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âNULL columns require additional space in the row to record whether their values are NULL. For MyISAM tables, each NULL column takes one bit extra, rounded up to the nearest byte.â
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mysql> select length(''),length(null),length(' ');
+------------+--------------+--------------+
| length('') | length(null) | length(' ') |
+------------+--------------+--------------+
| 0 | NULL | 2 |
+------------+--------------+--------------+
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