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` int(10) NOT NULL AUTO_INCREMENT COMMENT 'id',
  `name` varchar(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL COMMENT 'name',
  `type` varchar(12) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL COMMENT 'type',
  `peple` varchar(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL COMMENT 'peple',
  `age` int(3) NULL DEFAULT NULL COMMENT 'age',
  `sex` int(1) NULL DEFAULT NULL COMMENT 'sex',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 10 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of act
-- ----------------------------
INSERT INTO `act` VALUES (1, '张三', 'A', 'a', 10, 1);
INSERT INTO `act` VALUES (2, '张三', 'A', 'a', 10, 0);
INSERT INTO `act` VALUES (3, '王五', 'B', 'b', 10, 1);
INSERT INTO `act` VALUES (4, '赵六', 'B', 'b', 20, 0);
INSERT INTO `act` VALUES (5, '刘七', 'C', 'c', 30, 1);
INSERT INTO `act` VALUES (6, '阿三', 'D', 'd', 10, 0);
INSERT INTO `act` VALUES (7, '阿四', 'D', 'd', 20, 0);
INSERT INTO `act` VALUES (8, '阿五', 'A', 'a', 30, 1);
INSERT INTO `act` VALUES (9, '张三', 'A', 'a', 30, 1);

 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
                         HAVING COUNT(*) > 1
                       ) THEN name 
             WHEN id in ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.name = a.name
                         GROUP BY type
                         HAVING COUNT(*) = 1
                       ) 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
                         HAVING COUNT(*) >= 1
                       ) THEN type 
             WHEN id in ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type
                         GROUP BY type
                         HAVING COUNT(*) = 1  
                       ) 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
                         HAVING COUNT(*) >= 1
                       ) THEN age 
             WHEN id in ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.age = a.age
                         GROUP BY type
                         HAVING COUNT(*) = 1  
                       ) 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
                         HAVING COUNT(*) >= 1
                       ) THEN sex 
             WHEN id in ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.sex = a.sex
                         GROUP BY type
                         HAVING COUNT(*) = 1  
                       ) 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
                         HAVING COUNT(*) > 1
                       ) THEN name 
             WHEN id = ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type
                         GROUP BY type
                         HAVING COUNT(*) = 1
                       ) 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
                         HAVING COUNT(*) >= 1
                       ) THEN type 
             WHEN id = ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type
                         GROUP BY type
                         HAVING COUNT(*) = 1  
                       ) 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
                         HAVING COUNT(*) >= 1
                       ) THEN age 
             WHEN id = ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type
                         GROUP BY type
                         HAVING COUNT(*) = 1  
                       ) 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
                         HAVING COUNT(*) >= 1
                       ) THEN sex 
             WHEN id = ( SELECT MIN(id)
                         FROM   (select * from act) as a
                         WHERE  b.type = a.type
                         GROUP BY type
                         HAVING COUNT(*) = 1  
                       ) 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。