The null value represents missing or unknown information.
But using it brings complications to your SQL. In this video Chris discusses how to work with null to ensure you get the correct results.
Further reading:
Oracle Database SQL Reference on Nulls: http://docs.oracle.com/database/122/SQLRF/Nulls.htm#SQLRF30037
Null related functions: http://docs.oracle.com/database/122/SQLRF/Single-Row-Functions.htm#SQLRF51187
Is null condition: http://docs.oracle.com/database/122/SQLRF/Null-Conditions.htm#SQLRF52152
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All Things SQL blog: https://blogs.oracle.com/sql/
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The Magic of SQL with Chris Saxon
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