Union vs Union All in SQL: Key Differences and When to Use Each

SQL (structured query language) provides powerful tools to manipulate and retrieve data from databases. Among these tools, the UNION and UNION ALL operators are commonly used to combine results from two or more queries. While they might appear similar at first glance, their differences can significantly impact performance and results. This article dives into the key distinctions, use cases, and best practices for UNION and UNION ALL in SQL.

The UNION operator is used to combine the results of two or more SELECT statements into a single dataset. It automatically removes duplicate rows from the combined result.

Syntax

SELECT column1, column2
FROM table1
UNION
SELECT column1, column2
FROM table2;

Characteristics of UNION

1. Duplicate Removal: By default, UNION eliminates duplicate rows, ensuring a unique set of records in the final result.
2. Sorting: The result is implicitly sorted (by default, in ascending order) unless explicitly overridden using an ORDER BY clause.
3. Column Alignment: The number of columns and their data types must match across all SELECT statements.

Example

Table 1:

Table 2:

Using UNION:

SELECT ID, Name
FROM Table1
UNION
SELECT ID, Name
FROM Table2;

Result:

The UNION ALL operator also combines the results of two or more SELECT statements, but unlike UNION, it does not remove duplicate rows. All rows from each query, including duplicates, are included in the final result.

Syntax

SELECT column1, column2
FROM table1
UNION ALL
SELECT column1, column2
FROM table2;

Characteristics of UNION ALL

1. No Duplicate Removal: UNION ALL retains all rows, resulting in faster performance since no additional processing is required to eliminate duplicates.
2. Sorting: Similar to UNION, sorting occurs only when explicitly specified using an ORDER BY clause.
3. Column Alignment: The number of columns and their data types must still match across all SELECT statements.

Example

Using the same Table 1 and Table 2:

SELECT ID, Name
FROM Table1
UNION ALL
SELECT ID, Name
FROM Table2;

Result:

Key Differences Between UNION and UNION ALL

The choice between UNION and UNION ALL can have significant performance implications:

1. Duplicate Removal Cost:
   • UNION performs an internal DISTINCT operation to eliminate duplicates. This involves sorting and comparing rows, which can be computationally expensive for large datasets.
   • UNION ALL skips this step, making it more efficient, especially when working with large tables.
2. Index Usage:
   • Efficient indexing can reduce the cost of UNION operations, but UNION ALL generally requires fewer resources due to the absence of deduplication.
3. Memory and CPU Impact:
   • For queries involving millions of rows, UNION might consume more memory and processing power than UNION ALL.

Example Benchmark:

Consider two tables, each with 1 million rows. Combining them using:

• UNION: Requires sorting and comparing all rows, which can take significantly longer.
• UNION ALL: Simply appends the rows without additional computation.

Use UNION when:

Duplicate Rows are Undesirable: For example, generating a report where only unique records are needed.

Example:

SELECT CustomerID
FROM OnlineOrders
UNION
SELECT CustomerID
FROM InStoreOrders;

1. This ensures no customer is counted twice in the final result.
2. Data Consolidation: Merging data from different sources where duplicate records are not meaningful.

Use UNION ALL when:

1. Duplicates Are Acceptable: For example, consolidating logs or events where repeated entries are valid.

   Example:

SELECT EventID, Timestamp
FROM ApplicationLogs
UNION ALL
SELECT EventID, Timestamp
FROM SystemLogs;

This allows you to analyze all events, including duplicates.

1. Performance Is a Priority: When working with large datasets and duplicates are not a concern, UNION ALL is the better choice.
2. Intermediate Steps: In complex queries, you may use UNION ALL to combine data temporarily, followed by further filtering or aggregation.

1. Understand Your Data: Analyze the need for duplicate removal before choosing between UNION and UNION ALL.
2. Optimize Performance: Use UNION ALL for large datasets unless duplicates would cause issues.
3. Test Your Query: Run both UNION and UNION ALL during development to evaluate performance and correctness.
4. Use Indexes: Ensure the columns involved in UNION or UNION ALL operations are indexed to improve query performance.
5. Employ explicit Sorting: Use ORDER BY to control sorting explicitly, rather than relying on UNION’s implicit behavior.

Both UNION and UNION ALL are essential tools for combining datasets in SQL. While UNION ensures uniqueness at the cost of performance, UNION ALL provides a faster alternative when duplicates are acceptable. By understanding the nuances of these operators and aligning their usage with your data requirements, you can write more efficient and accurate SQL queries. Always evaluate the trade-offs between performance and result accuracy to make the best choice for your specific use case.

• Oracle. (2024). SQL UNION and UNION ALL operators. Retrieved from https://docs.oracle.com/en/database/
• Microsoft. (2024). UNION vs. UNION ALL in SQL. Retrieved from https://learn.microsoft.com/en-us/sql/
• W3Schools. (2024). SQL UNION and UNION ALL. Retrieved from https://www.w3schools.com/sql/sql_union.asp