Select user.userID
, username
, user.email
, registeredBy, registeredDate
, Case
When User.Level = 2 Then Concat(Level2Staff.firstName, ' ' , Level2Staff.lastName)
When User.Level = 1 Then Concat(customer.firstName, ' ' , customer.lastName)
End
AS staffName
FROM user
Inner Join level
On user.levelID = level.levelID
Inner Join staff
On user.registeredBy = staff.UserID
Left Join customer
On customer.userID = user.UserID
And user.level = 1
Left Join staff As Level2Staff
On user.userID = Level2Staff.UserID