In SQL, a comparison between a `null` value and any other value (including another `null`) using a comparison operator (eg `=`, `!=`, `&lt;`, etc) will result in a `null`, which is considered as `false` for the purposes of a where clause (strictly speaking, it&#39;s &quot;not true&quot;, rather than &quot;false&quot;, but the effect is the same).

The reasoning is that a `null` means &quot;unknown&quot;, so the result of any comparison to a `null` is also &quot;unknown&quot;. So you&#39;ll get no hit on rows by coding `where my_column = null`.

SQL provides the special syntax for testing if a column is `null`, via `is null` and `is not null`, which is a special condition to test for a `null` (or not a `null`).

Here&#39;s some SQL showing a variety of conditions and and their effect as per above.

    create table t (x int, y int);
    insert into t values (null, null), (null, 1), (1, 1);
    
    select &#39;x = null&#39; as test , x, y from t where x = null
    union all
    select &#39;x != null&#39;, x, y from t where x != null
    union all
    select &#39;not (x = null)&#39;, x, y from t where not (x = null)
    union all
    select &#39;x = y&#39;, x, y from t where x = y
    union all
    select &#39;not (x = y)&#39;, x, y from t where not (x = y);

returns only 1 row (as expected):

    TEST	X	Y
    x = y	1	1

See this running on [SQLFiddle][1]

  [1]: http://sqlfiddle.com/#!2/d885b/1

It&#39;s important to note, that **NULL doesn&#39;t equal NULL**.

`NULL` is not a value, and therefore cannot be compared to another value.

`where x is null` checks whether x is a null value.

`where x = null` is checking whether x equals NULL, which will never be true

First is correct way of checking whether a field value is `null` while later won&#39;t work the way you expect it to because `null` is special value which does not equal anything, so you can&#39;t use equality comparison using `=` for it.

So when you need to check if a field value is `null` or not, use:

    where x is null

instead of:

    where x = null

I think that equality is something that can be absolutely determined. The trouble with `null` is that it&#39;s inherently unknown. `Null` combined with any other value is `null` - unknown. Asking SQL &quot;Is my value equal to `null`?&quot; would be unknown every single time, even if the input is `null`. I think the implementation of `IS NULL` makes it clear.

I&#39;m currently using readonly=&quot;readonly&quot; to disable fields. I&#39;m now trying to style the attribute using CSS. I&#39;ve tried using

    input[readonly] {
      /* styling info here */
    }

but it is not working for some reason. I&#39;ve also tried

    input[readonly=&#39;readonly&#39;] {
      /* styling info here */
    }

that doesn&#39;t work either.

How can I style the readonly attribute with CSS?


How to style readonly attribute with CSS?

I have installed the new Visual Studio 2012 Ultimate.  

I have created a Test Project with my solution and it contains some default unit tests. However right clicking on new Methods doesn&#39;t show the &quot;Create Unit Tests&quot; context menu any longer.

**See Visual Studio 2010 Ultimate:**
![enter image description here][1]


**And Visual Studio 2012 Ultimate:**
![enter image description here][2]

Any idea why that could be?

**UPDATE:**

**Solution for Visual Studio 2012 and 2013:**

Microsoft has acknowledged it &lt;a href=&quot;http://visualstudio.uservoice.com/forums/121579-visual-studio/suggestions/2748862-add-create-unit-tests-menu-option-back-&quot;&gt;here&lt;/a&gt; and offers an &lt;a href=&quot;http://visualstudiogallery.msdn.microsoft.com/45208924-e7b0-45df-8cff-165b505a38d7?SRC=Home&quot;&gt;official work around patch&lt;/a&gt; to fix it. Download, install and Enjoy!

  [1]: http://i.stack.imgur.com/YgbQ3.png
  [2]: http://i.stack.imgur.com/TiEw6.png

Where is the &quot;Create Unit Tests&quot; selection?

