| 10. 18. 1. NULL means 'a missing unknown value' |
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The NULL value means "no data." |
NULL can be written in any lettercase. |
For operations performed with LOAD DATA INFILE or SELECT ... INTO OUTFILE, NULL is represented by the \N sequence. |
To test for NULL, you cannot use the arithmetic comparison operators such as =, <, or <>. |
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mysql>
mysql> SELECT 1 = NULL, 1 <> NULL, 1 < NULL, 1 > NULL;
+----------+-----------+----------+----------+
| 1 = NULL | 1 <> NULL | 1 < NULL | 1 > NULL |
+----------+-----------+----------+----------+
| NULL | NULL | NULL | NULL |
+----------+-----------+----------+----------+
1 row in set (0.00 sec)
mysql>
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