7. Explain the difference between UNION and UNION ALL in SQL and provide an example scenario for each.

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7. Explain the difference between UNION and UNION ALL in SQL and provide an example scenario for each.

Overview

Understanding the difference between UNION and UNION ALL in SQL is crucial for efficiently querying data from multiple tables, especially when dealing with large datasets. UNION is used to combine the results of two or more SELECT statements by removing duplicates, whereas UNION ALL combines all results, including duplicates. Knowing when to use each can significantly impact the performance and accuracy of your SQL queries.

Key Concepts

  1. Duplicate Removal: UNION automatically removes duplicate records, while UNION ALL does not.
  2. Performance Considerations: UNION ALL is faster than UNION because it doesn't check for duplicates.
  3. Use Cases: Choosing between UNION and UNION ALL depends on the requirement for distinct results or the need for performance optimization.

Common Interview Questions

Basic Level

  1. What is the difference between UNION and UNION ALL?
  2. Provide a simple example where UNION would be preferred over UNION ALL.

Intermediate Level

  1. How does UNION ALL affect performance compared to UNION?

Advanced Level

  1. In what scenarios would using UNION ALL and filtering duplicates in a subsequent step be more efficient than using UNION?

Detailed Answers

1. What is the difference between UNION and UNION ALL?

Answer: The primary difference lies in how duplicates are handled. UNION combines the result sets of two or more SELECT statements and removes duplicates to return only distinct values. On the other hand, UNION ALL combines all results, including duplicates, because it does not perform duplicate elimination.

Key Points:
- UNION performs duplicate removal, which can be more resource-intensive.
- UNION ALL is faster since it skips the step of checking for duplicates.
- Both operations require the same number and order of columns in the SELECT statements with compatible data types.

Example:

-- Assume we have two tables, Employees1 and Employees2, with the same structure.
-- Using UNION to combine results without duplicates:
SELECT Name FROM Employees1
UNION
SELECT Name FROM Employees2;

-- Using UNION ALL to combine results with duplicates:
SELECT Name FROM Employees1
UNION ALL
SELECT Name FROM Employees2;

2. Provide a simple example where UNION would be preferred over UNION ALL.

Answer: UNION is preferred when you need a result set of unique records from two or more tables. For example, when fetching a list of all employees from different department tables without any repetition.

Key Points:
- Use UNION for distinct records.
- UNION is suitable for scenarios where data integrity (no duplicates) is crucial.
- UNION may be slower than UNION ALL due to the deduplication process.

Example:

-- Assuming two tables, SalesDepartment(EmpID, Name) and HrDepartment(EmpID, Name)
-- Fetching unique employee names from both departments:
SELECT Name FROM SalesDepartment
UNION
SELECT Name FROM HrDepartment;

3. How does UNION ALL affect performance compared to UNION?

Answer: UNION ALL enhances performance by skipping the deduplication step that UNION performs. This makes UNION ALL significantly faster, especially in queries against large datasets where checking for duplicates can be resource-intensive.

Key Points:
- UNION ALL is faster due to no duplicate check.
- Ideal for performance-critical applications where duplicates are either unlikely or acceptable.
- The choice between UNION and UNION ALL should consider the impact on performance vs. the need for unique records.

Example:

-- Example demonstrating performance consideration:
-- Fetching all employee names from two tables, including duplicates, for a quick merge:
SELECT Name FROM TemporaryEmployees
UNION ALL
SELECT Name FROM PermanentEmployees;

4. In what scenarios would using UNION ALL and filtering duplicates in a subsequent step be more efficient than using UNION?

Answer: Using UNION ALL followed by filtering duplicates can be more efficient when working with large datasets, where the cost of deduplication upfront (as done by UNION) is higher than the combined cost of merging all data first and then removing duplicates. This approach can be beneficial when subsequent processing steps naturally eliminate duplicates or when the duplicates are concentrated in a small subset of the data.

Key Points:
- UNION ALL followed by explicit deduplication may outperform UNION for large or complex datasets.
- This approach allows for more control over the deduplication process, potentially optimizing it based on data characteristics.
- Efficiency gains depend on the specific dataset and subsequent processing steps.

Example:

-- Combining all records then filtering duplicates:
SELECT DISTINCT Name
FROM (
    SELECT Name FROM TemporaryEmployees
    UNION ALL
    SELECT Name FROM PermanentEmployees
) AS Combined;

This example demonstrates an efficient use of UNION ALL with a subsequent DISTINCT operation to ensure unique names, potentially optimizing performance for large datasets.

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