检查重复记录的最佳方法

Question

I have two tables A and B with a relationship of One-to-many from A to B .

A has 5 columns:

a1, a2, a3, a4, a5 

and B has 5 columns

b1, b2, b3, b4, a1. 

Note a1 is foreign key in table B.

I have a requirement to check duplicate records in the table ie no two records should have exactly same values for all the attributes.

The most efficient way I can think of for determining their uniqueness is by creating a checksum sort of value and keep it in every row of table A. But this requires extra space plus I will have to make sure that the checksum is really unique.

Is this the best way to go ahead or is there some other way I am unaware of?

For eg Lets say table A is Rules Table and Table B is Trigger table. Now Rules table has records of various rules created by different users.(This means that there will be a mapping to Users Table in Rules Table.). Now what I actually want is that a user should not be able to create identical rules. So when a user saves rules I run a query to check if there is record of identical checksum for this particular user if yes then I give the appropriate error otherwise I let the user to create the record.I guess this clears that why I can't put unique constraint on all records.

Answer 1

Do a SELECT with a GROUP BY clause. For example:

SELECT a1, a2, a3, a4, a5, COUNT(*) FROM #TempPersons GROUP BY a1, a2, a3, a4, a5 HAVING COUNT(*) > 1;

This will return a result with the a1, a2, a3, a4, a5 and a count of how many times that value appears

Answer 2

Having a UNIQUE constraint on those columns seems like the way to go.

However, just for the sake of answering your other remarks: I've worked with extra columns to check for changes in the past before. Back then I did something similar to this:

CONVERT([NVARCHAR](42),HASHBYTES('SHA1',CONCAT(Column1, '||', Column2, ...),(1))

I found it to be a rather nice way to concat many columns into a single hash, unique depending on it's contents & without it blowing out of proportion. (I used this in a datawarehousing environment, to check large tables for record level changes based on a business key. Stored this as a PERSISTED column to allow an index to run on this too).