MySql 5.6以下版本自定义函数返回VARCHAR的中文问题

-- 删除 
DROP FUNCTION IF EXISTS get_varchar_test; 
-- 创建 
CREATE FUNCTION get_varchar_test(_id INT) RETURNS VARCHAR(4) CHARACTER SET utf8 COLLATE utf8_general_ci
BEGIN   
    DECLARE content VARCHAR(4) CHARACTER SET utf8 COLLATE utf8_general_ci;

    # t_na 值为 ‘中文ab‘
    SELECT t_na INTO content FROM t1 WHERE t_id = _id;

    RETURN content; 
END; 
-- 测试
SELECT get_varchar_test(1);