为什么sql = FALSE总是返回true

Question

No matter what I do in MySQL 5.0 such as

SELECT 'test' = FALSE

SELECT '' = FALSE

I always get a 1 back in SQL. What is the reason for that? I was expecting a 0 or FALSE

EDIT adding context to the questions. This is how the problem came about, it happened that $name inadvertently became false making this join always pass, then I wondered why this works.

SELECT a.id
FROM user a 
INNER JOIN inventory b ON b.user_id = a.id AND b.name = $name

Answer 1

In MySql FALSE is a constant literal which is always evaluated as 0.

So you are checking if 'test' = 0 or if '' = 0 , and since you are comparing a string with an integer, MySql will try to cast the string to an integer.

If you try this query:

SELECT 'test' = FALSE

it will return 1 (TRUE) because 'test' will be converted to 0, while if you try this:

SELECT '1test' = FALSE

it will return 0 (FALSE) because '1test' will be converted to 1.

Answer 2

This has to do with MySQL's implicit conversion when using comparison operators (ie = )

Taken from the docs:

Strings are automatically converted to numbers and numbers to strings as necessary.

So, in your case:

• 'test' gets converted to 0
• FALSE is 0
• 0 = 0 is TRUE .