13.6.4.2 Local Variable Scope and Resolution

The scope of a local variable is the BEGIN ... END block within which 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.

Because local variables are in scope only during stored program execution, references to them are not permitted in prepare statements created within a stored program. Prepared statement scope is the current session, not the stored program, so the statement could be executed after the program ends, at which point the variables would no longer be in scope. For example, SELECT ... INTO local_var cannot be used as a prepared statement. This restriction also applies to stored procedure and function parameters. See Section 13.5.1, "PREPARE Syntax".

A local variable should not have the same name as a table column. 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 defintion 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 C.1, "Restrictions on Stored Programs".