MySql DB

MySQL NULL Values

Last modified on: December 11, 2024

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<h3>MySQL NULL Values: Understanding and Handling Missing Data</h3>
<p>In MySQL, <strong><code>NULL</code></strong> represents missing or unknown data. It is different from <strong>0</strong>, an empty string (<code>''</code>), or any other value. A <code>NULL</code> value indicates the absence of data in a column.</p>
<hr />
<h3>Key Points About NULL</h3>
<ol>
<li><strong><code>NULL</code> is Not Equal to Anything</strong>:
<ul>
<li>A <code>NULL</code> value is not the same as <code>0</code> or <code>''</code>.</li>
<li>Use <strong><code>IS NULL</code></strong> or <strong><code>IS NOT NULL</code></strong> to check for <code>NULL</code> values.</li>
</ul>
</li>
<li><strong>NULL in Comparisons</strong>:
<ul>
<li>Comparing <code>NULL</code> with any value using standard operators (<code>=</code>, <code>!=</code>, <code>&lt;</code>, etc.) results in <code>UNKNOWN</code>.</li>
</ul>
</li>
</ol>
<hr />
<h3>Checking for NULL Values</h3>
<h4>1. <strong>Using IS NULL</strong></h4>
<p>Retrieve rows where a column has a <code>NULL</code> value:</p>
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<pre class="language-php"><code>SELECT * 
FROM users 
WHERE phone IS NULL;
</code></pre>
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<h4>2. <strong>Using IS NOT NULL</strong></h4>
<p>Retrieve rows where a column does not have a <code>NULL</code> value:</p>
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<pre class="language-php"><code>SELECT * 
FROM users 
WHERE phone IS NOT NULL;
</code></pre>
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<h3>NULL in INSERT and UPDATE</h3>
<h4>1. <strong>Insert NULL Values</strong></h4>
<p>You can explicitly insert <code>NULL</code> into a column:</p>
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<pre class="language-php"><code>INSERT INTO users (name, phone) 
VALUES ('John Doe', NULL);
</code></pre>
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<h4>2. <strong>Update to NULL</strong></h4>
<p>Set a column's value to <code>NULL</code>:</p>
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<pre class="language-php"><code>UPDATE users 
SET phone = NULL 
WHERE id = 1;
</code></pre>
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<h3>NULL in Functions and Expressions</h3>
<h4>1. <strong>Using COALESCE()</strong></h4>
<p>The <strong><code>COALESCE()</code></strong> function returns the first non-<code>NULL</code> value from a list:</p>
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<pre class="language-php"><code>SELECT name, COALESCE(phone, 'No Phone') AS phone_display 
FROM users;
</code></pre>
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<h4>2. <strong>NULL and Aggregate Functions</strong></h4>
<ul>
<li>Aggregate functions (e.g., <code>SUM</code>, <code>COUNT</code>, <code>AVG</code>) ignore <code>NULL</code> values except for <code>COUNT(*)</code>.</li>
<li>Example:
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<pre class="language-php"><code>SELECT AVG(salary) AS average_salary 
FROM employees 
WHERE salary IS NOT NULL;
</code></pre>
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</li>
</ul>
<h4>3. <strong>IFNULL()</strong></h4>
<p>Replace <code>NULL</code> with a specified value:</p>
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<pre class="language-php"><code>SELECT name, IFNULL(phone, 'N/A') AS phone_display 
FROM users;
</code></pre>
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<h3>NULL in Conditional Statements</h3>
<h4>1. <strong>Handling NULL in WHERE Clause</strong></h4>
<p>Retrieve rows with conditional logic for <code>NULL</code>:</p>
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<pre class="language-php"><code>SELECT * 
FROM users 
WHERE role = 'admin' AND phone IS NOT NULL;
</code></pre>
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<h4>2. <strong>Using CASE for NULL Handling</strong></h4>
<p>Replace <code>NULL</code> values dynamically:</p>
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<pre class="language-php"><code>SELECT 
    name, 
    CASE 
        WHEN phone IS NULL THEN 'No Phone' 
        ELSE phone 
    END AS phone_display 
FROM users;
</code></pre>
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<h3>Practical Example</h3>
<p>Retrieve a list of users with a fallback message for missing phone numbers:</p>
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<pre class="language-php"><code>SELECT 
    name, 
    COALESCE(phone, 'Phone Not Provided') AS contact 
FROM users;
</code></pre>
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<h3>&nbsp;</h3>
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