Question

I have two tables that have the same columns. If a change occurs it is being recorded in table2 and then I compare table1 to table2 and see if they are different. If they are different I conclude there was a change and I would like to display that in my resulting table.

For example:

SELECT t1.name, t1.age, t1.profession, column_changed, old_value, new_value
FROM   table1 t1, table2 t2
WHERE  t1.column1<>t2.column1 
    OR t1.column2<>t2.column2 
    OR t1.column3<>t2.column3

Of course this query isn’t correct. I would like the column_changed, old_value, new_value display the relevant values.

Any ideas?

Answer 1

After further brain storming this I have concluded that ganders solution works better with slight improvment. The improvement is a while loop and a count variable. We need to have that in case there are several columns changed at once and not just one. It will, however, result in outputting NULL’s as well so you can just delete them. Here is the modified query:

WHILE @count<3
BEGIN
  Select t1.name, t1.age, t1.profession,     
    case when t1.column1 <> t2.column1 and @count = 1 then 'column1 changed'         
      when t1.column2 <> t2.column2 @count = 2 then 'column2 changed'         
      -- and so on...         
    end as [column_changed],     
    case when t1.column1 <> t2.column1 @count = 1 then t1.column1         
      when t1.column2 <> t2.column2 @count = 2 then t1.column2         
      -- and so on...         
    end as [old_value],     
    case when t1.column1 <> t2.column1 @count = 1 then t2.column1         
      when t1.column2 <> t2.column2 @count = 2 then t2.column2         
      -- and so on...         
    end as [new_value] 
  From table1 t1, table2 t2 
  Where t1.column1 <> t2.column1 Or t1.column2 <> t2.column2 Or t1.column3 <> t2.column3 
  SET @counter = @counter + 1
END