Sql-server – database design – psychometric testing layout

I would rename TreeData to measurements.

Also, unless you really need the Visits table, I wouldn't use it. If designed properly your database would look something like this if you included the visits table:

Using the visits table would complicate some of the queries that you're wanting answered. For example, to answer number one you might use something like this in sql server:

select
     t.treeid,
     max(m.height)-min(m.height) as Growth
from trees t
     inner join measurements m
          on t.treeid=m.treeid
     inner join visits v
          on m.visitid=t.visitid
where v.date>=dateadd(y,-2,getdate())
group by t.treeid

without the visits table you could rewrite that like this:

select
     m.treeid,
     max(m.height)-min(m.height) as Growth
from measurements m
where m.date>=dateadd(y,-2,getdate())
group by m.treeid

The main problem with your schema is the OrderDetail table: you have mixed data which is atomic (the date, the address) along with repeating data - the products ordered.

Your Orders table should be as follows

OrderId (primary key)
OrderNumber 
UserID (foreign key to users)
OrderDate

(address really belongs to the user)

whereas the OrderLines table should be as follows

OrderLine (primary key)
OrderId (foreign key to orders)
PartID (foreign key to products)
Quant
Price
Discount
Total (although this can be calculated as quant * price * (100 - discount) * 0.01)

Each order line has a total but there should not be a 'total' field in the orders table; instead, calculate this from the orderlines table.

The 'OrderLine' field might be redundant, if 'OrderId' and 'PartId' can be used as a composite primary key, which would mean that no part can appear twice in the same order. This might seem reasonable, but if you add fields such as 'supply date' (the date by which the customer wants to receive each part), you would have problems if a customer orders 100 widgets to be supplied by the end of December and another 100 widgets to be supplied by the end of January. The surrogate key (orderline) is probably the safest and easiest option.