12.7.3.4. Scope and Resolution of Local Variables

The scope of a local variable is within the BEGIN ... END block where it is declared. The variable can be referred to in blocks nested within the declaring block, except those blocks that declare a variable with the same name.

Local variables are within scope only during stored routine execution, so references to them are not permitted within prepared statements because those are global to the current session and the variables might have gone out of scope when the statement is executed. For example, SELECT ... INTO local_var cannot be used as a prepared statement.

Local variable names should not be the same as column names. If an SQL statement, such as a SELECT ... INTO statement, contains a reference to a column and a declared local variable with the same name, MySQL currently interprets the reference as the name of a variable. Consider the following procedure definition:

CREATE PROCEDURE sp1 (x VARCHAR(5))
BEGIN
  DECLARE xname VARCHAR(5) DEFAULT 'bob';
  DECLARE newname VARCHAR(5);
  DECLARE xid INT;

  SELECT xname, id INTO newname, xid
    FROM table1 WHERE xname = xname;
  SELECT newname;
END;

MySQL interprets xname in the SELECT statement as a reference to the xname variable rather than the xname column. Consequently, when the procedure sp1()is called, the newname variable returns the value 'bob' regardless of the value of the table1.xname column.

Similarly, the cursor definition in the following procedure contains a SELECT statement that refers to xname. MySQL interprets this as a reference to the variable of that name rather than a column reference.

CREATE PROCEDURE sp2 (x VARCHAR(5))
BEGIN
  DECLARE xname VARCHAR(5) DEFAULT 'bob';
  DECLARE newname VARCHAR(5);
  DECLARE xid INT;
  DECLARE done TINYINT DEFAULT 0;
  DECLARE cur1 CURSOR FOR SELECT xname, id FROM table1;
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;

  OPEN cur1;
  read_loop: LOOP
    FETCH FROM cur1 INTO newname, xid;
    IF done THEN LEAVE read_loop; END IF;
    SELECT newname;
  END LOOP;
  CLOSE cur1;
END;

See also Section E.1, “Restrictions on Stored Routines, Triggers, and Events”.

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