Question

说我有这样一张桌子：

model_id | object_id | date_modified
1        | 123       | NULL
1        | 123       | 2012-12-11 16:55:21
1        | 456       | 2012-12-11 16:52:21
1        | 789       | NULL

我想返回尚未修改的object_ids。这是我到目前为止所尝试的内容：

SELECT object_id from Table WHERE date_modified IS NULL

这会返回123和789，我只想要789。

SELECT DISTINCT object_id from Table WHERE date_modified IS NULL

这也会返回123和789，因为在WHERE过滤结果后应用了DISTINCT子句。

有没有办法通过单个查询实现这一点，还是应该返回一个不同的object_id列表并循环并以编程方式过滤？

Answer 1

SELECT object_id from Table GROUP BY object_id HAVING MAX(date_modified) IS NULL

Answer 2

嵌套查询会有所帮助。内部查询选择已修改的所有object_id，然后外部查询选择具有空日期但object_id不在修改列表中的所有记录。

SELECT object_id FROM table
  WHERE date_modified IS NULL AND object_id NOT IN (
    SELECT object_id FROM table
      WHERE date_modified IS NOT NULL
  )

Answer 3

查询：

SQLFIDDLEExample

SELECT DISTINCT t.object_id 
from Table1 t
WHERE t.object_id NOT IN (SELECT t1.object_id 
                          FROM Table1 t1
                          WHERE t1.date_modified IS NOT NULL)

结果：

| OBJECT_ID |
-------------
|       789 |

在WHERE之前应用DISTINCT

3 个答案: