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    <p>In order to understand what "Allow Null" means, you need to understand what <code>NULL</code> is, and understand that it is different to simply a zero or blank string.</p> <p><code>NULL</code> is a special value in SQL. It indicates data that does not exist. This is different to data that is blank.</p> <p>When you create a table in MySQL, it allows you to specify whether your fields are allowed to be <code>NULL</code>. If you specify "Allow Null", then it will be possible to create records with <code>NULL</code> values in those fields.</p> <p>This may be useful to you in some cases. For example, a Yes/No field may be a boolean value, but if the user didn't specify their preference, you may want to set to <code>NULL</code> to indicate this rather than defaulting to yes or no.</p> <p>In many cases though, allowing <code>NULL</code> can be problematic. Fields set to <code>NULL</code> can produce unexpected results in queries, because <code>NULL</code> does not follow the same rules as other values in all cases. eg: If you query the above example field <code>WHERE myfield != 1</code>, you may expect to get results for all the records which are not equal to 1. However <code>NULL</code> fields will not be returned; you need to write a special case for them.</p> <p><code>NULL</code> is also used as the default value when doing <code>JOIN</code> queries where some of the results do not have a record to join to. The unknown fields from the joined table will all be set to <code>NULL</code>.</p> <p>In general, if you're not sure what to do with "Allow Null", the safest option is to set it to no allow nulls.</p> <p>See the <a href="http://en.wikipedia.org/wiki/Null_%28SQL%29" rel="noreferrer">Wikipedia entry for NULL in SQL</a> for more info.</p> <p>Hope that helps.</p>
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    1. COThank you! Really nice explained.
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    2. CO+1 very easy to understand, thank you
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