Question

I have this sample data

+------+------------+------------+
| 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 |
+------+------------+------------+

DDL

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'})

I’m expected to get this data

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

currently in SQL Server 2005, I use this query

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

I’m cheating NULL value to date 9999-12-31

I’m looking for better solution for this

thank you

Answer 1

You could utilize the fact that the COUNT of NULL and NOT NULL columns will be different. So the script can go like this:

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