使用PHP / MySQL自动化电子邮件活动，不确定如何构建查询

I have two MySQL tables, one with subscriber details, the other where I keep track of who has had an email sent to them, which campaign, which version, etc (crawler and sent_mail, respectively.) I've built a script which can go through the crawler table and send emails based on the SendGrid API, a web form and some javascript. But I am trying to figure out how I can tell the script to pull emails from the crawler table which don't have the current campaign listed in the sent_mail table. I know I have to use a join, but am not really sure how to build the logic into the query. Thoughts? My attempt follows below:I have two MySQL tables, one with subscriber de