Find if any of the rows partially match a string
You can use LOCATE() – as in the other answer – or LIKE:

SELECT * 
FROM site_core_antispam_banned_domain
WHERE 'subdomain.com' LIKE CONCAT('%', domain_name, '%') ;


The above will look for domains that match any part of a given string. If you specifically wanted domains matching the right side of the string (for instance, if the domain to match against was somedomain.com.ro and you were only interested in *.com.ro results), you could make the pattern more specific:

SELECT * 
FROM site_core_antispam_banned_domain
WHERE 'subdomain.com' LIKE CONCAT('%', domain_name) ;