All about NULL

• In SQL, the NULL value is never true in comparison to any other value, even NULL. An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for the operators and functions involved in the expression.

• To search for column values that are NULL, you cannot use an expr = NULL test. To look forNULL values, you must use the IS NULL test.

• When using DISTINCT, GROUP BY, or ORDER BY, all NULL values are regarded as equal.

• When using ORDER BY, NULL values are presented first, or last if you specify DESC to sort in descending order.

• For some data types, MySQL handles NULL values specially. If you insert NULL into a TIMESTAMP column, the current date and time is inserted.

• If you insert NULL into an integer or floating-point column that has the AUTO_INCREMENTattribute, the next number in the sequence is inserted.

• A column that has a UNIQUE key defined can still contain multiple NULL values.