Find duplicates and remove it applying conditions SQL Server - sql

Question

I'm new to SQL Server, and I'm trying to remove duplicates from a table but with some conditions, and my doubt is on how to apply these conditions to the query.
I need to remove duplicates from the Users table, eg:
Id Code Name SysName
-----------------------------
1 D1 N1
2 D1
3 D1 N1 N-1
4 E2 N2
5 E2 N2
6 E2 N2
7 X3
8 X3 N-3
9
10
11 Z4 W2 N-4-4
12 Z4 W2 N-44
In the above table: for D1 code I want to keep the ID=3, which has all columns filled (Code, Name, and SysName) and delete ID=1 and ID=2
For E2 code, I want to keep any of these and remove the two duplicated ones
For X3 code, keep the one which has SysName=N-3
For ID=9, ID=10 (empty code and everything empty, remove all)
For Z4 code, remove ID=11 and keep N-44 Sysname
And the last thing, I've a FK with other table, so I think that I need first to get all Id's from Users, delete these ids from the second dependent table and finally delete from Users table.
Do you have any idea on how to achieve it? I do not pretend the solution but a structure code or examples/scenarios you have similar to it, any suggestion would be fine for me.
EDIT:
To resume.. I have Users table:
Id Code Name SysName
-----------------------------
1 D1 N1
2 D1
3 D1 N1 N-1
4 E2 N2
5 E2 N2
6 E2 N2
7 X3
8 X3 N-3
9
10
11 Z4 W2 N-4-4
12 Z4 W2 N-44
And I want to keep only:
Id Code Name SysName
-----------------------------
3 D1 N1 N-1
4 E2 N2
8 X3 N-3
12 Z4 W2 N-44

Answer 1

Are you looking for something like
SELECT Code,
MAX(ISNULL(Name, '')) Name,
MAX(ISNULL(SysName, '')) SysName
FROM T
WHERE Code IS NOT NULL
GROUP BY Code;
Returns:
+------+------+---------+
| Code | Name | SysName |
+------+------+---------+
| D1 | N1 | N-1 |
| E2 | N2 | |
| X3 | | N-3 |
| Z4 | W2 | N-4-4 |
+------+------+---------+
Demo

Answer 2

The next query show the list of ids to remove according to the next rules of importance:
1- If the user has all the fields empty/null will be deleted.
2- The user with more fields with errors will be considered first to remove (example SysName cannot contains two -).
3- The user with more fields empty/null will be considered first to remove.
;WITH
[Ids]
AS
(
SELECT
[U].[Id]
,[Importance] =
CASE
WHEN [X].[NumberOfFilledFields] = 0
THEN -1
ELSE ROW_NUMBER() OVER (PARTITION BY [U].[Code] ORDER BY [X].[NumberOfInvalidFields], [X].[NumberOfFilledFields] DESC)
END
FROM [Users] AS [U]
CROSS APPLY
(
SELECT
[NumberOfFilledFields] =
+ CASE WHEN NULLIF([U].[Code], '') IS NULL THEN 0 ELSE 1 END
+ CASE WHEN NULLIF([U].[Name], '') IS NULL THEN 0 ELSE 1 END
+ CASE WHEN NULLIF([U].[SysName], '') IS NULL THEN 0 ELSE 1 END
,[NumberOfInvalidFields] =
+ CASE WHEN [U].[SysName] LIKE '%-%-%' THEN 1 ELSE 0 END
) AS [X]
)
SELECT
[Id]
FROM [Ids]
WHERE (1 = 1)
AND ([Importance] = -1 OR [Importance] > 1);

Answer 3

This uses window functions and coalesce:
DECLARE #t TABLE ([Id] INT, [Code] CHAR(2), [Name] CHAR(2), [SysName] VARCHAR(10))
INSERT INTO #t values
(1 , 'D1', 'N1', Null ), (2 , 'D1', Null, Null ), (3 , 'D1', 'N1', 'N-1' ), (4 , 'E2', 'N2', Null ), (5 , 'E2', 'N2', Null ), (6 , 'E2', 'N2', Null )
, (7 , 'X3', Null, Null ), (8 , 'X3', Null, 'N-3' ) , (9 , Null, Null, Null ), (10, Null, Null, Null ), (11, 'Z4', 'W2', 'N-44'), (12, 'Z4', 'W2', 'N-44' )
;WITH t AS (
SELECT DISTINCT
[code]
, COALESCE([name], max([name]) OVER(PARTITION BY [code])) AS [Name]
, COALESCE([sysname], COALESCE(MAX([sysname]) OVER(PARTITION BY [code], [name]), MAX([sysname]) OVER(PARTITION BY [code]))) AS [SysName]
FROM #t
WHERE [code] IS NOT NULL)
SELECT MIN(t2.id), t.Code, t.Name, t.SysName
from #t t2
INNER JOIN t ON t.code = t2.code AND ISNULL(t.[Name], 'null') = ISNULL(t2.[Name], 'Null') AND ISNULL(t.[SysName], 'Null') = ISNULL(t2.[SysName], 'Null')
GROUP BY t.Code, t.Name, t.SysName

Answer 4

DEMO
(Any other answer: feel free to borrow the demos to test your answer or use it in yours! no need to duplicate effort!)
One could use an analytic function/window function like row_number() to assign a row to each record we want and keep all the #1 rows except for those where code is null... do this with a cte and then just delete.
We determine what to keep by looking at the record having the most data and in case of ties, use the earliest ID.
With cte as (
SELECT id, code, name, sysname,
row_number() over (partition by code order by (case when name is not null then 1 else 0 end + case when sysname is not null then 1 else 0 end) desc, ID) RN
FROM users)
Delete from cte where RN <> 1 or code is null;
Results in:
+----+----+------+------+---------+
| | ID | Code | Name | Sysname |
+----+----+------+------+---------+
| 1 | 3 | D1 | N1 | N-1 |
| 2 | 4 | E2 | N2 | NULL |
| 3 | 8 | X3 | NULL | N-3 |
| 4 | 11 | Z4 | W2 | N-4-4 |
+----+----+------+------+---------+
One could use the CTE and delete related FK records that would get purged
and then use the cte again and delete the users

Answer 5

you need to have knowledge of case
then you can change the condition accordingly
you can see the sample code bellow. just twist the case as per your requirements in where clause.
;with C as
(
select Dense_rank() over(partition by code order by id) as rn,*
from Users
)
delete from C
where rn =
(case
when (code = 'd1' and name is not null and sysname !='') then 0
when (code = 'E1' and rn = 1) then 0
when (code = 'X3' and sysname!='') then 0
when (code = 'z4' and name is not null and sysname !='') then 0
else rn
end )
Output:-
3 D1 N1 N-1
8 X3 N-3
11 Z4 W2 N-4-4
12 Z4 W2 N-44