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POA SELECT statement as subquery with two conditions
 

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  1. POA SELECT statement as subquery with two conditions
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    <p>I'm working on a ECG data that basically has the following schema</p> <pre><code>Col 1 = TIMESTAMP Col 2 = PATIENTID Col 3 = ECGVALUE </code></pre> <p>Now, I'm trying to write a SQL statement that should be able to select all the rows that satisfy the following condition</p> <pre><code>Row index &gt;= n and TIMESTAMP of xth row &lt;= TIMESTAMP of nth row + offset </code></pre> <p>To explaing it further, let say I've following data in my database</p> <pre><code>1.1 ANON 1.1 1.3 ANON 2.3 3.5 ANON 4.3 5.0 ANON 6.5 6.3 ANON 7.5 7.9 ANON 8 8.6 ANON 9.4 </code></pre> <p>Now, I want to select the data from 3rd row till 3 seconds of data has been collected, which means that my resultset should have</p> <pre><code>3.5 ANON 2.3 *//3rd row till TIMESTAMP &lt;= 3.5 + 3 &lt;= 6.5* 1.3 ANON 2.3 3.5 ANON 4.3 5.0 ANON 6.5 6.3 ANON 7.5 </code></pre> <p>Last two rows are neglected as difference between TIMESTAMP of first and last cannont go beyond 3. So If I go back to my condition, which is</p> <pre><code>Row index &gt;= n and TIMESTAMP of xth row &lt;= TIMESTAMP of nth row + offset </code></pre> <p>Here, </p> <pre><code>n: nth row from where data must be selected x: Any arbitary row in result set offset: Maximum difference between first and last TIMESTAMP of result set. </code></pre> <p>I've written a working SQL statement for the above condition, but I think its not that much optimized as I'm new to SQL.</p> <pre><code>SELECT TIMESTAMP, ECGVALUE FROM ( SELECT TIMESTAMP, ECGVALUE, ROW_NUMBER() OVER() AS RN FROM EKLUND.DEV_RAWECG ) WHERE RN &gt;= n AND TIMESTAMP &lt;= ( SELECT TIMESTAMP FROM ( SELECT TIMESTAMP, ROW_NUMBER() OVER() AS TM ) WHERE TM = n ) + offset; </code></pre>
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    1. USShivam
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    1. COAre you aware that the order of rows is not guaranteed unless specified explicitly with `ORDER BY`?
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      1. USAndriy M
    2. COYou sure? The data in database is already ordered by ASC order.
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      1. USShivam
    3. COWe are talking about the order of data as returned by the query, not about the order of data as stored physically in the database. Whatever the latter may be, it has no 100% influence on the former. The rows may be retrieved in parallel and the engine, for the sake of performance, will build the resulting set from the obtained chunks without paying attention to their particular order, in the absence of ORDER BY.
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      1. USAndriy M
 

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