Filtering for NULL in queries
The syntax for filtering for NULL (i.e. the absence of a value) in WHERE blocks is slightly different than filtering for specific values.
Note that because NULL is not equal to anything, not even to itself, using equality operators = NULL or <> NULL (or != NULL) will always yield the truth value of UNKNOWN which will be rejected by WHERE.
WHERE filters all rows that the condition is FALSE or UKNOWN and keeps only rows that the condition is TRUE.
Nullable columns in tables
When creating tables it is possible to declare a column as nullable or non-nullable.
By default every column (except those in primary key constraint) is nullable unless we explicitly set NOT NULL constraint.
Attempting to assign NULL to a non-nullable column will result in an error.