MySQL 5.7

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CREATE TABLE IF NOT EXISTS `sampletable` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `mid` varchar(50) DEFAULT NULL,
  `category` varchar(50) DEFAULT NULL,
  `createdttm` datetime DEFAULT current_timestamp(),
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=31826 DEFAULT CHARSET=latin1 ROW_FORMAT=COMPACT;
ALTER TABLE `sampletable` ADD COLUMN `created` DATE AS (DATE(`createdttm`));

Records: 0  Duplicates: 0  Warnings: 0


ALTER TABLE `sampletable` ADD UNIQUE `unique_index`(`mid`, `created`);

Records: 0  Duplicates: 0  Warnings: 0


INSERT INTO sampletable (`mid`, `createdttm`) VALUES
('mid1', '2021-08-02 15:00:00'),
('mid1', '2021-08-03 15:00:00')

Records: 2  Duplicates: 0  Warnings: 0


INSERT INTO sampletable (`mid`, `createdttm`) VALUES
('mid1', '2021-08-02 15:30:00')

Duplicate entry 'mid1-2021-08-02' for key 'unique_index'