MySQL如何从相同条件范围内的两个不同子集中选择和按条件顺序排序

I have real world mysql select problem with my website, I have members who's gender is either female or male, But they are not proportionally distributed, i.e 70% male, 30% female,

I want to do Select to the Display in my home page of the website, where

Select Random 50 members BUT make sure they are 50% male and 50% female
Order these results where male then female, next one male female, i.e m,f,m,f...etc

How do i achieve this via a Single Query ?

ex:

SELECT * FROM user u WHERE 1 ORDER BY rand() LIMIT 50

will not select the 50% ratio, i want, so please help if you can

Have you tried a union?

SELECT * FROM table WHERE gender = 1 LIMIT 25 
UNION
SELECT * FROM table WHERE gender = 2 LIMIT 25

SELECT RANDOM 50 MEMBERS
MAKE SURE THEY ARE 50% MALE AND 50% FEMALE
Order these results i.e M,F,M,F...ETC

Could you try this?

SET @odd := 1;
SET @even := 2;

SELECT *
FROM
(
    SELECT *
    FROM
    (
        SELECT @odd := @odd + 2 AS ord, users.*
        FROM users
        WHERE sex = 'M'
        ORDER BY RAND()
        LIMIT 25
    ) x

    UNION ALL

    SELECT *
    FROM
    (
        SELECT @even := @even + 2 AS ord, users.*
        FROM users
        WHERE sex = 'F'

        ORDER BY RAND()
        LIMIT 25
    ) y
) z
ORDER BY ord;

sample output

+------+------+------+
| ord  | id   | sex  |
+------+------+------+
|    3 |    1 | M    |
|    4 |    8 | F    |
|    5 |    2 | M    |
|    6 |    9 | F    |
|    7 |    3 | M    |
|    8 |   10 | F    |

My First Approach was like this

SET @odd := 1;
SET @even := 2;

SELECT *
FROM
(
    SELECT @odd := @odd + 2 AS ord, users.*
    FROM users
    WHERE sex = 'M'
    ORDER BY RAND()
    LIMIT 25


    UNION ALL

    SELECT @even := @even + 2 AS ord, users.*
    FROM users
    WHERE sex = 'F'
    ORDER BY RAND()
    LIMIT 25
) x
ORDER BY ord;

But I was getting an error as follows:

 ERROR 1221 (HY000): Incorrect usage of UNION and ORDER BY