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    <p>NULLs have special behavior: comparing anything with a NULL gives you back a <code>NULL</code>, which is something else than <code>false</code> or <code>0</code>. It means "unknown".</p> <p>For example, take this table:</p> <pre><code> user_id | gender ------------------ 1 | NULL 2 | 'M' 3 | 'F' 4 | 'F' </code></pre> <p><code>SELECT * FROM mytable WHERE gender = 'M'</code> will return 1 row, as expected</p> <p><code>SELECT * FROM mytable WHERE gender != 'M'</code> will return 2 rows, NOT 3 rows.</p> <p><code>SELECT * FROM mytable WHERE gender != 'M' OR gender IS NULL</code> will return the expected 3 rows.</p> <hr> <p>Edit: For some applications, using <code>0</code> (or, God forbid, another "magic number") instead of <code>NULL</code> is not even advisable (units or exact values are not relevant in this example): </p> <pre><code> Date | Temperature -------------------------- 2010-01-01 | 10 2010-01-02 | 4 2010-01-03 | 0 2010-01-04 | -22 2010-01-05 | -45 2010-01-06 | NULL 2010-01-07 | -34 </code></pre> <p>Here, the <code>NULL</code> on Jan 6th means "value unknown" - maybe because the temperature was so low that the thermometer probe stopped responding. However, it's a completely different meaning than Jan 3rd, when the temperature was <code>0</code>, that is, 0 degrees.</p> <p>Also, as @Bill Karwin mentions, NULLs behave specially in aggregate functions (<code>COUNT</code>,<code>SUM</code>,<code>AVG</code> etc.): calculating <code>AVG(Temperature)</code> on the above data would give you <code>-14.5</code>, as the NULL row is ignored.</p>
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    1. POWhat's the point of "NOT NULL DEFAULT '' "?
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    1. POWhat's the point of "NOT NULL DEFAULT '' "?
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    1. COI had no idea null would behave that way! I'm accepting this one as this is a crystal clear example of why to use not null.
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    2. CO+1 It's also worth mentioning that NULL is skipped for most aggregate functions. You probably don't want to count NULL as zero for `SELECT AVG(Temperature)` because it would skew your result. So NULL does have a useful purpose.
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