SQL Uniqueness

Brief interaction with Chat Jippitee on Uniqueness in SQL, when you need to make sure you get a certain column with only unique values… there are several approaches.

Using ROW_NUMBER

You can use the ROW_NUMBER window function to ensure a single row per App_Id. For example:

WITH RankedData AS (
    SELECT App_Id, 
           Owner, 
           Environment,
           ROW_NUMBER() OVER (PARTITION BY App_Id ORDER BY App_Id) AS RowNum
    FROM All_CIs_Table
)
SELECT App_Id, Owner, Environment
FROM RankedData
WHERE RowNum = 1;

Pros:

• Deterministic: You can control which row to keep using the ORDER BY clause
• Performance: Typically more efficient than DISTINCT for large datasets
• Flexibility: Allows you to specify criteria for prioritizing rows

Cons:

• Complexity: Slightly harder to read if you’re not familiar with window functions
• Data Assumptions: You need a sensible ORDER BY clause to pick the “first” row

Using GROUP BY with Aggregates

You can group by App_Id and pick one representative row for each group using aggregate functions.

SELECT App_Id,
       MAX(Owner) AS Owner,
       MAX(Environment) AS Environment
FROM All_CIs_Table
GROUP BY App_Id;

Pros:

• Simplicity: Concise and easy to understand
• Performance: GROUP BY is often faster than DISTINCT
• Deterministic: Explicitly defines how duplicates are resolved

Cons:

• Loss of Detail: You only get aggregated values
• Limited Control: You can’t prioritize which row is retained

Using OUTER APPLY or CROSS APPLY

You can use APPLY to fetch a single row for each App_Id.

SELECT App_Id, Owner, Environment
FROM All_CIs_Table AS A
OUTER APPLY (
    SELECT TOP 1 Owner, Environment
    FROM All_CIs_Table AS B
    WHERE A.App_Id = B.App_Id
    ORDER BY B.Owner, B.Environment
) AS Subquery;

Pros:

• Deterministic: Control over which row is selected with ORDER BY
• Flexibility: Can apply more complex logic in the subquery

Cons:

• Complexity: Harder to read and maintain than other approaches
• Performance: Can be slower for very large datasets

Comparison Table

Best Practice Recommendation

• Use ROW_NUMBER if you need precise control over which row to keep
• Use GROUP BY if you only need one row per App_Id without caring which specific row is chosen

Choose based on the specific constraints and requirements of your dataset and use case!

ROW_NUMBER Approach

The ROW_NUMBER approach provides excellent control over which row is kept by allowing you to customize the ORDER BY clause. Here’s a breakdown of how you can control the row selection and some strategies to prioritize rows:

Controlling by Null Values

If you want to prioritize rows with the least number of NULL values, you can calculate the count of NULL values and use that as part of the ORDER BY clause. For example:

WITH RankedData AS (
    SELECT App_Id, 
           Owner, 
           Environment,
           ROW_NUMBER() OVER (
               PARTITION BY App_Id 
               ORDER BY 
                   (CASE WHEN Owner IS NULL THEN 1 ELSE 0 END +
                    CASE WHEN Environment IS NULL THEN 1 ELSE 0 END) ASC,
                   App_Id -- Break ties if needed
           ) AS RowNum
    FROM All_CIs_Table
)
SELECT App_Id, Owner, Environment
FROM RankedData
WHERE RowNum = 1;

Explanation:

• The (CASE WHEN ... IS NULL THEN 1 ELSE 0 END) counts NULL values for each column
• Rows with fewer NULL values are prioritized (sorted in ascending order)
• Add additional ORDER BY criteria (e.g., App_Id) to resolve ties

Controlling by Specific Column Values

If you want to prioritize rows based on a specific column (e.g., Environment), you can use ORDER BY directly:

WITH RankedData AS (
    SELECT App_Id, 
           Owner, 
           Environment,
           ROW_NUMBER() OVER (
               PARTITION BY App_Id 
               ORDER BY 
                   Environment DESC, -- Example: prioritize specific values
                   App_Id
           ) AS RowNum
    FROM All_CIs_Table
)
SELECT App_Id, Owner, Environment
FROM RankedData
WHERE RowNum = 1;

Example Scenarios:

• Sort Environment values alphabetically or by specific logic
• Use a similar approach for other columns based on business rules

Controlling by Timestamp

To select the “latest” or “earliest” row for each App_Id, you can sort by a timestamp column:

WITH RankedData AS (
    SELECT App_Id, 
           Owner, 
           Environment,
           ROW_NUMBER() OVER (
               PARTITION BY App_Id 
               ORDER BY LastModifiedDate DESC -- Latest row
           ) AS RowNum
    FROM All_CIs_Table
)
SELECT App_Id, Owner, Environment
FROM RankedData
WHERE RowNum = 1;

Explanation:

• Replace LastModifiedDate with any timestamp column that represents the creation or update time of rows

Custom Tie-Breaking Logic

To ensure deterministic results when rows tie on other criteria, add secondary and tertiary sorting levels:

WITH RankedData AS (
    SELECT App_Id, 
           Owner, 
           Environment,
           ROW_NUMBER() OVER (
               PARTITION BY App_Id 
               ORDER BY 
                   LastModifiedDate DESC, -- Primary sort (e.g., latest row)
                   Environment,          -- Secondary sort (alphabetical)
                   Owner                 -- Tertiary sort (alphabetical)
           ) AS RowNum
    FROM All_CIs_Table
)
SELECT App_Id, Owner, Environment
FROM RankedData
WHERE RowNum = 1;

Controlling by Business Logic

Sometimes you may have complex criteria, such as prioritizing rows where Owner has a specific value. You can use a CASE statement in the ORDER BY clause:

WITH RankedData AS (
    SELECT App_Id, 
           Owner, 
           Environment,
           ROW_NUMBER() OVER (
               PARTITION BY App_Id 
               ORDER BY 
                   CASE WHEN Owner = 'PreferredOwner' THEN 1 ELSE 2 END ASC, -- Prioritize specific owners
                   LastModifiedDate DESC -- Tie-break by latest timestamp
           ) AS RowNum
    FROM All_CIs_Table
)
SELECT App_Id, Owner, Environment
FROM RankedData
WHERE RowNum = 1;

Explanation:

• Rows with Owner = 'PreferredOwner' are sorted first
• Additional criteria handle ties

General Guidelines for Using ORDER BY in ROW_NUMBER

• Combine Logic: Use CASE statements to encode priority rules
• Handle Ties Explicitly: Always include additional sorting fields to ensure deterministic results
• Performance Considerations: Optimize indexes on the columns used in ORDER BY where possible

These strategies allow you to tailor the row selection logic to your specific requirements while maintaining uniqueness on the App_Id column.