Mysql 双表条件查询

我在下面提到了两个表格：

Table1
ID        key          Val1         Date1
A-1       xyz-ze       AA           2018-10-01 15:16:20
A-1       xyz-ze       AC           2018-10-02 18:19:40
A-2       xyi-ze       BV           2018-07-01 15:16:20
A-2       xyi-ze       CR           2018-03-01 19:22:37
A-2       xyi-ze       BR           2018-10-03 21:17:24

Table2
ID        key          Val2         Date2
A-1       xyz-ze       AA           2018-10-01 15:16:20
A-2       xyi-ze       BR           2018-10-03 22:47:56

通过使用上面的表，我想获取Val1和Val2，但是考虑到Date1，Val1应该是最早的值，然后将Val1与Val2匹配，并基于此给出状态

所需输出：

ID    key     Val1  Val2 Date1                Date2                Status
A-1   xyz-ze  AA    AA   2018-10-01 15:16:20  2018-10-01 15:16:20  TRUE
A-2   xyi-ze  CR    BR   2018-03-01 19:22:37  2018-10-01 15:16:20  FALSE

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您可以使用自联接：

SELECT t1.ID, t1.`key`, t1.Val1,  t1.Date1, t2.Date1
FROM Table1 t1
LEFT JOIN Table1 t2 
   ON t1.ID = t2.ID AND t1.`key` = t2.`key` AND t2.Date1 < t1.Date1

通过这种方式，您可以使用以下各项确定最早的每组记录：

WHERE t2.Date1 IS NULL

因此，最终查询将是：

SELECT t1.ID, t1.`key`, t1.Val1, t3.Val2, t1.Date1, t3.Date2
FROM Table1 t1
LEFT JOIN Table1 t2 ON t1.ID = t2.ID AND t1.`key` = t2.`key` AND t2.Date1 < t1.Date1
INNER JOIN Table2 AS t3 ON t1.ID = t3.ID AND t1.`key` = t3.`key` 
WHERE t2.Date1 IS NULL

---

它给了我价值2的价值Null@VectorJX这是不可能的，因为表2内部连接到表1。可能是你打错了表格别名？

SELECT t1.ID, t1.`key`, t1.Val1, t3.Val2, t1.Date1, t3.Date2
FROM Table1 t1
LEFT JOIN Table1 t2 ON t1.ID = t2.ID AND t1.`key` = t2.`key` AND t2.Date1 < t1.Date1
INNER JOIN Table2 AS t3 ON t1.ID = t3.ID AND t1.`key` = t3.`key` 
WHERE t2.Date1 IS NULL