mysql将普通索引更改为唯一索引，且对重复的数据只保留一条

ALTER TABLE PAY_ORDER DROP INDEX IND_PAY_ORDER_1;                                                                     删除PAY_ORDER_1索引

ALTER IGNORE TABLE PAY_ORDER ADD UNIQUE INDEX IND_PAY_ORDER_1(ORDER_ID);               将ORDER_ID设为唯一索引（IND_PAY_ORDER_1）删除重复数据