How do you send NULL as a variable from PHP to SQL?

Question

I want to send a value of "NULL" to a column in mySQL.

   $ParentEventID = "NULL";

   mysql_query("UPDATE events SET 
     ParentEventID = '$ParentEventID'");

The column keeps defaulting to "0". I have the column set to accept NULL and I can edit the cell with the NULL check box.

I need to not set the default to NULL because it may effect code in other places.

+1 For a question that seems like a FAQ but I've never seen it on SO before. — Michael Berkowski
– Michael Berkowski, Commented Jun 18, 2011 at 20:55

Michael Berkowski · Accepted Answer · 2011-06-18 20:48:34Z

22

You are on the right track making "NULL" a string. If you need to set it NULL, then remove the qoutes around $ParentEventID.

mysql_query("UPDATE events SET ParentEventID = $ParentEventID");

However, before doing so make sure that the value of $ParentEventID === NULL.

answered Jun 18, 2011 at 20:48

Michael Berkowski

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4 Comments

Joel Over a year ago

Great! That did it. So...by removing the quotes what exactly is happening?-OK I see quotes make it a string. Thanks guys. I will accept your answer when SO lets me-in 10 minutes

Lightness Races in Orbit Over a year ago

What is happening is that null in SQL is not the string "NULL", but the keyword NULL. So you put that, without quotes, into the query. Just the same way you don't put quotes around the UPDATE keyword. Remember, you're not really "sending" anything: you're just forming an SQL query in a PHP string.

Michael Berkowski Over a year ago

@Joel by removing the single-quotes, the string NULL is passed in as the MySQL keyword NULL. If you keep the single-quotes as 'NULL', MySQL interprets it as a string value. If it is going into an integer column or a boolean column, I guess that's being interpreted as the value '0'.

nkorth Over a year ago

If you're expecting a possibility of other values it's a good idea to have quotes for everything other than NULL, even though it's a numeric field. (to prevent SQL injection)

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