Mastering SQL: A Complete Guide to Merging Query Results

The SQL UNION operator is a powerful tool that takes two or more SELECT statementsand combines their results into a single, unified result set. When you need to merge data from multiple queries efficiently, you could implement the following syntax:```sql SELECT * FROM table1 UNION SELECT * FROM table2; ```

sql
SELECT * FROM table1
UNION
SELECT * FROM table2;

This query combines and returns the results of both SELECT statements while automatically removing anyduplicate rows from the final output.Important:

Key Requirements for Using the UNION Operator

Each SELECT statement within the UNION operator must satisfy these critical requirements:- Have the same number of columns - Maintain matching data types for corresponding columns - Ensure columns appear in the same order - Follow consistent naming conventions ## Practical Examples using the SQL UNION Operator

• Have the same number of columns

• Maintain matching data types for corresponding columns

• Ensure columns appear in the same order

• Follow consistent naming conventions

Practical Examples of the SQL UNION Operator

Let's explore some real-world examples to demonstrate the UNION operator'sfunctionality. Considerthese two sample tables, users and orders, containing the following data:users name | age ---|--- John | 25 Jane | 30 John | 25 Bob | 20 orders name | order ---|--- John | Book Jane | Pen Bob | Pencil

Users Table

Orders Table

To merge these datasets effectively, you would execute:```sql SELECT name, age, NULL as order FROM users UNION SELECT name, NULL as age, order FROM orders; ```

sql
SELECT name, age, NULL AS order
FROM users
UNION
SELECT name, NULL AS age, order
FROM orders;

Thisoptimized query would generate the following result set:User ordersName | Age | Order ---|---|--- John | 25 | Book Jane | 30 | Pen John | 25 | NULL Bob | 20 | Pencil ## Advanced Tips for Using the SQL UNION Operator

Result Set

Advanced Tips for Using the SQL UNION Operator

While UNION is excellent for combining query results, it's important to understand its variants and alternatives: - Use UNION ALL when you want to keep duplicate rows - Consider INTERSECT for finding common records - Implement EXCEPT for identifying differences between queries - Index the columns involved in UNION operations for better performance - Always test with sample data before running on large datasets

• Use UNION ALL if you want to retain duplicate rows.

• Use INTERSECT to find common records between queries.

• Use EXCEPT to identify differences between queries.

• Index the columns involved in UNION operations to improve performance.

• Always test with sample data before running queries on large datasets.

For specific implementation details, consult your database management system's documentation, as syntax and optimization techniques may vary across different SQL platforms.