null-values.html

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            <h1>SQL NULL Values</h1>

            <h2>What is a NULL Value?</h2>
            <p>A field with a NULL value is a field with no value.</p>
            <p>If a field in a table is optional, it is possible to insert a new record or update a record without
                adding a value to this field. Then, the field will be saved with a NULL value.</p>

            <h2>How to Test for NULL Values?</h2>
            <p>It is not possible to test for NULL values with comparison operators, such as =, &lt;, or &lt;&gt;.</p>
            <p>We have to use the <strong>IS NULL</strong> and <strong>IS NOT NULL</strong> operators instead.</p>

            <h3>IS NULL Syntax</h3>
            <pre><code class="language-sql">SELECT column_names
FROM table_name
WHERE column_name IS NULL;</code></pre>

            <h3>IS NOT NULL Syntax</h3>
            <pre><code class="language-sql">SELECT column_names
FROM table_name
WHERE column_name IS NOT NULL;</code></pre>

            <h2>The IS NULL Operator</h2>
            <p>The IS NULL operator is used to test for empty values (NULL values).</p>
            <p>The following SQL lists all patients with a NULL value in the "allergies" field:</p>
            <pre><code class="language-sql">SELECT *
FROM patients
WHERE allergies IS NULL;</code></pre>
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