MYSQL随机加条件解决思路

MYSQL随机加条件
大家新年好。

mysql 数据库随机获取2条记录，没有红色代码时，无论怎么刷新，得到的记录数永远都是2条。
加了红色部分后，有时只能获取1条记录，代码哪里有问题？（注：ab值保存为是TEXT，储存格式不一定是数字。但是现在我想获取随机值时，大于5000）

SQL code

(SELECT * FROM `cms` AS t1 JOIN (SELECT ROUND(RAND() * (SELECT MAX(id) FROM `cms`)) AS id) AS t2 WHERE t1.id >= t2.id [color=#FF0000]AND ab>5000[/color] ORDER BY t1.id ASC LIMIT 1) UNION (SELECT * FROM `cms` AS t1 JOIN (SELECT ROUND(RAND() * (SELECT MAX(id) FROM `cms`)) AS id) AS t2 WHERE t1.id >= t2.id [color=#FF0000]AND ab>5000[/color] ORDER BY t1.id ASC LIMIT 1) LIMIT 2