mySQL returns all rows when field=0

Question

I was making some tests, and it was a surprise when i was querying a table, and the query SELECT * FROM table WHERE email=0 returned all rows from the table.

Answer 1

Your email column is a CHAR or VARCHAR type. When you use the condition email = 0, MySQL is casting the contents of the email column to an integer in order to compare them with the 0 you supplied. Had you surrounded your 0 in quotes, the query would work as expected. (email = '0')

Converting a non-numeric string to an integer in MySQL will result in 0.

mysql> SELECT CAST('email@example.com' AS SIGNED);
+-------------------------------------+
| CAST('email@example.com' AS SIGNED) |
+-------------------------------------+
|                                   0 |
+-------------------------------------+

In contrast, if you attempted the same thing with numeric strings, they may cast correctly:

mysql> SELECT CAST('12345' AS SIGNED);
+-------------------------+
| CAST('12345' AS SIGNED) |
+-------------------------+
|                   12345 |
+-------------------------+