My SQL Database: How to use UNION and UNION ALL?
When working with MySQL databases, the UNION and UNION ALL operators are powerful tools for combining the results of multiple SELECT statements. In this blog post, we will explore how to use UNION and UNION ALL in MySQL, along with practical examples and common use cases.
1. UNION vs. UNION ALL
The UNION operator is used to combine the result sets of two or more SELECT statements into a single result set. It removes duplicate rows from the final result set. On the other hand, the UNION ALL operator also combines the result sets of multiple SELECT statements but includes all rows, including duplicates.
Syntax:
```sql SELECT column1, column2 FROM table1 UNION SELECT column1, column2 FROM table2; ```Example:
```sql SELECT name, age FROM employees UNION SELECT name, age FROM contractors; ```In this example, the results of the SELECT statements from the 'employees' and 'contractors' tables will be combined, and duplicate rows will be removed.
2. Common Use Cases
Some common use cases for using UNION and UNION ALL in MySQL include:
- Combining data from multiple tables with similar structures
- Merging results from different queries
- Creating reports that require data from multiple sources
3. Importance in Interviews
Understanding how to use UNION and UNION ALL in MySQL is a valuable skill for database developers and analysts. It demonstrates the ability to manipulate and combine data from multiple sources efficiently, which is a common requirement in many data-related roles.
Conclusion
In conclusion, the UNION and UNION ALL operators in MySQL are powerful tools for combining and merging data from multiple sources. By mastering these operators, you can efficiently work with complex data sets and create insightful reports. Remember to use UNION when you want to remove duplicates and UNION ALL when duplicates are acceptable.
