Illegal mix of collations for operation 'UNION'

Question

 SELECT a.paid, a.total, a.remain, a.date, a.patient_id FROM (
(SELECT paid, total, remain, date, patient_id FROM payment ) 
UNION 
(SELECT paid, total_need AS total, remain, date, patient_id FROM patient_vouchers ) 
 ) a
 WHERE a.patient_id ='.$patient_id.'
ORDER BY a.date ASC

I need to get records from two different tables order by date, so what is wrong in this sql statement to get me that error?!

Answer 1

查找具有不同排序规则的变量并对齐它们，例如使用COLLATE utf8_general_ci