以下是一个使用JOIN、LEFT JOIN和UNION子查询对表a进行排序的代码示例：

-- 创建并插入数据到表a
CREATE TABLE a (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    age INT
);

INSERT INTO a (id, name, age)
VALUES (1, 'John', 30),
       (2, 'Alice', 25),
       (3, 'Bob', 35);

-- 使用JOIN子查询对表a按照age字段进行排序
SELECT a.id, a.name, a.age
FROM a
JOIN (
    SELECT id, age
    FROM a
    ORDER BY age
) AS subq ON a.id = subq.id
ORDER BY subq.age;

-- 使用LEFT JOIN子查询对表a按照age字段进行排序
SELECT a.id, a.name, a.age
FROM a
LEFT JOIN (
    SELECT id, age
    FROM a
    ORDER BY age
) AS subq ON a.id = subq.id
ORDER BY subq.age;

-- 使用UNION子查询对表a按照age字段进行排序
SELECT id, name, age
FROM (
    SELECT id, name, age
    FROM a
    UNION
    SELECT id, name, age
    FROM a
) AS subq
ORDER BY subq.age;

这些示例分别使用JOIN、LEFT JOIN和UNION子查询将表a按照age字段进行排序。请根据实际需求选择适合的方法。