mysql: why comparing a 'string' to 0 gives true?

Solution 1:

MySQL automatically casts a string to a number:

SELECT '1string' = 0 AS res; -- res = 0 (false)
SELECT '1string' = 1 AS res; -- res = 1 (true)
SELECT '0string' = 0 AS res; -- res = 1 (true)

and a string that does not begin with a number is evaluated as 0:

SELECT 'string' = 0 AS res;  -- res = 1 (true)

Of course, when we try to compare a string with another string there's no conversion:

SELECT '0string' = 'string' AS res; -- res = 0 (false)

but we can force a conversion using, for example, a + operator:

SELECT '0string' + 0 = 'string' AS res; -- res = 1 (true)

last query returns TRUE because we ar summing a string '0string' with a number 0, so the string has to be converted to a number, it becomes SELECT 0 + 0 = 'string' and then again the string 'string' is converted to a number before being compared to 0, and it then becomes SELECT 0 = 0 which is TRUE.

This will also work:

SELECT '1abc' + '2ef' AS total; -- total = 1+2 = 3

and will return the sum of the strings converted to numbers (1 + 2 in this case).

Solution 2:

"Strings are automatically converted to numbers and numbers to strings as necessary." This means that in order to compare a string to a number, it tries to parse a number from the start of the string. In this case there is no number there, so it converts to 0, and 0 = 0 is true.