domain.com is different than www.domain.com. domain.com will not work on www.domain.com and vice versa.

I want to purchase SSL Certificate. Can I use it for both https://domain.com and https://www.domain.com?

Yes, the certificate is issued on exact domain name and the www prefix is never a domain rather a subdomain. If you’ll buy the certificate on domain.com then it would work on www.domain.com as well but throw a warning message that this certificate is not for the www.domain.com rather domain.com. If you want the same certificate to work for domain.com/www.domain.com/*.domain.com then you’ll have to buy a Widlcard Certificate which works on all subdomain on a domain.

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