“The database principal owns a database role and cannot be dropped. (Microsoft SQL Server, 15421)”

So you may want to find which role the user owns and how to fix this error.

Run the script below to find the roles user owns:

SELECT dp.[name] AS [Owner], dp2.[name] AS [Role]
FROM sys.database_principals dp JOIN sys.database_principals dp2 ON dp.principal_id = dp2.owning_principal_id
WHERE dp.[name] = ‘UserToDelete’

In the above query replace <’UserToDelete’> with the user that you wish to delete.

For each role in the query result, you can modify its owner in the Property window to [dbo].Or you can run the script below:

ALTER AUTHORIZATION ON ROLE::[ROLE_InResult] TO [dbo]

Now you’re ready to delete the user. 

Or like this table Product

Some one inserted Product 1, 2, 3 later again and made the first 4 rows reduntant. You will find these by quering:

/*————————
 SELECT p.ProductName, COUNT(*) AS Num
 FROM Product p
 GROUP BY p.ProductName
————————*/
ProductName                                        Num
————————————————– ———–
Product1                                           3
Product2                                           2
Product3                                           2
Product4                                           1

So when cleaning comes to play, we’d like to keep the last 4  rows only. How could you find and keep the lastest item only in each group? Or how could you delete the redundant ones?

Tip: use non-equi self-join

/*————————
SELECT DISTINCT p.ProductId, p.ProductName
  FROM Product p JOIN Product p2 ON p.ProductId < p2.ProductId AND p.ProductName = p2.ProductName
————————*/
ProductId   ProductName
———– ————————————————–
1           Product1
2           Product2
3           Product3
4           Product1

So only task left is to remove them:

/*————————
DELETE
FROM   Product
WHERE  ProductId IN (SELECT DISTINCT p.ProductId
                     FROM   Product p
                            JOIN Product p2
                                 ON  p.ProductId > p2.ProductId
                                 AND p.ProductName = p2.ProductName)
————————*/

(4 row(s) affected)