如果我将NULL提供给MySQL的CHAR_LENGTH()函数，它会返回什么？

In this case, the output of CHAR_LENGTH() function depends on the condition that whether we are providing NULL as a string or we are providing simply NULL to it. Following example will demonstrate the difference −

mysql> Select CHAR_LENGTH(NULL);
+-------------------+
| CHAR_LENGTH(NULL) |
+-------------------+
| NULL              |
+-------------------+
1 row in set (0.00 sec)

mysql> Select CHAR_LENGTH('NULL');
+---------------------+
| CHAR_LENGTH('NULL') |
+---------------------+
| 4                   |
+---------------------+
1 row in set (0.00 sec)

从上面的结果集中可以观察到，当我们将NULL作为字符串提供时，CHAR_LENGTH()函数将返回字符的数量，即4个字符，否则当我们仅提供NULL时，它将返回NULL作为输出。