Syntax:
[begin_label:] BEGIN
    [statement_list]
END [end_label]

[begin_label:] LOOP
    statement_list
END LOOP [end_label]

[begin_label:] REPEAT
    statement_list
UNTIL search_condition
END REPEAT [end_label]

[begin_label:] WHILE search_condition DO
    statement_list
END WHILE [end_label]

Labels are permitted for BEGIN ... END blocks and for the LOOP, REPEAT,
and WHILE statements. Label use for those statements follows these
rules:

o begin_label must be followed by a colon.

o begin_label can be given without end_label. If end_label is present,
  it must be the same as begin_label.

o end_label cannot be given without begin_label.

o Labels at the same nesting level must be distinct.

o Labels can be up to 16 characters long.

To refer to a label within the labeled construct, use an ITERATE or
LEAVE statement. The following example uses those statements to
continue iterating or terminate the loop:

CREATE PROCEDURE doiterate(p1 INT)
BEGIN
  label1: LOOP
    SET p1 = p1 + 1;
    IF p1 < 10 THEN ITERATE label1; END IF;
    LEAVE label1;
  END LOOP label1;
END;

The scope of a block label does not include the code for handlers
declared within the block. For details, see [HELP DECLARE HANDLER].

URL: https://dev.mysql.com/doc/refman/8.0/en/statement-labels.html

