Query database for multiple columns from a key value pair table

Question

I am working on defining a sql query for use in sql server compact edition 3.5 on a windows mobile handset. I am going to need to get back a result set from three tables.

I don't exactly remember all the column names, as I'm asking this question at home, but here is a good example of the tables I'm dealing with.

Table 1: Customers
Table 2: PresoldOrders
Table 3: CustomerDetails

*

 ________________________________________
|                                        |
|--------------- Customers --------------|
|________________________________________|
|                                        |
| PK    int            CustomerNumber    |
|       varchar(125)   FirstName         |
|       varchar(125)   LastName          |
|       varchar(125)   Email             |
|       varchar(200)   Address1          |
|       varchar(200)   Address2          |
|       varchar(200)   City              |
|       varchar(2)     State             |
|       varchar(5)     Zip               |
|________________________________________|

*

 ________________________________________
|                                        |
|------------ CustomerDetails -----------|
|________________________________________|
|                                        |
| PK    int            CustomerDetailsId |
| FK    int            CustomerNumber    |
|       varchar(255)   FieldName         |
|       varchar(255)   FieldValue        |
|________________________________________|

*

 ________________________________________
|                                        |
|------------ PresoldOrders -------------|
|________________________________________|
|                                        |
| PK    int            PresoldOrderId    |
| FK    int            CustomerNumber    |
|       int            OrderNumber       |
|       int            RouteStopNumber   |
|       datetime       DeliveryDate      |
|       varchar(100)   Other1            |
|       varchar(100)   Other2            |
|________________________________________| 

Now, the query should return all records that exist in customers even if they don't exist in 'PresoldOrderHeaders' table. This part of it is pretty easy, I plan to just use a left outer join. The second part of the query is a bit more complex.

Here is the query I've constructed so far.

SELECT c.CustomerNumber
       c.FirstName
       c.LastName
       c.Email
       c.Address1
       c.Address2
       c.City
       c.State
       c.Zip
       po.OrderNumber
       po.DeliveryDate
       po.Other1
       po.Other2
FROM Customer c
LEFT OUTER JOIN PresoldOrders po on c.CustomerNumber = po.CustomerNumber
ORDER BY po.RouteStopNumber; 

Tricky part is the CustomerDetails table. Here is an example of some data

 _________________________________________________________
|       |                 |              |                |
| PK    | CustomerNumber  | FieldName    | FieldValue     |
|-------|-----------------|--------------|----------------|
| 1     | 1               | A            | 125            |
|-------|-----------------|--------------|----------------|
| 2     | 1               | B            | 126            |
|-------|-----------------|--------------|----------------|
| 3     | 1               | C            | 127            |
|-------|-----------------|--------------|----------------|
| 4     | 2               | A            | 138            |
|-------|-----------------|--------------|----------------|
| 5     | 2               | B            | 140            |
|-------|-----------------|--------------|----------------|
| 6     | 2               | C            | 143            |
|-------|-----------------|--------------|----------------|
|_________________________________________________________|

For the information that I will be displaying in the Component One Flex Grid, the FieldName's listed in the CustomerDetails table will be fixed.

Here is want I want to archive:

 _____________________________________________________________________________________________________________________
|                 |           |          |     |                     |                     |                          |
| CustomerNumber  | FirstName | LastName | ... | FieldName A's value | FieldName B's Value | FieldName C's Value      |
|-----------------|-----------|----------|-----|---------------------|---------------------|--------------------------|
| 1               | John      | Someone  | ... | 125                 | 126                 | 127                      |
|-----------------|-----------|----------|-----|---------------------|---------------------|--------------------------|
| 2               | Dan       | Other    | ... | 138                 | 140                 | 143                      |
|-----------------|-----------|----------|-----|---------------------|---------------------|--------------------------|
|_____________________________________________________________________________________________________________________|

Normally, I'd have column names for A, B, and C defined in the 'CustomerDetails' table; however, this table can't be changed, so I must work with what I've been given. The requirements in the spec for my task is to have 15 plus columns to be displayed in a grid on a mobile device; not something I'd go for but those are the requirements.

Ok finally, the question:

Can one use sql to query a key value pairing table and have those key's value's displayed in columns like the above? This is the requirement I have and I'm thinking I'll need to create one query with my join on presoldorders table and then get a list of all details for each customer in a list and iterate through and combine into data table in code on handheld.

Answer 1

If you know in advance all key values, you can pivot resultset. I'm not sure if sql server compact supports PIVOT , but you can do: select CustomerNumber,
Max(Case when FieldName='A' then FieldValue end) as a_value,
// the same for B, C, all keys.
From CustomerDetails
Group by CustomerNumber select CustomerNumber,
Max(Case when FieldName='A' then FieldValue end) as a_value,
// the same for B, C, all keys.
From CustomerDetails
Group by CustomerNumber
For simplicity I don't join other tables, but I hope it gives you an idea how to turn rows into columns.