17.8 SIGNAL

SIGNAL condition_value
    [SET signal_information_item
    [, signal_information_item] ...]

condition_value: {
    SQLSTATE [VALUE] sqlstate_value
  | condition_name
}

signal_information_item:
    condition_information_item_name = simple_value_specification

condition_information_item_name: {
    CLASS_ORIGIN
  | SUBCLASS_ORIGIN
  | MESSAGE_TEXT
  | MYSQL_ERRNO
  | CONSTRAINT_CATALOG
  | CONSTRAINT_SCHEMA
  | CONSTRAINT_NAME
  | CATALOG_NAME
  | SCHEMA_NAME
  | TABLE_NAME
  | COLUMN_NAME
  | CURSOR_NAME
}

condition_name, simple_value_specification:
    (see following discussion)

SIGNAL是一种返回错误的方法。提供错误信息给：处理器、外部应用程序、客户端。它还可以控制错误特征（error number, SQLSTATE value, message）。

执行SIGNAL不需要任何权限。

详细说明参考：https://dev.mysql.com/doc/refman/5.5/en/signal.html