行到列的部分转置

Question

The question is very simple , but I am having troubles with realization.

The current table looks like this:

ID  Value

A   1
A   2
A   3
B   1
B   2
C   1

And i need it like this:

ID Value1 Value2 Value3 Value...
A   1      2      3      NULL
B   1      2      NULL   NULL
C   1     NULL    NULL   NULL

Answer 1

If the value columns are a known\\reasonable set range ie 1-5 you can do something like the following:

Select ID, 
    MAX(CASE WHEN Value = 1 Then 1 Else Null End) as Value1,
    MAX(CASE WHEN Value = 2 Then 2 Else Null End) as Value2,
    MAX(CASE WHEN Value = 3 Then 3 Else Null End) as Value3,
    MAX(CASE WHEN Value = 4 Then 4 Else Null End) as Value4,
    MAX(CASE WHEN Value = 5 Then 5 Else Null End) as Value5
From Table
Group By ID

If you don't know the number of columns at the outset ie they are dynamic you will then have to write a dynamic sql pivot. There are plenty of stack examples showing this:

• this one
• or this one
• and this one

Answer 2

OK, with your help, and a help from a friend, i solved the problem like this.

Select ROW_NUMBER() 
        OVER (PARTITION BY Field1 
        ORDER BY Field1) 
        AS order_num, Field1, Value
into #tab1
from Source_Table
Order by Field1


Select * 
from #tab1
    PIVOT
        (Max(Value)  
            FOR order_num IN ([1], [2], [3], [4], [5])) AS PVT


drop table #tab1

I still have to fully understand how it works, but it works. I hope it helps someone else too.