SQL difference of two rows

Question

I have a table, Table1

Table1
ID  REG  VALUE
1   54   500
2   54   1700
3   60   5000
4   60   5500

Now, i need to copy rows from this table to a second one, Table2 , but for entries with the same REG i want to make single row in the second table,with the difference between the highest VALUE and the lower VALUE , something like this:

Table2
ID  REG  VALUE
1   54   1200
2   60   500

How can I do this?

Answer 1

You could use an insert-select statement:

INSERT INTO table2 (reg, value)
SELECT   reg, MAX(value) - MIN(value)
FROM     table1
GROUP BY reg
HAVING   COUNT(*) > 1

EDIT:

If the requirement is to also copy values that appear on a single reg row too, this can be done using a case expression:

INSERT INTO table2 (reg, value)
SELECT   reg, 
         CASE COUNT(*) WHEN 1 THEN MAX(value) ELSE MAX(value) - MIN(value) END
FROM     table1
GROUP BY reg