Displaying the correct condition found in WHERE clause - tsql

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

Does Age, name, profession form a primary key (or at least a unique key?):
If so, you could do something like:
SELECT
t1.name, t1.age, t1.profession,
t1.column1 as t1column1, t2.column1 as t2column1,
t1.column2 as t1column2, t2.column2 as t2column2,
FROM
table1 t1, table2 t2
WHERE
(t1.name = t2.name and t1.age = t2.age and t1.profession = t2.profession) and
(
t1.column1<>t2.column1
OR t1.column2<>t2.column2
OR t1.column3<>t2.column3)
Of course, that requires a unique key which is the same across both tables. Also, I clearly changed the results of the query to show all columns instead of just the one that changed. Identifying the one that changed like that in a single T-SQL Query is awkward (but possible) so my recommendation would be to return it like this and depending on your use case have the application/presentation layer handle finding which column changed or just scan it by eye.
If you really want to do it in T-SQL, you could do it with UNIONS, like:
SELECT
t1.name, t1.age, t1.profession,
'column1' as ColumnChanged,
t1.column1 as oldValue,
t2.column1 as newValue
FROM
table1 t1, table2 t2
WHERE
(t1.name = t2.name and t1.age = t2.age and t1.profession = t2.profession) and
t1.column1<>t2.column1
UNION ALL #better peformance than UNION, which would enforce uniqueness
SELECT
t1.name, t1.age, t1.profession,
'column2' as ColumnChanged,
t1.column2 as oldValue,
t2.column2 as newValue
FROM
table1 t1, table2 t2
WHERE
(t1.name = t2.name and t1.age = t2.age and t1.profession = t2.profession) and
t1.column2<>t2.column2
.......

Answer 2

Try this:
Select t1.name, t1.age, t1.profession,
case when t1.column1 <> t2.column1 then 'column1 changed'
when t1.column2 <> t2.column2 then 'column2 changed'
-- and so on...
end as [column_changed],
case when t1.column1 <> t2.column1 then t1.column1
when t1.column2 <> t2.column2 then t1.column2
-- and so on...
end as [old_value],
case when t1.column1 <> t2.column1 then t2.column1
when t1.column2 <> t2.column2 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

Answer 3

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

Answer 4

Here's your solution rewritten as a single SELECT statement (or, if you like, #ganders's rewritten to support several changes in the same row):
SELECT
x.column_changed,
t1.column1 AS old_value,
t2.column1 AS new_value
FROM table1 t1
INNER JOIN table2 t2 ON t1.name = t2.name
INNER JOIN (
SELECT 'column1' UNION ALL
SELECT 'column2' UNION ALL
…
) x (column_changed) ON (
x.column_changed = 'column1' AND t1.column1 <> t2.column1 OR
x.column_changed = 'column2' AND t1.column2 <> t2.column2 OR
…
)

Answer 5

I added a join #T2 t2 on t2.name = t1.name to your query using the name column. Making the assumption that it can be used to link the records between the two tables.
select
t1.name, t1.age, t1.profession,
column_change =
case when t1.column1 <> t2.column1 then 'column1'
when t1.column2 <> t2.column2 then 'column2'
when t1.column3 <> t2.column3 then 'column3'
end,
old_value =
case when t1.column1 <> t2.column1 then t1.column1
when t1.column2 <> t2.column2 then t1.column2
when t1.column3 <> t2.column3 then t1.column3
end,
new_value =
case when t1.column1 <> t2.column1 then t2.column1
when t1.column2 <> t2.column2 then t2.column2
when t1.column3 <> t2.column3 then t2.column3
end
from #T1 t1
join #T2 t2 on t2.name = t1.name
where
(t1.column1 <> t2.column1
or t1.column2 <> t2.column2
or t1.column3 <> t2.column3)