已经唯一索引了，为什么还要普通索引？

来源：4-3 设计品类和参数表

弱弱小乖乖

2020-10-19

DROP TABLE IF EXISTS `t_spec_group`;
CREATE TABLE `t_spec_group`  (
  `id` int(10) UNSIGNED NOT NULL AUTO_INCREMENT COMMENT '主键',
  `spg_id` int(10) UNSIGNED NOT NULL COMMENT '品类编号',
  `name` varchar(200) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL COMMENT '品类名称',
  `is_delted` tinyint(1) NOT NULL DEFAULT 0 COMMENT '逻辑删除',
  PRIMARY KEY (`id`) USING BTREE,
  UNIQUE INDEX `unq_spg_id`(`spg_id`) USING BTREE,
  UNIQUE INDEX `unq_name`(`name`) USING BTREE,
  INDEX `idx_spg_id`(`spg_id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 6 CHARACTER SET = utf8 COLLATE = utf8_general_ci COMMENT = '品类表' ROW_FORMAT = Dynamic;

spg_id 这个字段已经设置了唯一索引，为什么还要给他设置普通索引？

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