Question

假设我有一张桌子：

CREATE TABLE T
(
    TableDTM  TIMESTAMP  NOT NULL,
    Code      INT        NOT NULL
);

我插入一些行：

INSERT INTO T (TableDTM, Code) VALUES ('2011-01-13 10:00:00', 5);
INSERT INTO T (TableDTM, Code) VALUES ('2011-01-13 10:10:00', 5);
INSERT INTO T (TableDTM, Code) VALUES ('2011-01-13 10:20:00', 5);
INSERT INTO T (TableDTM, Code) VALUES ('2011-01-13 10:30:00', 5);
INSERT INTO T (TableDTM, Code) VALUES ('2011-01-13 10:40:00', 0);
INSERT INTO T (TableDTM, Code) VALUES ('2011-01-13 10:50:00', 1);
INSERT INTO T (TableDTM, Code) VALUES ('2011-01-13 11:00:00', 1);
INSERT INTO T (TableDTM, Code) VALUES ('2011-01-13 11:10:00', 1);
INSERT INTO T (TableDTM, Code) VALUES ('2011-01-13 11:20:00', 0);
INSERT INTO T (TableDTM, Code) VALUES ('2011-01-13 11:30:00', 5);
INSERT INTO T (TableDTM, Code) VALUES ('2011-01-13 11:40:00', 5);
INSERT INTO T (TableDTM, Code) VALUES ('2011-01-13 11:50:00', 3);
INSERT INTO T (TableDTM, Code) VALUES ('2011-01-13 12:00:00', 3);
INSERT INTO T (TableDTM, Code) VALUES ('2011-01-13 12:10:00', 3);

所以我最终得到了一张类似于：

的表格

2011-01-13 10:00:00, 5
2011-01-13 10:10:00, 5
2011-01-13 10:20:00, 5
2011-01-13 10:30:00, 5
2011-01-13 10:40:00, 0
2011-01-13 10:50:00, 1
2011-01-13 11:00:00, 1
2011-01-13 11:10:00, 1
2011-01-13 11:20:00, 0
2011-01-13 11:30:00, 5
2011-01-13 11:40:00, 5
2011-01-13 11:50:00, 3
2011-01-13 12:00:00, 3
2011-01-13 12:10:00, 3

如何选择每组相同数字的第一个日期，所以我最终得到了这个：

2011-01-13 10:00:00, 5
2011-01-13 10:40:00, 0
2011-01-13 10:50:00, 1
2011-01-13 11:20:00, 0
2011-01-13 11:30:00, 5
2011-01-13 11:50:00, 3

在一天中的大部分时间里，我一直在搞乱子查询等等，出于某种原因我似乎无法破解它。我确定在某个地方有一个简单的方法！

我可能想从结果中排除0，但现在这并不重要..

Answer 1

PostgreSQL支持窗口函数，请查看this

[编辑] 请尝试以下方法：

SELECT TableDTM, Code FROM
(
    SELECT TableDTM,
           Code,
           LAG(Code, 1, NULL) OVER (ORDER BY TableDTM) AS PrevCode
    FROM   T
)
WHERE PrevCode<>Code OR PrevCode IS NULL;

Answer 2

试试这个：

SELECT MIN(TableDTM) TableDTM, Code
FROM
(
    SELECT T1.TableDTM, T1.Code, MIN(T2.TableDTM) XTableDTM
    FROM T T1
    LEFT JOIN T T2
    ON T1.TableDTM <= T2.TableDTM
    AND T1.Code <> T2.Code
    GROUP BY T1.TableDTM, T1.Code
) X
GROUP BY XTableDTM, Code
ORDER BY 1;

Answer 3

你可以试试像

这样的东西

"SELECT DISTINCT Code, (SELECT MIN(TableDTM) FROM T AS Q WHERE Q.Code = T.Code) As TableDTM FROM T;"

如果您需要排除0，请将其更改为：

 SELECT DISTINCT Code, (SELECT MIN(TableDTM) FROM T AS Q WHERE Q.Code = T.Code) As TableDTM FROM T WHERE Code <> 0;

SQL查找表中第一次出现的数据集

4 个答案: