How to find missing data from duplicate table without using 'EXCEPT' - sql

Question

I used to use 'EXCEPT' to find missing data from 2 tables that should have the same data but was told not to use it anymore. I found a solution but I'm not entirely sure how it works. Could someone explain it to me or help me with another solution?
This is a basic example of my query:
SELECT MIN(C.TABLE_NAME) as TABLE_NAME,columnid,column
FROM
(
SELECT DISTINCT 'Source' as TABLE_NAME,columnid,column
FROM table1
UNION ALL
SELECT DISTINCT 'Output' as TABLE_NAME,columnid,column
FROM table2
) AS C
GROUP BY columnid,column
HAVING COUNT(*) = 1;
The output result shouldn't display any rows if the data is matching. The above code works as intended as I tested it on a table where I know the data is matching and not matching. I'm just not sure how it works. Sorry for the simple question. I'm new to this.
Edit:
I quickly made some sample data if it helps.
WITH salesman AS
(
SELECT 5005 AS id, 'Pit Alex' AS [name]
UNION ALL
SELECT 5006 AS id, 'Mc Lyon' AS [name]
UNION ALL
SELECT 5011 AS id, 'Lauson Hen' AS [name]
UNION ALL
SELECT 5007 AS id, 'Paul Adam' AS [name]
) ,
salesmancopy AS
(
SELECT 5005 AS id, 'Pit Alex' AS [name]
UNION ALL
SELECT 5006 AS id, 'Mc Lyon' AS [name]
UNION ALL
SELECT 5010 AS id, 'Lauson Hen' AS [name]
)
SELECT MIN(C.TABLE_NAME) as TABLE_NAME,id,[name]
FROM
(
SELECT DISTINCT 'original' as TABLE_NAME,id,[name]
FROM salesman
UNION ALL
SELECT DISTINCT 'copy' as TABLE_NAME,id,[name]
FROM salesmancopy
) AS C
GROUP BY id,[name]
HAVING COUNT(*) = 1;

Answer 1

If you want rows from table1 that are not in table2 then your solution will work only if table2 does not contain some unique rows. In other words, rows in table2 have to exist in table1. Another solution is to use NOT EXISTS
select *
from table1 t1
where not exists (
select 1
from table2 t2
where t1.columnid = t2.columnid and
t1.column = t2.column
)
Here you can see a comparison of different approaches to this problem where NOT EXISTS solution is prefered over LEFT JOIN + IS NULL solution.

Answer 2

Except is the fastest method to determinate if exists on one side.
But if you want to check both tables on single go you could use FULL OUTER JOIN
IF OBJECT_ID('tempdb..#t1') IS NOT NULL
DROP TABLE #t1;
IF OBJECT_ID('tempdb..#t2') IS NOT NULL
DROP TABLE #t2;
SELECT *
INTO #t1
FROM (SELECT 1 AS num UNION SELECT 2 AS num UNION SELECT 3 AS num) d;
SELECT *
INTO #t2
FROM (SELECT 1 AS num UNION SELECT 2 AS num UNION SELECT 5 AS num) d;
SELECT *
FROM #t1
FULL OUTER JOIN #t2
ON #t2.num = #t1.num
WHERE #t1.num IS NULL
OR #t2.num IS NULL;
Output:

Answer 3

To get around the issue mentioned by Radim Bača, ie if you would need to figure out the differences between two tables, when rows dont exist in table2 but they exist in table1, you can choose the following option.
1.Create two column to indicate if the record is from original or copy
2.group by the columns you wish to compare.
3.use the clause having count(orig)<> count(copy)
use the same query as before with small changes,
WITH salesman AS
(
SELECT 5005 AS id, 'Pit Alex' AS [name]
UNION ALL
SELECT 5006 AS id, 'Mc Lyon' AS [name]
UNION ALL
SELECT 5011 AS id, 'Lauson Hen' AS [name]
UNION ALL
SELECT 5007 AS id, 'Paul Adam' AS [name]
) ,
salesmancopy AS
(
SELECT 5005 AS id, 'Pit Alex' AS [name]
UNION ALL
SELECT 5006 AS id, 'Mc Lyon' AS [name]
UNION ALL
SELECT 5010 AS id, 'Lauson Hen' AS [name]
)
SELECT c.id
,c.name
,count(orig) as present_in_orig
,count(copy) as present_in_copy
FROM
(
SELECT 'original' as orig
,null as copy
,id
,[name]
FROM salesman
UNION ALL
SELECT null as orig
,'copy' as copy
,id
,[name]
FROM salesmancopy
) AS C
GROUP BY id
,[name]
HAVING COUNT(copy)<> count(orig)
order by 1,2
See the following link from Stew who details this method very nicely.
https://stewashton.wordpress.com/2014/02/04/compare-and-sync-tables-tom-kyte-and-group-by/