A clarification. If a string value begins with a digit, MySQL does not
convert it to zero (FALSE), but rather to an integer made up of the
leading digits in the value (until a non-digit is reached). This can be
shown by using the CAST() function.
mysql> SELECT '21b' = 0, CAST('21b' AS SIGNED);
+-----------+-----------------------+
| '21b' = 0 | CAST('21b' AS SIGNED) |
+-----------+-----------------------+
| 0 | 21 |
+-----------+-----------------------+
1 row in set (0.00 sec)
However, if the string value begins with a non-digit, then it is coerced
or cast to zero:
mysql> SELECT 'b21' = 0, CAST('b21' AS SIGNED);
+-----------+-----------------------+
| 'b21' = 0 | CAST('b21' AS SIGNED) |
+-----------+-----------------------+
| 1 | 0 |
+-----------+-----------------------+
1 row in set (0.00 sec)
It is still true that Przemyslaw was assuming that a non-empty string
would always evaluate as TRUE, and this isn't necessarily the case. So
the best strategy is to compare directly with the empty string, as
already discussed.
Thanks to Paul DuBois for reminding me of this. My apologies to anyone
whom I might have misled.
--
Jon Stephens, Technical Writer
MySQL AB www.mysql.com
Office: +61 (07) 3388 2228
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