Why checksum() returns the same value for different string

Question

The following SQL and its result shows the different string got the same checksum result. Why?

select  str ,
        binary_checksum(str) binary_checksum,
        checksum(str) checksum,
        hashbytes('md5', str) md5
from    ( values ( '2Volvo Director 20'), ( '3Volvo Director 30'), ( '4Volvo Director 40') ) 
        t ( str ) 

str                binary_checksum checksum    md5
------------------ --------------- ----------- --------------------------------------------
2Volvo Director 20 -1356512636     -383039272  0xB9BD78BCF70FAC36AF14FFF589767278
3Volvo Director 30 -1356512636     -383039272  0xF039462F3D15B162FFCDB6125D290826
4Volvo Director 40 -1356512636     -383039272  0xFAF315CDA6E453CCC09838CFB129EE74

Answer 1

SQL CHECKSUM() and MD5 are Hash functions. Hashing is a one way algorithm which can take any number of chars/bytes and return a fixed number of chars/bytes.

It mean no matter if your input is 1 character or a complete book (War and Peace) you will get back the same length of response. So the input is infinite number of combinations meanwhile the output is finite. Based on that it is inevitable to get the same Hash for different values. It is called Hash collision. Good Hash algorithms try to mitigate this to make it hard to find these colliding values.

But enough theory about hashing. Here is exactly the answer to your question. What is the issue with CHECKSUM()?