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 disallowed 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. For example, in the following statement, xname is interpreted as a reference to the xname variable rather than the xname column:

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;

When this procedure is called, the newname variable returns the value 'bob' regardless of the value of the table1.xname column.

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