&gt; **Possible Duplicate:**  
&gt; [what is &amp;ldquo;=null&amp;rdquo; and &amp;ldquo; IS NULL&amp;rdquo;](https://stackoverflow.com/questions/2749044/what-is-null-and-is-null)  
&gt; [Is there any difference between IS NULL and =NULL](https://stackoverflow.com/questions/3777230/is-there-any-difference-between-is-null-and-null)  

&lt;!-- End of automatically inserted text --&gt;

What is the difference between

    where x is null

and 

    where x = null

and why does the latter not work?

SQL is null and = null

> Possible Duplicate: 
> <a href="https://stackoverflow.com/questions/2749044/what-is-null-and-is-null" target="_blank" rel="noopener noreferrer">what is “=null” and “ IS NULL”</a> 
> <a href="https://stackoverflow.com/questions/3777230/is-there-any-difference-between-is-null-and-null" target="_blank" rel="noopener noreferrer">Is there any difference between IS NULL and =NULL</a>

What is the difference between
<pre><code class="hljs language-csharp">where x is null
</code></pre>
and
<pre><code class="hljs language-csharp">where x = null
</code></pre>
and why does the latter not work?

Here is a demo query, notice it is very simple, Fetches only where base_price is 0,
And still, it chooses the condition 3:

    SELECT
       CASE course_enrollment_settings.base_price
    	WHEN course_enrollment_settings.base_price = 0 	 	THEN 1
        WHEN course_enrollment_settings.base_price&lt;101		THEN 2
    	WHEN course_enrollment_settings.base_price&gt;100 AND   
                          course_enrollment_settings.base_price&lt;201	THEN 3
            ELSE 6
       END AS &#39;calc_base_price&#39;,
       course_enrollment_settings.base_price
    FROM
    	course_enrollment_settings
    WHERE course_enrollment_settings.base_price = 0

`base_price` is `decimal(8,0)`

When run this on my DB, I get:

&gt; 3 0   
3 0  
3 0   
3 0  
3 0



How do I use properly CASE..WHEN in MySQL

I want to find the longest `VARCHAR` in a specific column of a SQL Server table.

Here&#39;s an example:

    ID = INT IDENTITY
    DESC = VARCHAR(5000)

    ID | Desc
    ---|-----
    1  | a
    2  | aaa
    3  | aa

What&#39;s the SQL to return 3? Since the longest value is 3 characters?

Retrieve the maximum length of a VARCHAR column in SQL Server

I have a column called CODE in a MySQL table which can be NULL. Say I have some rows with CODE=&#39;C&#39; which I want to ignore in my select result set. I can have either CODE=NULL or CODE!=&#39;C&#39; in my result set.

The following query does not return a row with CODE as NULL:

    SELECT * from TABLE where CODE!=&#39;C&#39;


But this query works as expected and I know it is the right way to do it.

    SELECT * from TABLE where CODE IS NULL OR CODE!=&#39;C&#39;

My question is why does having only CODE!=&#39;C&#39; does not return rows where CODE=NULL? Definitely &#39;C&#39; is not NULL. We are comparing no value to a character here. Can someone throw some light as why it doesn&#39;t work that way?

MySQL comparison with null value

This seems fairly simple but I can&#39;t get it to turn up on Google.

If I have:

    class City &lt; ActiveRecord::Base
      has_many :photos
    end
    
    class Photo &lt; ActiveRecord::Base
      belongs_to :city
    end

I want to find all cities that have no photos. I&#39;d love to be able to call something like...

    City.where( photos.empty? )

...but that doesn&#39;t exist. So, how do you do this kind of query?

- - - - -

**Update:**
Having now found an answer to the original question, I&#39;m curious, how do you construct the inverse?

IE: if I wanted to create these as scopes:

    scope :without_photos, includes(:photos).where( :photos =&gt; {:city_id=&gt;nil} )
    scope :with_photos, ???

Rails find record with zero has_many records associated

Disregarding performance, will I get the same result from query A and B below? How about C and D?

