sql-server - 聚合函数返回 null

Question

我有这个示例数据

+------+------------+------------+
| CODE | START_DATE | END_DATE   |
+------+------------+------------+
| 0001 | 2012-01-01 | 2012-01-31 |
+------+------------+------------+
| 0001 | 2012-02-01 | 2012-02-29 |
+------+------------+------------+
| 0001 | 2012-03-01 | NULL       |
+------+------------+------------+
| 0002 | 2012-02-01 | 2012-02-29 |
+------+------------+------------+
| 0002 | 2012-03-01 | 2012-03-31 |
+------+------------+------------+
| 0002 | 2012-04-01 | NULL       |
+------+------------+------------+
| 0003 | 2012-02-01 | 2012-02-29 |
+------+------------+------------+
| 0003 | 2012-03-01 | 2012-03-31 |
+------+------------+------------+

动态链接

CREATE TABLE SAMPLE
(
    CODE VARCHAR(4),
    START_DATE DATETIME,
    END_DATE DATETIME
)

INSERT INTO SAMPLE (CODE, START_DATE, END_DATE) VALUES ('0001', {d '2012-01-01'}, {d '2012-01-31'})
INSERT INTO SAMPLE (CODE, START_DATE, END_DATE) VALUES ('0001', {d '2012-02-01'}, {d '2012-02-29'})
INSERT INTO SAMPLE (CODE, START_DATE, END_DATE) VALUES ('0001', {d '2012-03-01'}, NULL)

INSERT INTO SAMPLE (CODE, START_DATE, END_DATE) VALUES ('0002', {d '2012-02-01'}, {d '2012-02-29'})
INSERT INTO SAMPLE (CODE, START_DATE, END_DATE) VALUES ('0002', {d '2012-03-01'}, {d '2012-03-31'})
INSERT INTO SAMPLE (CODE, START_DATE, END_DATE) VALUES ('0002', {d '2012-04-01'}, NULL)

INSERT INTO SAMPLE (CODE, START_DATE, END_DATE) VALUES ('0003', {d '2012-02-01'}, {d '2012-02-29'})
INSERT INTO SAMPLE (CODE, START_DATE, END_DATE) VALUES ('0003', {d '2012-03-01'}, {d '2012-03-31'})

我应该得到这些数据

+------+------------+------------+
| CODE | START_DATE | END_DATE   |
+------+------------+------------+
| 0001 | 2012-01-01 | NULL       |
+------+------------+------------+
| 0002 | 2012-02-01 | NULL       |
+------+------------+------------+
| 0003 | 2012-02-01 | 2012-03-31 |
+------+------------+------------+

目前在 SQL Server 2005 中，我使用这个查询

SELECT  CODE,
        MIN(START_DATE) AS START_DATE,
        CASE
            WHEN    MAX(ISNULL(END_DATE, {d '9999-12-31'})) = {d '9999-12-31'}
            THEN    NULL
        ELSE
            MAX(END_DATE)
        END AS END_DATE
FROM    SAMPLE
GROUP BY    CODE

我在作弊 截至日期 9999-12-31 的 NULL 值

我正在为此寻找更好的解决方案

谢谢

Answer 1

您可以利用 NULL 和 NOT NULL 列的 COUNT 不同这一事实。所以脚本可以像这样:

SELECT  CODE,
        MIN(START_DATE) AS START_DATE,
        CASE 
            WHEN    COUNT(*) > COUNT(END_DATE)
            THEN    NULL
        ELSE
            MAX(END_DATE)
        END AS END_DATE
FROM    SAMPLE
GROUP BY    CODE