Union vs Union All: Understanding the Difference
What is Union?
Union is a SQL operator used to combine the result sets of two or more SELECT statements into a single result set. It removes duplicate rows from the combined result and generates a distinct result set.
Examples of Union:
SELECT column1 FROM table1 UNION SELECT column1 FROM table2;
Uses of Union:
– Merging similar data from different tables or views
– Combining results of similar queries into a single result set
What is Union All?
Union All is also a SQL operator used to combine the result sets of two or more SELECT statements into a single result set. However, unlike Union, it does not remove duplicate rows from the combined result set. It includes all rows, whether they are duplicates or not.
Examples of Union All:
SELECT column1 FROM table1 UNION ALL SELECT column1 FROM table2;
Uses of Union All:
– Merging multiple result sets without removing duplicates
– Combining results where duplicates are expected and needed
Differences Between Union and Union All:
| Difference Area | Union | Union All |
|---|---|---|
| Result Set | Generates a distinct result set without duplicate rows. | Includes all rows, whether they are duplicates or not. |
| Performance | Performs slightly slower due to the overhead of removing duplicates. | Performs faster as it does not require removing duplicates. |
| Memory Usage | Consumes more memory as it needs to hold the distinct result set. | Consumes less memory as it includes all rows, including duplicates. |
| Syntax | Uses the keyword “UNION”. | Uses the keyword “UNION ALL”. |
| Column Number | The SELECT statements in the union must have an equal number of columns. | The SELECT statements can have a different number of columns. |
| Duplicate Rows | Eliminates duplicate rows from the result set. | Includes all rows, including duplicates, in the result set. |
| Query Optimization | May optimize queries due to the removal of duplicates. | No duplicate removal occurs, so less query optimization may take place. |
| Combining Result Sets | Merges and orders the result sets based on the column list. | Merges the result sets as they are, without any ordering. |
| Usage Frequency | Used less frequently when duplicates need to be removed. | Used more often when duplicates are expected or needed. |
| Analyzing Data | Produces a cleaner and more organized data output. | Preserves the original data structure, including duplicates, for analysis. |
Conclusion:
In conclusion, Union and Union All are SQL operators used to combine the result sets of multiple SELECT statements. The main difference lies in how they handle duplicate rows. Union removes duplicates, while Union All retains them. Consider your data requirements and the purpose of your query before deciding which operator to use.
People Also Ask:
Q: When should I use Union or Union All in my SQL query?
A: Use Union when you want to eliminate duplicate rows and Union All when you want to include all rows, including duplicates.
Q: How do Union and Union All affect query performance?
A: Union may have slightly slower performance due to the removal of duplicates, while Union All performs faster as it does not require duplicate removal.
Q: Can the SELECT statements in a Union or Union All have a different number of columns?
A: No, Union requires an equal number of columns in each SELECT statement, while Union All allows different column numbers within the SELECT statements.
Q: Does Union All optimize queries like Union does?
A: Union All does not optimize queries as it does not involve duplicate removal. Union may perform query optimization based on the elimination of duplicates.
Q: Which operator is more commonly used in SQL queries?
A: Union All is more commonly used when duplicates are expected or needed, while Union is used when duplicate rows need to be removed from the result set.
