North Yorkshire Police say it took place in Coney Street in York city centre and a man was elbowed in the face by another man outside Blue Heaven bar and grill.
It happened on Saturday, October 26 between 9.45pm and 10.30pm.
A police spokesman said: "We have already launched an investigation and are now appealing for information from anyone who has yet to come forward.
"If you believe you may have witnessed the incident, please email katie.hicks@northyorkshire.police.uk Alternatively you can call North Yorkshire police on 101 select option 2 and ask for Katie Hicks, or contact Crimestoppers anonymously on 0800 555 111 or via their website."
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