Running SQL Server on a front-end Web server is not recommended

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If you run SQL Server on an Office SharePoint Server front-end Web server (for example, in a single-server evaluation deployment), the ASP.NET cache will release system memory at a lower threshold than SQL Server, which could result in InfoPath Forms Services memory starvation.

 

Taken from TechNet

https://technet.microsoft.com/en-us/library/cc261832.aspx