MySQL :: MySQL 8.4 Reference Manual :: 15.6.7.5 SIGNAL Statement (original) (raw)

15.6.7.5 SIGNAL Statement

SIGNAL is the way to“return” an error.SIGNAL provides error information to a handler, to an outer portion of the application, or to the client. Also, it provides control over the error's characteristics (error number, SQLSTATE value, message). Without SIGNAL, it is necessary to resort to workarounds such as deliberately referring to a nonexistent table to cause a routine to return an error.

SIGNAL Overview

The conditionvalue in aSIGNAL statement indicates the error value to be returned. It can be anSQLSTATE value (a 5-character string literal) or a conditionname that refers to a named condition previously defined withDECLARE ... CONDITION (see Section 15.6.7.1, “DECLARE ... CONDITION Statement”).

An SQLSTATE value can indicate errors, warnings, or “not found.” The first two characters of the value indicate its error class, as discussed in Signal Condition Information Items. Some signal values cause statement termination; seeEffect of Signals on Handlers, Cursors, and Statements.

The SQLSTATE value for aSIGNAL statement should not start with '00' because such values indicate success and are not valid for signaling an error. This is true whether the SQLSTATE value is specified directly in theSIGNAL statement or in a named condition referred to in the statement. If the value is invalid, a Bad SQLSTATE error occurs.

To signal a generic SQLSTATE value, use'45000', which means “unhandled user-defined exception.”

The SIGNAL statement optionally includes a SET clause that contains multiple signal items, in a list of_conditioninformationitemname_ =simplevaluespecification assignments, separated by commas.

Each_conditioninformationitemname_ may be specified only once in the SET clause. Otherwise, a Duplicate condition information item error occurs.

Valid simplevaluespecification designators can be specified using stored procedure or function parameters, stored program local variables declared with DECLARE, user-defined variables, system variables, or literals. A character literal may include a charset introducer.

For information about permissible_conditioninformationitemname_ values, seeSignal Condition Information Items.

The following procedure signals an error or warning depending on the value of pval, its input parameter:

CREATE PROCEDURE p (pval INT)
BEGIN
  DECLARE specialty CONDITION FOR SQLSTATE '45000';
  IF pval = 0 THEN
    SIGNAL SQLSTATE '01000';
  ELSEIF pval = 1 THEN
    SIGNAL SQLSTATE '45000'
      SET MESSAGE_TEXT = 'An error occurred';
  ELSEIF pval = 2 THEN
    SIGNAL specialty
      SET MESSAGE_TEXT = 'An error occurred';
  ELSE
    SIGNAL SQLSTATE '01000'
      SET MESSAGE_TEXT = 'A warning occurred', MYSQL_ERRNO = 1000;
    SIGNAL SQLSTATE '45000'
      SET MESSAGE_TEXT = 'An error occurred', MYSQL_ERRNO = 1001;
  END IF;
END;

If pval is 0, p() signals a warning because SQLSTATE values that begin with '01' are signals in the warning class. The warning does not terminate the procedure, and can be seen with SHOW WARNINGS after the procedure returns.

If pval is 1, p() signals an error and sets the MESSAGE_TEXT condition information item. The error terminates the procedure, and the text is returned with the error information.

If pval is 2, the same error is signaled, although the SQLSTATE value is specified using a named condition in this case.

If pval is anything else,p() first signals a warning and sets the message text and error number condition information items. This warning does not terminate the procedure, so execution continues and p() then signals an error. The error does terminate the procedure. The message text and error number set by the warning are replaced by the values set by the error, which are returned with the error information.

SIGNAL is typically used within stored programs, but it is a MySQL extension that it is permitted outside handler context. For example, if you invoke the mysql client program, you can enter any of these statements at the prompt:

SIGNAL SQLSTATE '77777';

CREATE TRIGGER t_bi BEFORE INSERT ON t
  FOR EACH ROW SIGNAL SQLSTATE '77777';

CREATE EVENT e ON SCHEDULE EVERY 1 SECOND
  DO SIGNAL SQLSTATE '77777';

SIGNAL executes according to the following rules:

If the SIGNAL statement indicates a particular SQLSTATE value, that value is used to signal the condition specified. Example:

CREATE PROCEDURE p (divisor INT)
BEGIN
  IF divisor = 0 THEN
    SIGNAL SQLSTATE '22012';
  END IF;
END;

If the SIGNAL statement uses a named condition, the condition must be declared in some scope that applies to the SIGNAL statement, and must be defined using anSQLSTATE value, not a MySQL error number. Example:

CREATE PROCEDURE p (divisor INT)
BEGIN
  DECLARE divide_by_zero CONDITION FOR SQLSTATE '22012';
  IF divisor = 0 THEN
    SIGNAL divide_by_zero;
  END IF;
END;

If the named condition does not exist in the scope of theSIGNAL statement, anUndefined CONDITION error occurs.

