Windows users

Exqlite uses an Erlang NIF under the hood.
Means calling a native implementation in C.

For Windows users this means compiling Exqlite does not magically just work.
Under the hood mix will try to compile the library with NMAKE on Windows.
For this, NMAKE and C++ build tools needs to be available.

Of course, using WSL 2 can be an alternative if things below doesn't work.

Requirements

Install Microsoft Visual C++ build tools

Download page:
visualstudio.microsoft.com/visual-cpp-build-tools

Alternative direct download link:
aka.ms/vs/16/release/vs_buildtools.exe
(aligned with Visual Studio 2019 - version 16)

You need to install the C++ build tools workload with the default optional components.

Building environment

Start command prompt with necessary environment

Assuming you are building for Windows x64.

Within Windows start menu search for:
x64 Native Tools Command Prompt

Starting this command prompt all necessary environment variables
for compiling should be ready within the prompt.

Ready to run:

mix deps.compile exqlite

# or
mix compile
mix test
...

Alternative way to start prompt

Assuming you have latest version of Build Tools, aligned with Visual Studio 2019,
installed in its default installation path.

cmd /k "C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Auxiliary\Build\vcvars64.bat"

Visual Studio Code users using ElixirLS

Assuming you have latest version of Build Tools, aligned with Visual Studio 2019,
installed in its default installation path.

Start Visual Studio Code from a PowerShell prompt within your project folder.

cmd /k '"C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Auxiliary\Build\vcvars64.bat" && code .'

With starting Visual Studio Code this way, ElixirLS should work
and even your integrated terminal should be aware of the build tools.

Probably make yourself a shortcut for this.

Integrated terminal only

Within your global settings.json or your workspace .vscode\settings.json add:

{
  "terminal.integrated.shell.windows": "cmd.exe",
  "terminal.integrated.shellArgs.windows": [
     "/k",
     "C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\BuildTools\\VC\\Auxiliary\\Build\\vcvars64.bat"
  ]
}