【关于mysql：SQL中的左外连接，右表中缺少数据的左表列也显示出来】

左表

CREATE TABLE `branch`  (
  `id` varchar(36) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL DEFAULT '' COMMENT 'id',
  `sort` int(3) NOT NULL COMMENT '单位排序',
  `name` varchar(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL DEFAULT '' COMMENT '所属单位名称',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci COMMENT = '所属单位表' ROW_FORMAT = Dynamic;

CREATE TABLE `letter_data`  (
  `id` bigint(36) NOT NULL AUTO_INCREMENT COMMENT 'id，后端生成',
  `source` int(2) NOT NULL COMMENT '信件来源',
  `number` varchar(100) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL DEFAULT '' COMMENT '信件编号',
  `day` varchar(128) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL DEFAULT '' COMMENT '来件日期',
  `letter_type` varchar(30) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT '' COMMENT '信件类别',
  `branch_id` varchar(150) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT '' COMMENT '责任单位',
  `create_time` datetime(0) NOT NULL DEFAULT CURRENT_TIMESTAMP(0) COMMENT '创建时间',
  `update_time` datetime(0) NOT NULL DEFAULT CURRENT_TIMESTAMP(0) ON UPDATE CURRENT_TIMESTAMP(0) COMMENT '更新时间',
  PRIMARY KEY (`id`) USING BTREE,
  INDEX `branch_id_index`(`branch_id`) USING BTREE,
  INDEX `source_index`(`source`) USING BTREE,
  INDEX `day_index`(`day`) USING BTREE,
  INDEX `index_Source`(`source`) USING BTREE,
  INDEX `index_LetterType`(`letter_type`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 8574042 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci COMMENT = '信件详情表' ROW_FORMAT = Dynamic;

当你需要查询letter_data中branch_id中的数量并通过branch表连接查询，
查询语句

SELECT DISTINCT
        b.name ,
        count(*) AS count
        FROM
        branch  AS b
        LEFT JOIN letter_data AS ld ON ld.branch_id = b.id 
			where 1=1
			and ld.`day` between '2023-03-04 00:00:00' and '2023-03-04 12:00:00'
        GROUP BY
        b.name
        ORDER BY
        count DESC,b.name DESC

查询出来的视图

这种left join on后的where条件会使原本在on之后查询左表的字段筛选掉，从而即使左表的字段所count的值为null也不显示出来

解决的办法有三种：

第一种

将左连接表后的where条件加到on后面，使得不筛掉左表的值

SELECT DISTINCT
        b.name ,
        count(branch_id) AS count
        FROM
        branch  AS b
        LEFT JOIN letter_data AS ld ON ld.branch_id = b.id  and ld.`day` between '2023-03-04 00:00:00' and '2023-03-04 12:00:00'
        GROUP BY
        b.name
        ORDER BY
        count DESC,b.name DESC

第二种

在where条件后加 or branch_id is null，为的是显示右表没有数据的字段也显示出来，在count函数中的括号里面填branch_id字段，也就是count（branch_id）

SELECT DISTINCT
        b.name ,
        count(branch_id) AS count
        FROM
        branch  AS b
        LEFT JOIN letter_data AS ld ON ld.branch_id = b.id  
				  where 1=1 and ld.`day` between '2023-03-04 00:00:00' and '2023-03-04 12:00:00' 
				or branch_id is null
        GROUP BY
        b.name
        ORDER BY
        count DESC,b.name DESC

第三种

通过后端去筛选，将右表branch_id没有的数据设为0并加入到列表。

**BranchRankingListVo.java**
@Data
public class BranchRankingListVo {
    @ApiModelProperty(value = "责任单位")
    private String name;
    private Integer count;
}

**BranchEntity.java**
@Data
@NoArgsConstructor
@AllArgsConstructor
@TableName("branch")
@ApiModel(value="Branch对象", description="所属单位表")
public class BranchEntity {

    @ApiModelProperty(value = "id")
    @TableField("id")
    private String id;

    @ApiModelProperty(value = "单位排序")
    @TableField("sort")
    private Integer sort;

    @ApiModelProperty(value = "所属单位名称")
    @TableField("name")
    private String name;

}

实现方法

List<BranchRankingListVo> BranchRankinglist = letterDataMapper.getBranchRankinglist(startDate, endDate);
        // 获取责任单位列表
        List<BranchEntity> branches = branchMapper.getBranches();
        branches.forEach(f ->{
            boolean tempFlag = true;
            for (BranchRankingListVo branchRankingListVo : BranchRankinglist) {
                if (f.getName().equals(branchRankingListVo.getName())) {
                    tempFlag = false ;
                    break;
                }
            }
            if (tempFlag){
                BranchRankingListVo temp = new BranchRankingListVo();
                temp.setName(f.getName());
                temp.setCount(0);
                BranchRankinglist.add(temp);
            }
        });