If SIGNAL refers to a named condition that is defined with a MySQL error number rather than an SQLSTATE value, aSIGNAL/RESIGNAL can only use a CONDITION defined with SQLSTATE error occurs. The following statements cause that error because the named condition is associated with a MySQL error number:

DECLARE no_such_table CONDITION FOR 1051;
SIGNAL no_such_table;

If a condition with a given name is declared multiple times in different scopes, the declaration with the most local scope applies. Consider the following procedure:

CREATE PROCEDURE p (divisor INT)
BEGIN
  DECLARE my_error CONDITION FOR SQLSTATE '45000';
  IF divisor = 0 THEN
    BEGIN
      DECLARE my_error CONDITION FOR SQLSTATE '22012';
      SIGNAL my_error;
    END;
  END IF;
  SIGNAL my_error;
END;

If divisor is 0, the firstSIGNAL statement executes. The innermost my_error condition declaration applies, raising SQLSTATE '22012'.

If divisor is not 0, the secondSIGNAL statement executes. The outermost my_error condition declaration applies, raising SQLSTATE '45000'.

For information about how the server chooses handlers when a condition occurs, see Section 15.6.7.6, “Scope Rules for Handlers”.

Signals can be raised within exception handlers:

CREATE PROCEDURE p ()
BEGIN
  DECLARE EXIT HANDLER FOR SQLEXCEPTION
  BEGIN
    SIGNAL SQLSTATE VALUE '99999'
      SET MESSAGE_TEXT = 'An error occurred';
  END;
  DROP TABLE no_such_table;
END;

CALL p() reaches theDROP TABLE statement. There is no table named no_such_table, so the error handler is activated. The error handler destroys the original error (“no such table”) and makes a new error with SQLSTATE '99999' and message An error occurred.

Signal Condition Information Items

The following table lists the names of diagnostics area condition information items that can be set in aSIGNAL (orRESIGNAL) statement. All items are standard SQL except MYSQL_ERRNO, which is a MySQL extension. For more information about these items see Section 15.6.7.7, “The MySQL Diagnostics Area”.

Item Name             Definition
---------             ----------
CLASS_ORIGIN          VARCHAR(64)
SUBCLASS_ORIGIN       VARCHAR(64)
CONSTRAINT_CATALOG    VARCHAR(64)
CONSTRAINT_SCHEMA     VARCHAR(64)
CONSTRAINT_NAME       VARCHAR(64)
CATALOG_NAME          VARCHAR(64)
SCHEMA_NAME           VARCHAR(64)
TABLE_NAME            VARCHAR(64)
COLUMN_NAME           VARCHAR(64)
CURSOR_NAME           VARCHAR(64)
MESSAGE_TEXT          VARCHAR(128)
MYSQL_ERRNO           SMALLINT UNSIGNED

The character set for character items is UTF-8.

It is illegal to assign NULL to a condition information item in a SIGNAL statement.

A SIGNAL statement always specifies an SQLSTATE value, either directly, or indirectly by referring to a named condition defined with an SQLSTATE value. The first two characters of an SQLSTATE value are its class, and the class determines the default value for the condition information items:

MESSAGE_TEXT = 'Unhandled user-defined warning condition';  
MYSQL_ERRNO = ER_SIGNAL_WARN  
MESSAGE_TEXT = 'Unhandled user-defined not found condition';  
MYSQL_ERRNO = ER_SIGNAL_NOT_FOUND  
MESSAGE_TEXT = 'Unhandled user-defined exception condition';  
MYSQL_ERRNO = ER_SIGNAL_EXCEPTION

For legal classes, the other condition information items are set as follows:

CLASS_ORIGIN = SUBCLASS_ORIGIN = '';
CONSTRAINT_CATALOG = CONSTRAINT_SCHEMA = CONSTRAINT_NAME = '';
CATALOG_NAME = SCHEMA_NAME = TABLE_NAME = COLUMN_NAME = '';
CURSOR_NAME = '';

The error values that are accessible afterSIGNAL executes are theSQLSTATE value raised by theSIGNAL statement and theMESSAGE_TEXT andMYSQL_ERRNO items. These values are available from the C API:

At the SQL level, the output from SHOW WARNINGS and SHOW ERRORS indicates the MYSQL_ERRNO and MESSAGE_TEXT values in theCode and Message columns.

To retrieve information from the diagnostics area, use theGET DIAGNOSTICS statement (seeSection 15.6.7.3, “GET DIAGNOSTICS Statement”). For information about the diagnostics area, see Section 15.6.7.7, “The MySQL Diagnostics Area”.

Effect of Signals on Handlers, Cursors, and Statements

Signals have different effects on statement execution depending on the signal class. The class determines how severe an error is. MySQL ignores the value of thesql_mode system variable; in particular, strict SQL mode does not matter. MySQL also ignores IGNORE: The intent ofSIGNAL is to raise a user-generated error explicitly, so a signal is never ignored.

In the following descriptions, “unhandled” means that no handler for the signaled SQLSTATE value has been defined withDECLARE ... HANDLER.