SQL 2相同的行

我的问题是：

SELECT CONCAT(f.name, ' ', f.parent_names) AS FullName,
       stts.name AS 'Status',
       u.name AS Unit,
       city.name AS City,
       hus.mobile1 AS HusbandPhone,
       wife.mobile1 AS WifePhone,
       f.phone AS HomePhone,
       f.contact_initiation_date AS InitDate,
       fh.created_at AS StatusChangeDate,
       cmt.created_at AS CommentDate,
       cmt.comment AS LastComment
FROM families f
JOIN categories stts ON f.family_status_cat_id = stts.id
JOIN units u ON f.unit_id = u.id
JOIN categories city ON f.main_city_cat_id = city.id
JOIN contacts hus ON f.husband_id = hus.id
JOIN contacts wife ON f.wife_id = wife.id
JOIN comments cmt ON f.id = cmt.commentable_id
AND cmt.created_at =
    (SELECT MAX(created_at)
     FROM comments
     WHERE commentable_id = f.id)
JOIN family_histories fh ON f.id = fh.family_id
AND fh.created_at =
    (SELECT MAX(created_at)
     FROM family_histories
     WHERE family_id = f.id
         AND family_history_cat_id = 1422)
WHERE f.id = 17883

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问题：结果是2行-但它们是相同的。为什么我得到两个结果而不是一个？

可能在评论或家庭历史记录上存在双重关系，您可以通过返回

*

作为结果来检查。应该在某个地方有所区别

提供完整的结果以发现问题

您可以（但不推荐）通过设置

DISTINCT

来解决此问题

SELECT DISTINCT CONCAT(f.name, ' ', f.parent_names) AS FullName, stts.name AS 'Status', u.name AS Unit, city.name AS City, hus.mobile1 AS HusbandPhone, wife.mobile1 AS WifePhone, f.phone AS HomePhone, f.contact_initiation_date AS InitDate, fh.created_at AS StatusChangeDate, cmt.created_at AS CommentDate, cmt.comment AS LastComment
FROM families f JOIN categories stts ON f.family_status_cat_id = stts.id
JOIN units u ON f.unit_id = u.id
JOIN categories city ON f.main_city_cat_id = city.id
JOIN contacts hus ON f.husband_id = hus.id
JOIN contacts wife ON f.wife_id = wife.id
JOIN comments cmt ON f.id = cmt.commentable_id AND cmt.created_at = (SELECT MAX(created_at) FROM comments WHERE commentable_id = f.id)
JOIN family_histories fh ON f.id = fh.family_id AND fh.created_at = (SELECT MAX(created_at) FROM family_histories WHERE family_id = f.id AND family_history_cat_id = 1422)
WHERE f.id = 17883

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当您连接存在一对多关系的表时，您会看到返回了多行，而事实上该行在主表中只存在一次，但在一个子表中有多行相关数据

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还要注意，由于子表中没有匹配的记录，因此可能会丢失所需的数据。在这种情况下，请使用。

这取决于表的结构和表中的数据。你能在你的问题中添加相关表格结构的描述吗？这是由于JOIN。使用

DISTINCT

关键字获取不同的行。连接中使用的哪些ID保证（而不仅仅是假定的）是唯一的？我认为仅使用DISTINCT不是一个好方法。如果存在重复数据，而您对此无能为力，那么您真的没有选择的余地。但是，如果复制是因为没有正确过滤而发生的，那么使用distinct只会隐藏一个错误，这个错误可能会在将来产生不良后果。