Probieren Sie es aus (ohne Unterabfrage )
SELECT a.name, b.cars, count(c.id) as number
FROM users a
INNER JOIN data_tmp b
on a.who = b.who
LEFT JOIN data c
on a.who = c.who AND
a.name = c.name
WHERE a.who = 2
GROUP BY a.name, b.cars
das funktioniert auf verschiedenen Servern:
MSSQL-SERVER @ SQLFIDDLE
MYSQL @ SQLFIDDLE