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    <p>Here's one way (lameness quotient calculation left as an exercise to the reader):</p> <pre><code>CREATE TABLE scores (user_id VARCHAR2(32), test_date DATE, score NUMBER); INSERT INTO scores VALUES('U1',SYSDATE-61, 85); INSERT INTO scores VALUES('U1',SYSDATE-31, 89); INSERT INTO scores VALUES('U1',SYSDATE, 92); INSERT INTO scores VALUES('U2',SYSDATE-61, 65); INSERT INTO scores VALUES('U2',SYSDATE-31, 89); INSERT INTO scores VALUES('U2',SYSDATE, 84); COMMIT; SELECT s1.user_id, s1.test_date, s2.score-s1.score delta FROM scores s1 JOIN (SELECT user_id, trunc(test_date,'MM') test_date, score FROM scores) s2 ON (s1.user_id = s2.user_id AND trunc(add_months(s1.test_date,1),'MM') = s2.test_date); USER_ID TEST_DATE DELTA -------------------------------- --------- ---------- U1 9/15/2009 3 U1 8/16/2009 4 U2 9/18/2009 -5 U2 8/19/2009 24 </code></pre> <p><strong>EDIT</strong>: It's a slow afternoon, so I decided to look into this analytic function stuff that 10g offers (further dragging myself into the current century ;-), and rewrote the above using the LAG function:</p> <pre><code>SELECT user_id, test_date, score , LAG(score, 1, NULL) OVER (PARTITION BY user_id ORDER BY test_date DESC) - score delta , LAG(score, 1, NULL) OVER (PARTITION BY user_id ORDER BY test_date DESC) AS next_score FROM scores ORDER BY 1, 2 DESC; </code></pre> <p>Which produces:</p> <pre><code>USER_ID TEST_DATE SCORE DELTA NEXT_SCORE -------------------------------- ----------- ---------- ---------- ---------- U1 10/19/2009 92 U1 9/18/2009 89 3 92 U1 8/19/2009 85 4 89 U2 10/19/2009 84 U2 9/18/2009 89 -5 84 U2 8/19/2009 65 24 89 </code></pre> <p>Look, Ma! No self-join! Now THAT's slick ;-) (As an aside, the explain plans indicate the self-join is not as efficient).</p> <p>As a springboard, I started with this <a href="http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:16389714357623" rel="nofollow noreferrer">asktom.com question</a>.</p>
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    1. POHow do I get entries in the next month from oracle sql?
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    1. POHow do I get entries in the next month from oracle sql?
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    1. COI think this is quite slick
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      1. USSteven Noble
    2. COOne caveat: if the table has a lot of data, appropriate indexes might become necessary.
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    3. COYou're missing a test case -- if you put in more than one user, you'll find your query interleaves the different user's test results because the only ordering is on test_date. You should put a PARTITION BY user_id in front of the ORDER BY test_date.
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      1. USWilliam Rose
 

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