systemctl enable differs from systemctl start, how?

mysqld.service is a "virtual" unit – it doesn't exist on the filesystem, it's just part of systemd's compatibility layer. You can start it and systemd will run the legacy /etc/rc.d/mysqld initscript, but you cannot systemctl enable it because you need a real .service file which could be symlinked into the proper place.

You can write such a unit yourself and put it in /etc/systemd/system/mysqld.service:

[Unit]
Description=MySQL Server
After=network.target

[Service]
ExecStart=/usr/bin/mysqld --defaults-file=/etc/mysql/my.cnf --datadir=/var/lib/mysql --socket=/var/run/mysqld/mysqld.sock
User=mysql
Group=mysql
WorkingDirectory=/usr

[Install]
WantedBy=multi-user.target

Run systemctl daemon-reload after creating/modifying.

Alternatively, you can install the initscripts-systemd package, which includes arch-daemons.target for automatically starting services defined in rc.conf. However, this package might go away soon, and it's always better to have native configuration files for the init system in use.

@Grawity's answer is correct and probably better than this, but I did get it solved yesterday basically by passing through to the rc.d script...

/lib/systemd/system/mysqld.service

[Unit]
Description=MySQL Server
After=network.target

[Service]
Type=forking
ExecStart=/etc/rc.d/mysqld start
ExecStop=/etc/rc.d/mysqld stop

[Install]
WantedBy=multi-user.target

Nota bene: Remember to put host-specific unit files under /etc/systemd/system/ and NOT /lib/systemd/system/.

The latter is for distro-specific stuff; the former is for host-specific stuff that you configure yourself. It's like /usr/bin/ vs. /usr/local/bin/, respectively.

So unless a package installs unit files by itself (under /lib/systemd/system/), put your own "custom" stuff under /etc/systemd/system/.