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POLEFT OUTER JOIN vs SUBSELECT in MySQL
 

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  1. POLEFT OUTER JOIN vs SUBSELECT in MySQL
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    <p>I have a table say <code>table1</code> which has 3 columns <code>column1, column2 and column3</code>.</p> <p>The <code>column1</code> and <code>column2</code> are a <code>FOREIGN KEY</code> with 2 other tables. However the data in <code>column3</code> is from n number of tables. </p> <p>For e.g. Let us consider Facebook. To display the activities it might maintain a table which could have <code>user1 photoliked photo1</code> or <code>user1 statusliked status1</code>. So in this case <code>column3</code> cannot be a <code>FOREIGN KEY</code> with a specific table.</p> <p>Now there are 2 ways of getting real data -</p> <p>1st way -</p> <pre><code>SELECT user_id, verb_id, CASE WHEN verb_id = photoliked THEN (SELECT photo_name FROM photos WHERE photo_id = column3) -- getting the desired data from the third column WHEN verb_id = statusliked THEN (SELECT status FROM statustable WHERE status_id = column3) ELSE '' END AS performedon FROM table1 JOIN table2 ON user_id = user_id -- joining the first column JOIN table3 ON verb_id = verb_id -- joining the second column </code></pre> <p>2nd way -</p> <pre><code>SELECT user_id, verb_id, CASE WHEN verb_id = photoliked THEN p.photo_name WHEN verb_id = statusliked THEN s.status ELSE '' END AS performedon FROM table1 JOIN table2 ON user_id = user_id -- joining the first column JOIN table3 ON verb_id = verb_id -- joining the second column LEFT JOIN photos p ON p.photo_id = column3 -- joining the column3 with specific table LEFT JOIN statustable s ON s.status_id = column3 </code></pre> <p><strong>Question</strong></p> <p>Which of the 2 ways is better to retrieve data? and which of the 2 queries is less expensive? </p>
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    1. COPossible duplicate http://stackoverflow.com/a/10684539/1239506
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    2. CONo it is not duplicate. In that question there is a `IN` in the `WHERE` clause and the `SUBSELECT` does not depend on any column of the main query.
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    3. COThe second query is better...
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