Have you ever encountered a situation where you needed to fix inconsistent data in your SQL Server database? In this article, we will explore how the OVER() clause, along with its constituent PARTITION BY and ORDER BY components, can be used to solve such problems.

The Problem

Imagine you are working with a large resource planning system that uses an attribute-based model instead of the traditional row model. This means that instead of storing data in rows, the system stores attribute types in one table and their corresponding values in another table.

However, you discover that there are inconsistencies in the version numbers of the attribute values. Some records have multiple attribute values with the same version number or gaps in the version number. This becomes a problem when you need to identify the current version of a field using the MAX(FieldVersion) clause in queries.

Using the OVER() Clause to Fix the Problem

The solution to this problem lies in using the OVER() clause along with the ROW_NUMBER() function. Here’s how it works:

1. Split each individual field into its own mini-set of data, using ContactId and FieldTypeId.
2. Order the records by their StartDate to determine the version number order.
3. Use ROW_NUMBER() with the OVER() clause to assign the correct version numbers.

Let’s take a look at an example:

WITH BadVersions (ContactId, FieldId, FieldTypeId, FieldVersion, CorrectVersion, StartDate, EndDate)
AS
(
SELECT BAD.ContactId, BAD.FieldId, BAD.FieldTypeId, BAD.FieldVersion, BAD.CorrectVersion, BAD.StartDate, BAD.EndDate
FROM
(
SELECT CF.ContactId, CF.FieldId, CF.FieldTypeId, CF.FieldVersion, CF.StartDate, CF.EndDate,
ROW_NUMBER() OVER (PARTITION BY CF.ContactId, CF.FieldTypeId ORDER BY CF.ContactId, CF.FieldTypeId, CF.StartDate) AS CorrectVersion
FROM ContactFields CF
INNER JOIN
(
SELECT ContactId, FieldTypeId
FROM ContactFields
GROUP BY ContactId, FieldTypeId HAVING COUNT(*) > 1
) MULTIPLES
ON CF.ContactId = MULTIPLES.ContactId AND CF.FieldTypeId = MULTIPLES.FieldTypeId
) BAD
WHERE BAD.FieldVersion != BAD.CorrectVersion
)
SELECT ContactId, FieldId, FieldTypeId, FieldVersion, CorrectVersion, StartDate, EndDate
INTO #BadRecords
FROM BadVersions BV;

In this example, we first identify the bad rows using a CTE (Common Table Expression) named BadVersions. We use the PARTITION BY clause in the ROW_NUMBER() function to assign the correct version numbers to each row. The identified rows are then stored in a temporary table called #BadRecords.

Finally, we can update the incorrect version numbers with the correct ones using an UPDATE statement:

UPDATE F
SET F.FieldVersion = BR.CorrectVersion
FROM ContactFields F
INNER JOIN #BadRecords BR ON F.FieldId = BR.FieldId;

After the update, the field versions will be correctly ordered, resolving the inconsistency issue.

Conclusion

The OVER() clause, when used in conjunction with the PARTITION BY and ORDER BY components, can be a powerful tool for fixing data inconsistencies in SQL Server. By breaking down the data into smaller sets and assigning the correct version numbers using ROW_NUMBER(), you can easily solve problems like inconsistent version numbers.

Next time you encounter a similar issue, consider using the OVER() clause to save time and effort in fixing your data. It’s a handy technique that every SQL Server developer should have in their toolkit.