    -- A
    select *
    from   a left join b
               on &lt;blahblah&gt;
           left join c
               on &lt;blahblan&gt;
    
    
    -- B
    select *
    from   a left join c
               on &lt;blahblah&gt;
           left join b
               on &lt;blahblan&gt;  
    
    -- C
    select *
    from   a join b
               on &lt;blahblah&gt;
           join c
               on &lt;blahblan&gt;
    
    
    -- D
    select *
    from   a join c
               on &lt;blahblah&gt;
           join b
               on &lt;blahblan&gt;  

Does the join order matter in SQL?

I have the following code:

    return this.ObjectContext.BranchCostDetails.Where(
        b =&gt; b.TarrifId == tariffId &amp;&amp; b.Diameter == diameter
            || (b.TarrifId==tariffId &amp;&amp; !string.IsNullOrWhiteSpace(b.Diameter))
            || (!b.TarrifId.HasValue) &amp;&amp; b.Diameter==diameter);

And I get this error when I try to run the code:

&gt; LINQ to Entities does not recognize the method &#39;Boolean
&gt; IsNullOrWhiteSpace(System.String)&#39; method, and this method cannot be
&gt; translated into a store expression.&quot;

How can I solve this problem and write code better than this?

String.IsNullOrWhiteSpace in LINQ Expression

In PHP, what is the difference between `is_null` and `==null` in PHP? What are the qualifications for both to return true?

PHP is_null() and ==null

I love the null-coalescing operator because it makes it easy to assign a default value for nullable types.

     int y = x ?? -1;

That&#39;s great, except if I need to do something simple with `x`. For instance, if I want to check `Session`, then I usually end up having to write something more verbose.

I wish I could do this:

    string y = Session[&quot;key&quot;].ToString() ?? &quot;none&quot;;

But you can&#39;t because the `.ToString()` gets called before the null check so it fails if `Session[&quot;key&quot;]` is null. I end up doing this:

    string y = Session[&quot;key&quot;] == null ? &quot;none&quot; : Session[&quot;key&quot;].ToString();

It works and is better, in my opinion, than the three-line alternative:

    string y = &quot;none&quot;;
    if (Session[&quot;key&quot;] != null)
        y = Session[&quot;key&quot;].ToString();

Even though that works I am still curious if there is a better way. It seems no matter what I always have to reference `Session[&quot;key&quot;]` twice; once for the check, and again for the assignment. Any ideas?

What is the proper way to check for null values?

I&#39;m trying to select the max date in three different fields in each record (MySQL)
So, in each row, I have date1, date2 and date3: date1 is always filled, date2 and date3 can be NULL or empty
The GREATEST statement is simple and concise but has no effects on NULL fields, so this doesn&#39;t work well:

    SELECT id, GREATEST(date1, date2, date3) as datemax FROM mytable

I tried also more complex solutions like this:

    SELECT
        CASE
            WHEN date1 &gt;= date2 AND date1 &gt;= date3 THEN date1
            WHEN date2 &gt;= date1 AND date2 &gt;= date3 THEN date2
            WHEN date3 &gt;= date1 AND date3 &gt;= date2 THEN date3
            ELSE                                        date1
        END AS MostRecentDate

Same problem here: NULL values are a GREAT problem in returning the right records

Please, have you got a solution?
Thanks in advance....



Content Type	Original Author	Original Content on Stackoverflow
Question	Sachin Kainth	View Question on Stackoverflow
Solution 1 - Sql	Bohemian	View Answer on Stackoverflow
Solution 2 - Sql	Curtis	View Answer on Stackoverflow
Solution 3 - Sql	Sarfraz	View Answer on Stackoverflow
Solution 4 - Sql	Vikram	View Answer on Stackoverflow

SQL is null and = null

Sql Problem Overview

Sql Solutions

Solution 1 - Sql

Solution 2 - Sql

Solution 3 - Sql

Solution 4 - Sql

Where is the "Create Unit Tests" selection?

How to style readonly attribute with CSS?

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