CODEWITHSUNDEEP
mysql
Trung Tran
Trung Tran

Reputation: 299

Why this condition 0='=1' is true?

I have seen some programmers using this condition 0='"="1' and don't know why it returns true ?Can anyone explain it to me ? Thank you!

Upvotes: 2

Views: 110

Answers (2)

Bohemian
Bohemian

Reputation: 425003

In order to compare an number with a string the string is cast to a number.

When casting a string to a number, mysql accepts all leading numbers and throws away the rest. When there are no leading numbers, the string is cast to zero:

0 = 'abc' -- true: string cast to 0
1 = '1abc' -- true: string cast to 1

Your string is evaluated as being 0 because there are no leading numbers.

Upvotes: 4

Alma Do
Alma Do

Reputation: 37365

The issue has no relation with "unusual" looks of the string. Yes. it's just primitive implicit type-conversion. You can do:

mysql> select 0='blablabla';
+---------------+
| 0='blablabla' |
+---------------+
|             1 |
+---------------+
1 row in set, 1 warning (0.00 sec)

And see your reason:

mysql> show warnings;
+---------+------+-----------------------------------------------+
| Level   | Code | Message                                       |
+---------+------+-----------------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: 'blablabla' |
+---------+------+-----------------------------------------------+
1 row in set (0.00 sec)

So same with your '"="1' - it's just string and will be truncated during conversion to DOUBLE causing zero-value.

Upvotes: 1

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