echo 'deb [arch=amd64] https://packages.microsoft.com/debian/8/prod jessie main' > /etc/apt/sources.list.d/mssql-release.list

Add the key

curl https://packages.microsoft.com/keys/microsoft.asc | sudo apt-key add -

then update and install

sudo apt update
sudo apt install msodbcsql mssql-tools

If you try and connect to sql now you get an obscure error like this:

/opt/mssql-tools/bin/sqlcmd -S 10.0.2.13 -U xxx -P yyy
Sqlcmd: Error: Microsoft ODBC Driver 13 for SQL Server : Can't open lib '/opt/microsoft/msodbcsql/lib64/libmsodbcsql-13.1.so.9.1' : file not found.

But the file exists:

$ file /opt/mssql-tools/bin/sqlcmd
/opt/mssql-tools/bin/sqlcmd: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=21a353af470e7849544daab892ec9b1bfc36dc87, not stripped

This somewhat misleading error is actually due to the libmsodbcsql lib being linked against a very specific version of libssl. Check the output of ldd:

$ ldd /opt/microsoft/msodbcsql/lib64/libmsodbcsql-13.1.so.9.1 | grep 'not found'
libcrypto.so.1.0.0 => not found
libssl.so.1.0.0 => not found

But it turns out this can easily be solved by manually installing the libssl package from Debian Jessie:

wget "http://security.debian.org/debian-security/pool/updates/main/o/openssl/libssl1.0.0_1.0.1t-1+deb8u7_amd64.deb"
sudo apt install ./libssl1.0.0_1.0.1t-1+deb8u7_amd64.deb

then you should be back in business.