Database: UNION and UNION ALL

When working with databases, the UNION and UNION ALL operators are commonly used to combine the results of two or more SELECT statements. In this blog post, we will explore the differences between UNION and UNION ALL, their use cases, practical applications, and their importance in interviews.

UNION Operator

The UNION operator is used to combine the result sets of two or more SELECT statements, removing any duplicate rows from the final result set. Here's an example:

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

In this example, the UNION operator will combine the results of the two SELECT statements from table1 and table2, removing any duplicate rows from the final result.

Example:

```sql SELECT employee_id, employee_name FROM employees UNION SELECT employee_id, employee_name FROM contractors; ```

In this example, the UNION operator will combine the results of the two SELECT statements from the employees and contractors tables, removing any duplicate rows based on the employee_id and employee_name columns.

UNION ALL Operator

The UNION ALL operator is similar to the UNION operator, but it does not remove duplicate rows from the final result set. Here's an example:

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

In this example, the UNION ALL operator will combine the results of the two SELECT statements from table1 and table2, including any duplicate rows in the final result set.

Example:

```sql SELECT product_id, product_name FROM products UNION ALL SELECT product_id, product_name FROM archived_products; ```

In this example, the UNION ALL operator will combine the results of the two SELECT statements from the products and archived_products tables, including any duplicate rows in the final result set.

Common Use Cases

The UNION and UNION ALL operators are commonly used in various scenarios, such as:

• Combining data from multiple tables with similar structures
• Merging results from different queries into a single result set
• Generating reports that require data from multiple sources

Importance in Interviews

Understanding the differences between the UNION and UNION ALL operators is essential for database developers and analysts, as they are frequently asked about these operators in interviews. Employers often use these questions to test a candidate's understanding of SQL fundamentals and their ability to write efficient queries.

Conclusion

In conclusion, the UNION and UNION ALL operators are powerful tools for combining data from multiple sources in a database. By understanding the differences between these operators and their common use cases, you can improve your SQL skills and tackle interview questions with confidence.

Tags:

Database, SQL, UNION, UNION ALL, Interview Questions