SOQL and SOSL Reference

Introduction to SOQL and SOSL
Salesforce Object Query Language (SOQL)
Typographical Conventions in This Document
Quoted String Escape Sequences
Reserved Characters
Alias Notation
SOQL SELECT Syntax
SELECT
TYPEOF
USING SCOPE
WHERE
Comparison Operators
Logical Operators
Date Formats and Date Literals in WHERE
Filter on Polymorphic Relationship Fields
Query Multi-Select Picklists
Use null in WHERE in WHERE
WITH
GROUP BY
HAVING
ORDER BY
LIMIT
OFFSET
FOR VIEW and and FOR REFERENCE
UPDATE
SET OPTIONS
FOR UPDATE
SOQL SELECT Examples
SOQL SELECT Functions
Relationship Queries
Change the Batch Size in Queries
SOQL Object Limits and Limitations
Syndication Feed SOQL and Mapping Syntax
Location-Based SOQL Queries
Considerations for Querying Currency Data Using SOQL
Salesforce Object Search Language (SOSL)
PDF

Use null in WHERE in WHERE

You can search for null values by using the null keyword in SOQL queries.
This example query returns the account IDs of all events with a non-null activity date. 
1SELECT AccountId
2FROM Event
3WHERE ActivityDate != null
If you run a query on a boolean field, null matches FALSE values. For example, if Test_c is a boolean field, then the following query returns all the account records where Test_c is false:
1SELECT Id, Name Test_c
2FROM Account
3WHERE Test_c = null

The clause WHERE Test_c = null is equivalent to WHERE Test_c = false. The clause WHERE Test_c != null is equivalent to WHERE Test_c = true.

The WHERE clause behaves in two different ways, depending on the version, when handling null values in a parent field for a relationship query. In a WHERE clause that checks for a value in a parent field, the record is returned even if the parent does not exist. 
1SELECT Id
2FROM Case
3WHERE Contact.LastName = null
Case record Id values are returned.