What is a distinct merge in SQL?

In SQL, a "distinct merge" isn't a standard, built-in feature or a specific keyword like JOIN or UNION. However, the concept refers to the process of merging data from multiple sources (tables) while ensuring that the resulting dataset contains only unique or distinct rows. This typically involves combining data from different tables and then removing any duplicate entries.

Here's how you can achieve a distinct merge in SQL:

1. Using UNION with ALL to Merge Data:

- First, you can use UNION ALL to combine data from different tables. UNION ALL simply appends the result sets of multiple SELECT statements without removing duplicates.

2. Applying DISTINCT to Remove Duplicates:

- After merging the data with UNION ALL, you can wrap the entire query in another SELECT statement and use the DISTINCT keyword to eliminate duplicate rows.

3. Example Scenario:

- Suppose you have two tables, Customers1 and Customers2, each containing customer information, and you want to create a single list of unique customer entries.

4. SQL Query Example:

SELECT DISTINCT CustomerID, FirstName, LastName
FROM (
  SELECT CustomerID, FirstName, LastName FROM Customers1
  UNION ALL
  SELECT CustomerID, FirstName, LastName FROM Customers2
) AS CombinedCustomers;

- In this example, UNION ALL combines the rows from Customers1 and Customers2. The outer SELECT DISTINCT then ensures that only unique rows are returned, effectively performing a distinct merge.

5. Alternative Approach Using Temporary Tables or CTEs:

- You could also create a temporary table or use a Common Table Expression (CTE) to store the merged data and then apply DISTINCT.

- Example using a CTE:

WITH CombinedCustomers AS (
  SELECT CustomerID, FirstName, LastName FROM Customers1
  UNION ALL
  SELECT CustomerID, FirstName, LastName FROM Customers2
)
SELECT DISTINCT CustomerID, FirstName, LastName
FROM CombinedCustomers;

6. Performance Considerations:

- Using DISTINCT can be resource-intensive, especially on large datasets, because it requires sorting and comparing all rows. Ensure your tables are properly indexed to improve query performance.

- If you know that duplicates are unlikely or that performance is critical, you might consider other strategies, such as pre-processing the data or using more specific filtering criteria.

In summary, a "distinct merge" in SQL involves combining data from multiple tables and then removing any duplicate rows to ensure the final result set contains only unique entries. This is typically achieved by using UNION ALL followed by SELECT DISTINCT.

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