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            <h1>SQL EXISTS Operator</h1>

            <p>The EXISTS operator is used to test for the existence of any record in a subquery.</p>
            <p>The EXISTS operator returns TRUE if the subquery returns one or more records.</p>
            <p><strong>Note:</strong> SQL statements that use the EXISTS condition are very inefficient since the
                sub-query is rerun for EVERY row in the outer query's table. There are more efficient ways to write most
                queries, that do not use the EXISTS condition.</p>

            <h2>EXISTS Syntax</h2>
            <pre><code class="language-sql">SELECT column_name(s)
FROM table_name
WHERE EXISTS
  (SELECT column_name FROM table_name WHERE condition);</code></pre>

            <h2>SQL EXISTS Examples</h2>
            <p>The following SQL statement returns all patients who were diagnosed with pregnancy:</p>
            <pre><code class="language-sql">SELECT * FROM patients
WHERE EXISTS (SELECT diagnosis FROM admissions
              WHERE patients.patient_id = admissions.patient_id
              AND diagnosis = 'Pregnancy');</code></pre>

            <p>The above SQL statement can be written more efficiently as:</p>
            <pre><code class="language-sql">SELECT * FROM patients
JOIN admissions ON patients.patient_id = admissions.patient_id
WHERE diagnosis = 'Pregnancy';</code></pre>
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