mysql语句中的局部变量

DELIMITER //
CREATE PROCEDURE calculate_avg_salary()
BEGIN
    DECLARE total_salary INT;
    DECLARE employee_count INT;
    DECLARE avg_salary FLOAT;
    
    SELECT SUM(salary) INTO total_salary FROM employees;
    SELECT COUNT(*) INTO employee_count FROM employees;
    
    SET avg_salary = total_salary / employee_count;
    
    SELECT avg_salary;
END //
DELIMITER ;

SET @column_name := 'name';
SET @table_name := 'customers';

SET @sql := CONCAT('SELECT ', @column_name, ' FROM ', @table_name);
PREPARE stmt FROM @sql;
EXECUTE stmt;

SET @debug := 1;

IF @debug = 1 THEN
    SELECT * FROM debug_table;
ELSE
    SELECT * FROM production_table;
END IF;