}" DROP COLUMN "{}";'.format(table_name,
            column_name)
    else:
        query_str = 'ALTER TABLE "{}"."{}" DROP COLUMN "{}";'.format(schema,
            table_name, column_name)
    conn.execute(query_str)
