$list = Db::name('user_0')->union('SELECT * FROM rx_user_1')->union('SELECT * FROM rx_user_2')->limit(10)->select();

执行的sql 是 ：
SELECT * FROM `rx_user_0` LIMIT 10 UNION SELECT * FROM rx_user_1 UNION SELECT * FROM rx_user_2

报错：
General error: 1221 Incorrect usage of UNION and LIMIT

把执行sql改为：
SELECT * FROM `rx_user_0` UNION SELECT * FROM rx_user_1 UNION SELECT * FROM rx_user_2 LIMIT 10
就正确了