CAML: Select SharePoint List Items with not empty column values

Once I needed a CAML query for selecting elements with non-empty column. I wanted to select all the users with name detected (filled) and with OS "Windows".

First I decided to use <Neq> construction and <Value Type='Text'></Value>. I thought it’s an analog for var != "". But the result was wrong.

<Where>
        <And>
            <Eq>
                    <FieldRef Name="OS " />
                    <Value Type='Text'>Windows</Value>
            </Eq>
            <Neq>
                    <FieldRef Name="UserName />
                    <Value Type='Text'></Value>
            </Neq>
        </And>
</Where>

So I watched CAML documentation and found <IsNotNull> construction. It appeared to be right:

<Where>
        <And>
            <Eq>
                    <FieldRef Name="OS " />
                    <Value Type='Text'>Windows</Value>
            </Eq>
            <IsNotNull>
                    <FieldRef Name="UserName" />
            </IsNotNull>
        </And>
</Where>

This query returns list items where "OS" is "Windows" and UserName not null.

If you need to select elements with Null-values, you should use <IsNull> construction.

<Where>
        <And>
            <Eq>
                    <FieldRef Name="OS " />
                    <Value Type='Text'>Windows</Value>
            </Eq>
            <IsNull>
                    <FieldRef Name="UserName" />
            </IsNull>
        </And>
</Where>