Syntax
------

SIGNAL error_condition
  [SET error_property
  [, error_property] ...]

error_condition:
  SQLSTATE [VALUE] 'sqlstate_value'
 | condition_name

error_property:
  error_property_name = <error_property_value>

error_property_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

SIGNAL empties the diagnostics area and produces a custom error. This
statement can be used anywhere, but is generally useful when used inside a
stored program. When the error is produced, it can be caught by a HANDLER. If
not, the current stored program, or the current statement, will terminate with
the specified error.

Sometimes an error HANDLER just needs to SIGNAL the same error it received,
optionally with some changes. Usually the RESIGNAL statement is the most
convenient way to do this.

error_condition can be an SQLSTATE value or a named error condition defined
via DECLARE CONDITION. SQLSTATE must be a constant string consisting of five
characters. These codes are standard to ODBC and ANSI SQL. For customized
errors, the recommended SQLSTATE is '45000'. For a list of SQLSTATE values
used by MariaDB, see the MariaDB Error Codes page. The SQLSTATE can be read
via the API method mysql_sqlstate( ).

To specify error properties user-defined variables and local variables can be
used, as well as character set conversions (but you can't set a collation).

The error properties, their type and their default values are explained in the
diagnostics area page.

Errors
------

If the SQLSTATE is not valid, the following error like this will be produced:

ERROR 1407 (42000): Bad SQLSTATE: '123456'

If a property is specified more than once, an error like this will be produced:

ERROR 1641 (42000): Duplicate condition information item 'MESSAGE_TEXT'

If you specify a condition name which is not declared, an error like this will
be produced:

ERROR 1319 (42000): Undefined CONDITION: cond_name

If MYSQL_ERRNO is out of range, you will get an error like this:

ERROR 1231 (42000): Variable 'MYSQL_ERRNO' can't be set to the value of '0'

Examples
--------

Here's what happens if SIGNAL is used in the client to generate errors:

SIGNAL SQLSTATE '01000';
Query OK, 0 rows affected, 1 warning (0.00 sec)

SHOW WARNINGS;

+---------+------+------------------------------------------+
| Level   | Code | Message                                  |
+---------+------+------------------------------------------+
| Warning | 1642 | Unhandled user-defined warning condition |
+---------+------+------------------------------------------+
1 row in set (0.06 sec)

SIGNAL SQLSTATE '02000';
ERROR 1643 (02000): Unhandled user-defined not found condition

How to specify MYSQL_ERRNO and MESSAGE_TEXT properties:

SIGNAL SQLSTATE '45000' SET MYSQL_ERRNO=30001, MESSAGE_TEXT='H
ello, world!';

ERROR 30001 (45000): Hello, world!

The following code shows how to use user variables, local variables and
character set conversion with SIGNAL:

CREATE PROCEDURE test_error(x INT)
BEGIN
 DECLARE errno SMALLINT UNSIGNED DEFAULT 31001;
 SET @errmsg = 'Hello, world!';
 IF x = 1 THEN
   SIGNAL SQLSTATE '45000' SET
   MYSQL_ERRNO = errno,
   MESSAGE_TEXT = @errmsg;
 ELSE
   SIGNAL SQLSTATE '45000' SET
   MYSQL_ERRNO = errno,
   MESSAGE_TEXT = _utf8'Hello, world!';
 END IF;
END;

How to use named error conditions:

CREATE PROCEDURE test_error(n INT)
BEGIN
 DECLARE `too_big` CONDITION FOR SQLSTATE '45000';
 IF n > 10 THEN
   SIGNAL `too_big`;
 END IF;
END;

In this example, we'll define a HANDLER for an error code. When the error
occurs, we SIGNAL a more informative error which makes sense for our procedure:

CREATE PROCEDURE test_error()
BEGIN
 DECLARE EXIT HANDLER
 FOR 1146
 BEGIN
   SIGNAL SQLSTATE '45000' SET
   MESSAGE_TEXT = 'Temporary tables not found; did you call init()
procedure?';
 END;
 -- this will produce a 1146 error
 SELECT `c` FROM `temptab`;
END;

URL: https://mariadb.com/kb/en/signal/