Does NULL mean “no value”? Or does it mean “an unknown value”? Its semantics and history are ambiguous and inconsistent.
Read moreMySQL: IS NULL conditions and indexes
It is quite common to use an equality condition and an IS NULL condition on the same columns. Unfortunately, this could make queries very slow.
Read moreNULL comparisons in MySQL, PostgreSQL, and SQLite
Any comparison involving at least one NULL operand will return NULL. However, SQL supports operators to easily write NULL-safe comparisons.
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