检测OLD值是否不等于NEW值并且OLD值为NULL

使用

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SELECT NOT 1 <=> 1,NOT NULL <=> NULL, NOT 1 <=> NULL, NOT 1 <=> 2, 1 <> 1, NULL <> NULL, 1 <> NULL, 1 <>2;+-------------+-------------------+----------------+-------------+--------+--------------+-----------+-------+| NOT 1 <=> 1 | NOT NULL <=> NULL | NOT 1 <=> NULL | NOT 1 <=> 2 | 1 <> 1 | NULL <> NULL | 1 <> NULL | 1 <>2 |+-------------+-------------------+----------------+-------------+--------+--------------+-----------+-------+|0 |      0 |   1 |1 |      0 | NULL         | NULL      |     1 |+-------------+-------------------+----------------+-------------+--------+--------------+-----------+-------+

PS。抱歉，在发布问题之前应该已经阅读了手册，但希望对您有所帮助。