code: func (c *Column) CheckNotNull(data types.Datum) error {
  	if (mysql.HasNotNullFlag(c.Flag) || mysql.HasPreventNullInsertFlag(c.Flag)) && data.IsNull() {
  		return ErrColumnCantNull.GenWithStackByArgs(c.Name)
  	}
  	return nil
  }
docs: // CheckNotNull checks if nil value set to a column with NotNull flag is set.