When working with SQL in data engineering or data warehousing engagements, there are various techniques and operators available to combine and manipulate data from multiple tables or queries. Two commonly used operators for data consolidation are UNION and UNION ALL. While they may appear similar at first glance, there are crucial differences between the two. In this article, we will explore the differences between UNION and UNION ALL in SQL and when to use each operator.
- UNION Operator:
The UNION operator in SQL is used to combine the result sets of two or more SELECT statements into a single result set. The key characteristic of the UNION operator is that it removes duplicate rows from the final result set. When using UNION, the columns in the SELECT statements must have the same data type and be in the same order. The resulting data set will include distinct rows from all the SELECT statements involved.
Example: Consider the following two tables:
Table A:
| ID | Name |
|---|---|
| 1 | John |
| 2 | Alice |
| 3 | Emma |
Table B:
| ID | Name |
|---|---|
| 2 | Alice |
| 4 | Peter |
| 5 | Sarah |
Using the UNION operator: SELECT * FROM TableA UNION SELECT * FROM TableB;
Result:
| ID | Name |
|---|---|
| 1 | John |
| 2 | Alice |
| 3 | Emma |
| 4 | Peter |
| 5 | Sarah |
Note: The duplicate row with ID 2 and Name “Alice” is removed from the final result set.
- UNION ALL Operator:
The UNION ALL operator, on the other hand, combines the result sets of two or more SELECT statements into a single result set without removing duplicate rows. Unlike the UNION operator, UNION ALL does not perform any duplicate elimination. This means that all rows from each SELECT statement are included in the final result set, regardless of whether they are duplicates or not.
Example (continued from the previous example): Using the UNION ALL operator: SELECT * FROM TableA UNION ALL SELECT * FROM TableB;
Result:
| ID | Name |
|---|---|
| 1 | John |
| 2 | Alice |
| 3 | Emma |
| 2 | Alice |
| 4 | Peter |
| 5 | Sarah |
Note: The duplicate row with ID 2 and Name “Alice” is retained in the final result set.
- When to Use UNION and UNION ALL:
The choice between UNION and UNION ALL depends on the specific requirements of your SQL query.
Here are some considerations:
- Use UNION when you want to eliminate duplicate rows and obtain a distinct result set.
- Use UNION ALL when you want to combine all rows from multiple SELECT statements, including duplicates.
- UNION can have a performance impact as it involves an additional step to remove duplicates, while UNION ALL is generally faster because it does not perform duplicate elimination.
- If you are certain that there are no duplicate rows or you want to preserve duplicates for a specific reason, using UNION ALL is more efficient.
Understanding the difference between UNION and UNION ALL in SQL is crucial for data consolidation and result set manipulation. UNION removes duplicate rows, while UNION ALL retains all rows, including duplicates. Consider your specific requirements and performance considerations when choosing between these operators. Mastering the appropriate usage of UNION and UNION ALL empowers SQL developers to efficiently combine and manipulate data from multiple sources.
