Move to VPS has created a database error

[North East Man]

My host moved my site from shared hosting to a VPS yesterday.  All apparently went well except that there was this error thrown up after login
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Expression #3 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'xxxxxx_smf225.mes.subject' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
File: /home/xxxxx/forum.mysite.co.uk/Sources/BoardIndex.php
Line: 133
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The central list of all discussion boards did not show.  Essemntially all i had was the top row of tabs.

They logged in and fixed the problem by "Turning off the ONLY_FULL_GROUP_BY functionality in sql_mode".  Everything worked as it should.

The problem is, it has come back again this morning.  Same error.  As I am on day one of having this VPS I am at a loss as to where to go and what to do to fix this.  I have messaged the hosts support team, but they have not got back to me yet.

In very simple terms, can anyone advise what steps I need to take to solve this?  How do I turn off ONLY_FULL_GROUP_BY functionality in sql mode?

[Arantor]

You change the my.cnf file to not include it. Where that will be and exactly how to change it is dependent mostly on your server distro...

[North East Man]

Many thanks.  The hosting company have just gone in and made that adjustment for me.  Now I just need to learn how to do that for myself as part of the learning process.