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            <h1>SQL NOT NULL Constraint</h1>

            <p>By default, a column can hold NULL values.</p>
            <p>The <strong>NOT NULL</strong> constraint enforces a column to NOT accept NULL values.</p>
            <p>This ensures a field always contains a value, which means you cannot insert a new record or update a
                record without providing a value for this field.</p>

            <h2>NOT NULL on CREATE TABLE</h2>
            <p>The following SQL ensures that the "ID", "LastName", and "FirstName" columns will NOT accept NULL values
                when creating the "Persons" table:</p>
            <pre><code class="language-sql">CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255) NOT NULL,
    Age int
);</code></pre>

            <h2>NOT NULL on ALTER TABLE</h2>
            <p>Note: We do not support this syntax directly.</p>
            <p>To create a NOT NULL constraint on the "Age" column after the table is already created:</p>
            <pre><code class="language-sql">ALTER TABLE Persons
MODIFY Age int NOT NULL;</code></pre>
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