mysql將普通索引更改爲唯一索引，且對重複的數據只保留一條

ALTER TABLE PAY_ORDER DROP INDEX IND_PAY_ORDER_1;                                                                     刪除PAY_ORDER_1索引
ALTER IGNORE TABLE PAY_ORDER ADD UNIQUE INDEX IND_PAY_ORDER_1(ORDER_ID);               將ORDER_ID設爲唯一索引（IND_PAY_ORDER_1）刪除重複數據