Any version of SQL Server from 2008 and above will do; I tend to use 2008 R2 as it is the smallest download, and is not TOO memory hungry.
Microsoft SQL Server 2008 Express R2 can be downloaded from
Unhelpfully, Microsoft don't provide a direct link - you have to select which variant you want to install. Most modern PCs are 64bit, in which case you want SQLEXPRWT_x64_ENU.exe If your PC is 32 bit, download SQLEXPRWT_x86_ENU.exe
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