JQL will give you always a list of issues - and only that. With SQL you can do more, aggregations, search for other things (Users, Projects, ...), etc.
See here for useful examples:https://confluence.atlassian.com/display/JIRA041/Example+SQL+queries+for+JIRA
Furthermore the differences mentioned by Udo, I would add one more:
SQL is standard while JQL is proprietary. It means that there are a lot of tools supporting SQL whereas JQL is restricted to JIRA.
Fortunately, there is an add-on (SQL for JIRA) which brings all the power of JQL to SQL. Therefore users can take all the benefits of SQL and JQL working together.
Badges are a great way to show off community activity, whether you’re a newbie or a Champion.Learn more
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG