Mysql重复数据查询置为空

　　前两天产品有个需求，相同的商品因为价格不同而分开展示，但是明细还是算一条明细，具体区分展示出商品的价格和数量信息，其他重复的商品信息要置空。

　　需求并不难，用程序代码循环处理就可以了。但是后面涉及到打印报表，只能用纯sql语句生成。开始自己用了union 写的复杂了一些，后来百度了下文章看到一种实现思路，在此记录一下。下面的内容是根据其思路分析自己修改实现的一个demo，也简单扩展了一点实现功能。

直接上代码好了：

-- ----------------------------
-- Table structure for act
-- ----------------------------
DROP TABLE IF EXISTS `act`;
CREATE TABLE `act`  (
  `id` ) NOT NULL AUTO_INCREMENT COMMENT 'id',
  `name` ) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL COMMENT 'name',
  `type` ) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL COMMENT 'type',
  `peple` ) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL COMMENT 'peple',
  `age` ) NULL DEFAULT NULL COMMENT 'age',
  `sex` ) NULL DEFAULT NULL COMMENT 'sex',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE  CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of act
-- ----------------------------
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生成表数据：

下面案列是以type相同与否作为判断条件分组，peple就是type的小写，方便对照观察。后面的Bk结尾的别名字段也是方便对照观察加上的。

实现sql:

SELECT
id,
CASE
               WHEN id in ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.name = a.name
                         GROUP BY type

                       ) THEN name
             WHEN id in ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.name = a.name
                         GROUP BY type

                       ) THEN name
             ELSE ''
        END AS 'name' ,
        CASE WHEN id in ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type

                         GROUP BY type

                       ) THEN type
             WHEN id in ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type
                         GROUP BY type

                       ) THEN type
             ELSE ''
        END AS type,
                 CASE WHEN id in ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.age = a.age

                         GROUP BY type

                       ) THEN age
             WHEN id in ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.age = a.age
                         GROUP BY type

                       ) THEN age
             ELSE ''
        END AS age,
                CASE WHEN id in ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.sex = a.sex

                         GROUP BY type

                       ) THEN sex
             WHEN id in ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.sex = a.sex
                         GROUP BY type

                       ) THEN sex
             ELSE ''
        END AS sex, 

         peple,name as nameBk,age as ageBk,sex as sexBk
FROM    (select * from act order by type,id asc) as b

执行结果：

如果干脆要把任意字段置空（不管是不是重复的）也可以：

SELECT
id,
CASE
               WHEN id = ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type
                         GROUP BY type

                       ) THEN name
             WHEN id = ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type
                         GROUP BY type

                       ) THEN name
             ELSE ''
        END AS name,
        CASE WHEN id = ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type

                         GROUP BY type

                       ) THEN type
             WHEN id = ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type
                         GROUP BY type

                       ) THEN type
             ELSE ''
        END AS type,
                 CASE WHEN id = ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type

                         GROUP BY type

                       ) THEN age
             WHEN id = ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type
                         GROUP BY type

                       ) THEN age
             ELSE ''
        END AS age,
                CASE WHEN id = ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type

                         GROUP BY type

                       ) THEN sex
             WHEN id = ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type
                         GROUP BY type

                       ) THEN sex
             ELSE ''
        END AS sex, 

         peple,name as nameBk,age as ageBk,sex as sexBk
FROM    (select * from act order by type,id asc) as b

执行结果：

上面案例都是以type作为判断分组条件，如果是有多个字段，直接在后面接着增加就可以了。