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    <blockquote> <p>The value 2 (did) with the update instruction shouldn't be 4?</p> </blockquote> <p><strong>No, it is 4, and it should.</strong></p> <p>Because the <code>UPDATE</code> statements in SQL, are <strong>All-at-once</strong> operations. Meaning that all the expressions that you have, in the same logical phase are evaluated as if at the same point in time. In your example:</p> <pre><code>UPDATE discipline SET did = did + did WHERE did=2; </code></pre> <p>the <code>did</code> will equal to <code>4</code> because the expression <code>did = did + did</code> is evaluated wit the value <code>did = 2</code> at the same time.</p> <p><a href="http://www.sqlfiddle.com/#!2/a1b10/4" rel="nofollow"><strong>SQL Fiddle Demo</strong></a></p> <p>This will make your table looks like:</p> <pre><code>| DID | CODE | UNIVERSITY | NUMBER_STUDENTS | --------------------------------------------- | 1 | BD | U1 | 30 | | 3 | SIBD | U3 | 30 | | 4 | IBD | U2 | 30 | &lt;&lt;&lt;&lt;&lt; </code></pre> <hr> <blockquote> <p>I don't understand why the result of the instruction SELECT SUM (DISTINCT did) is 6 and isn't 9 (1+4+3).</p> </blockquote> <p>The values of <code>Did</code> after update will be:</p> <pre><code>1 3 4 </code></pre> <p>Therefore:</p> <pre><code>SELECT SUM(DISTINCT did) FROM discipline </code></pre> <p>Will equal to <code>8</code>.</p> <p><a href="http://www.sqlfiddle.com/#!2/a1b10/5" rel="nofollow"><strong>SQL Fiddle Demo</strong></a></p> <hr> <p>In your updated example, you won't be able to <code>UPDATE did</code>, becuase of the foreign key constraint and you will got an exception:</p> <blockquote> <p>Cannot delete or update a parent row: a foreign key constraint fails (<code>db_2_65e0c</code>.<code>precedence</code>, CONSTRAINT <code>precedence_ibfk_2</code> FOREIGN KEY (<code>advanced</code>) REFERENCES <code>discipline</code> (<code>did</code>))</p> </blockquote> <p><a href="http://www.sqlfiddle.com/#!2/14345" rel="nofollow"><strong>SQL Fiddle Demo</strong></a></p> <p>In this case, you will need to add the <code>ON DELETE CASCADE ON UPDATE CASCADE</code> to the other foreign key constraint:</p> <pre><code> FOREIGN KEY (advanced) REFERENCES discipline(did) </code></pre> <p>Then the <code>UPDATE</code>:</p> <pre><code>UPDATE discipline SET did=did + did WHERE did=2; </code></pre> <p>will be the same as explained before, and </p> <pre><code>SELECT SUM(DISTINCT did) FROM discipline </code></pre> <p>Will equal to <code>8</code>.</p> <p><a href="http://www.sqlfiddle.com/#!2/82c86/3" rel="nofollow"><strong>SQL fiddle demo of the two table(working)</strong></a></p>
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    1. COAnd with the table precedence the value will change? Thanks.
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    2. CO@user1966757 Sorry, What do you mean?
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    3. COI have edited my post I am using a table named precedence and I haven't put it on my post. This will change the result?
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