union.html

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            <h1>SQL UNION Operator</h1>

            <p>The UNION operator is used to combine the result-set of two or more SELECT statements.</p>

            <p>- Every SELECT statement within UNION must have the same number of columns</p>
            <p>- The columns must also have similar data types</p>
            <p>- The columns in every SELECT statement must also be in the same order</p>

            <h2>UNION Syntax</h2>
            <pre><code class="language-sql">SELECT column_name(s) FROM table1
UNION
SELECT column_name(s) FROM table2;</code></pre>

            <h2>UNION ALL Syntax</h2>
            <p>The UNION operator selects only distinct values by default. To allow duplicate values, use UNION ALL:</p>

            <pre><code class="language-sql">SELECT column_name(s) FROM table1
UNION ALL
SELECT column_name(s) FROM table2;</code></pre>

            <p><strong>Note:</strong> The column names in the result-set are usually equal to the column names in the
                first SELECT statement.</p>

            <h2>SQL UNION Example</h2>
            <p>The following SQL statement returns the first_names (only distinct values) from both the "patients" and
                the "doctors" table:</p>

            <pre><code class="language-sql">SELECT first_name FROM patients
UNION
SELECT first_name FROM doctors
ORDER BY first_name;</code></pre>

            <h2>SQL UNION ALL Example</h2>
            <p>The following SQL statement returns the first_names (duplicate values also) from both the "patients" and
                the "doctors" table:</p>

            <pre><code class="language-sql">SELECT first_name FROM patients
UNION ALL
SELECT first_name FROM doctors
ORDER BY first_name;</code></pre>
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