CODEWITHSUNDEEP
mysqlnull
The.Anti.9
The.Anti.9

Reputation: 44648

MySQL: selecting rows where a column is null

I'm having a problem where when I try to select the rows that have a NULL for a certain column, it returns an empty set. However, when I look at the table in phpMyAdmin, it says null for most of the rows.

My query looks something like this:

SELECT pid FROM planets WHERE userid = NULL

Empty set every time.

A lot of places said to make sure it's not stored as "NULL" or "null" instead of an actual value, and one said to try looking for just a space (userid = ' ') but none of these have worked. There was a suggestion to not use MyISAM and use innoDB because MyISAM has trouble storing null. I switched the table to innoDB but now I feel like the problem may be that it still isn't actually null because of the way it might convert it. I'd like to do this without having to recreate the table as innoDB or anything else, but if I have to, I can certainly try that.

Upvotes: 326

Views: 502220

Answers (9)

MKJ07
MKJ07

Reputation: 1

Replace the code with this one it will work

SELECT pid FROM planets WHERE userid IS NULL

Upvotes: -1

user
user

Reputation: 101

SELECT pid FROM planets WHERE userid is null;

Upvotes: 10

Marc B
Marc B

Reputation: 360562

SQL NULL's special, and you have to do WHERE field IS NULL, as NULL cannot be equal to anything,

including itself (ie: NULL = NULL is always false).

See Rule 3 https://en.wikipedia.org/wiki/Codd%27s_12_rules

Upvotes: 613

RichKenny
RichKenny

Reputation: 179

Had the same issue where query:

SELECT * FROM 'column' WHERE 'column' IS NULL;

returned no values. Seems to be an issue with MyISAM and the same query on the data in InnoDB returned expected results.

Went with:

SELECT * FROM 'column' WHERE 'column' = ' ';

Returned all expected results.

Upvotes: 9

minhas23
minhas23

Reputation: 9671

Info from http://w3schools.com/sql/sql_null_values.asp:

1) NULL values represent missing unknown data.

2) By default, a table column can hold NULL values.

3) NULL values are treated differently from other values

4) It is not possible to compare NULL and 0; they are not equivalent.

5) It is not possible to test for NULL values with comparison operators, such as =, <, or <>.

6) We will have to use the IS NULL and IS NOT NULL operators instead

So in case of your problem:

SELECT pid FROM planets WHERE userid IS NULL

Upvotes: 12

obe
obe

Reputation: 7788

There's also a <=> operator:

SELECT pid FROM planets WHERE userid <=> NULL

Would work. The nice thing is that <=> can also be used with non-NULL values:

SELECT NULL <=> NULL yields 1.

SELECT 42 <=> 42 yields 1 as well.

See here: https://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#operator_equal-to

Upvotes: 27

Mo514
Mo514

Reputation: 11

I had the same issue when converting databases from Access to MySQL (using vb.net to communicate with the database).

I needed to assess if a field (field type varchar(1)) was null.

This statement worked for my scenario:

SELECT * FROM [table name] WHERE [field name] = ''

Upvotes: 0

Aniket Kulkarni
Aniket Kulkarni

Reputation: 12983

As all are given answers I want to add little more. I had also faced the same issue.

Why did your query fail? You have,

SELECT pid FROM planets WHERE userid = NULL;

This will not give you the expected result, because from mysql doc

In SQL, the NULL value is never true in comparison to any other value, even NULL. An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for the operators and functions involved in the expression.

Emphasis mine.

To search for column values that are NULL, you cannot use an expr = NULL test. The following statement returns no rows, because expr = NULL is never true for any expression

Solution

SELECT pid FROM planets WHERE userid IS NULL;

To test for NULL, use the IS NULL and IS NOT NULL operators.

Upvotes: 49

Michael Pakhantsov
Michael Pakhantsov

Reputation: 25370

SELECT pid FROM planets WHERE userid IS NULL

Upvotes: 94

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