sql.h not found when installing PyODBC on Heroku
The other answers are more or less correct; you're missing the unixodbc-dev[el] package for your operating system; that's what pip needs in order to build pyodbc from source.
However, a much easier option is to install pyodbc via the system package manager. On Debian/Ubuntu, for example, that would be apt-get install python-pyodbc. Since pyodbc has a lot of compiled components and interfaces heavily with the UnixODBC OS-level packages, it is probably a better fit for a system package rather than a Python/pip-installed one.
You can still list it as a dependency in your requirements.txt files if you're making code for distribution, but it'll usually be easier to install it via the system PM.
I'm trying to install PyODBC on Heroku, but I get fatal error: sql.h: No such file or directory in the logs when pip runs.  How do I fix this error?