Diff between two tables (using sql) -> incremental changes - sql

Question

I have a need to identify differences between two tables. I have looked at sql query to return differences between two tables but it was a bit too different for me to extrapolate with my current SQL skills.
Table A is a snapshot of a certain group of people where the snapshot was taken yesterday, where each row is a unique person and certain characteristics about the person. Table B is the same snapshot taken 24 hours later. Within the 24 hour period:
New people may have been added.
People from yesterday may have been removed.
People from yesterday may have changed (i.e., original row is there, but one or more column values have changed).
My output should have the following:
a row for each new person added
a row for each person removed
a row for each person who has changed
I would grateful for any ideas. Thanks!

Answer 1

This type of problem has a very simple and efficient solution that does not use joins (it doesn't even use a union of the results of two MINUS operations) - it just uses one union and a GROUP BY operation. The solution was developed in a thread on AskTom many years ago, it is surprising that it is not more widely known and used. For example (but not only): https://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:24371552251735
In your case, assuming there is a primary key constraint on PERSON_ID (which makes the solution simpler):
select max(flag) as flag, PERSON_ID, first_name, last_name, (etc. - all the columns)
from ( select 'old' as flag, t1.*
from old_table t1
union all
select 'new' as flag, t2.*
from new_table t2
)
group by PERSON_ID, first_name, last_name, (etc.)
having count(*) = 1
order by PERSON_ID -- optional
;
If for a PERSON_ID all the data is the same in both tables, that will result in a count of 2 for that group. So it won't pass the HAVING condition. The only groups that will have a count of 1 (and therefore will be just one row each!) are either rows that are in one table but not the other. If a person was added, that will show only one row, with the flag = 'new'. If a person was deleted, you will get only one row, with the flag 'old'. If there were updates, the same PERSON_ID will appear twice, but since at least one field is different, the two rows (one with flag 'new' and the other with 'old') will be in different groups, they will pass the HAVING filter, and they will BOTH be in the output.
Which is slightly different from what you requested; you will get both the old AND the new information for updates, labeled as 'old' and 'new'. You said you wanted only one of those but didn't state which one. This will give you both (which makes more sense anyway), but if you really only want one, it can be done easily in the query above.
Note - the outer select must have max(flag) rather than flag because flag is not a GROUP BY column; but it's the max() over exactly one row, so it WILL be the flag for that row anyway.
Added - OP indicated he would like to get only the "new" row for a person with updated (changed, modified) data. The approach shown below will change the flag to "changed" in this case.
with old_table ( person_id, first_name, last_name ) as (
select 101, 'John', 'Smith' from dual union all
select 102, 'Mary', 'Green' from dual union all
select 103, 'July', 'Dobbs' from dual union all
select 104, 'Will', 'Scott' from dual
),
new_table ( person_id, first_name, last_name ) as (
select 101, 'Joe' , 'Smith' from dual union all
select 102, 'Mary', 'Green' from dual union all
select 104, 'Will', 'Scott' from dual union all
select 105, 'Andy', 'Brown' from dual
)
-- end of test data; solution (SQL query) begins below this line
select case ct when 1 then flag else 'changed' end as flag,
person_id, first_name, last_name
from (
select max(flag) as flag, person_id, first_name, last_name,
count(*) over (partition by person_id) as ct,
row_number() over (partition by person_id order by max(flag)) as rn
from ( select 'old' as flag, t1.*
from old_table t1
union all
select 'new' as flag, t2.*
from new_table t2
)
group by person_id, first_name, last_name
having count(*) = 1
)
where rn = 1
order by person_id -- ORDER BY clause is optional
;
Output:
FLAG PERSON_ID FIRS_NAME LAST_NAME
------- ---------- --------- ---------
changed 101 Joe Smith
old 103 July Dobbs
new 105 Andy Brown

Answer 2

The first 2 parts are easy:
select 'New', name from B where not exists (select name from A where A.name=B.name)
union select 'Removed', name from A where not exists (select name from B where B.name = A.name)
The last one is where you need to compare characteristics. How many of them are there? Do you want to list what has changed or only that they have changed?
For argument's sake, let us only say that the characteristics are address and telephone #:
union select 'Phone', name from A,B where A.name = B.name and A.telephone != B.telephone
union select 'Address', name from A,B where A.name = B.name and A.address != B.address

Answer 3

Note: The question isn't currently tagged with the dbms. I use sql-server, so that's what I used to write the below. There may be slight differences in another dbms.
You can do something along these lines:
select *
from TableA a
left join TableB b on b.ID = a.ID
where a.ID is null -- added since yesterday
union
select *
from TableA a
left join TableB b on b.ID = a.ID
where b.ID is null -- removed since yesterday
union
select *
from TableA a
inner join TableB b on b.ID = a.ID -- restrict to records in both tables
where a.SomeValue <> b.SomeValue
or a.SomeOtherValue <> b.SomeOtherValue
--etc
Each select handles one portion of your expected output. In this manner, they'd all be joined into 1 result set. If you drop the union, you'll end up with a separate set for each.

Answer 4

I suggest to use Except to get the changed records. The below query should work if the db is sql server.
-- added since yesterday
SELECT B.*
FROM TableA A
LEFT Outer Join TableB B on B.ID = A.ID
WHERE A.ID IS NULL
UNION
-- removed since yesterday
SELECT A.*
FROM TableA A
LEFT OUTER JOIN TableB B on B.ID = A.ID
WHERE B.ID IS NULL
UNION
-- Those changed with values from yesterdady
SELECT B.* FROM TableB B WHERE EXISTS(SELECT A.ID FROM TableA A WHERE A.ID = B.ID)
EXCEPT
SELECT A.* FROM TableA A WHERE EXISTS(SELECT B.ID FROM TableB B WHERE B.ID = A.ID)

Answer 5

Assuming you have a unique id for each person in the able, you can use full outer join:
select coalesce(ty.customerid, tt.customerid) as customerid,
(case when ty.customerid is null then 'New'
when tt.customerid is null then 'Removed'
else 'Modified'
end) as status
from tyesterday ty full outer join
ttoday tt
on ty.customerid= tt.customerid
where ty.customerid is null or
tt.customerid is null or
(tt.col1 <> ty.col1 or tt.col2 <> ty.col2 or . . . ); -- may need to take `NULL`s into account

Answer 6

mathguy provided a successful answer to my initial problem. I asked him for a revision (to make it even better). He provided a revision, but I am getting a "missing keyword" error when executing against my code. Here is my code:
select case when ct = 1 then flag else 'changed' as flag, PERSON_ID, FIRSTNAME, LASTNAME
from (
select max(flag), PERSON_ID, FIRSTNAME, LASTNAME
count() over (partition by PERSON_ID) as ct,
row_number() over (partition by PERSON_ID
order by case when flag = 'new' then 0 end) as rn
from ( select 'old' as flag, t1.*
from YESTERDAY_TABLE t1
union all
select 'new' as flag, t2.*
from TODAY_TABLE t2
)
group by PERSON_ID, FIRSTNAME, LASTNAME
having count(*) = 1
)
where rn = 1
order by PERSON_ID;