How to Use "Union" In Oracle Sql?

In Oracle SQL, the "UNION" operator is used to combine the result sets of two or more SELECT statements into a single result set. The UNION operator removes duplicate rows from the combined result set.

To use the UNION operator, you simply need to write the SELECT statements that you want to combine, separated by the UNION keyword. Each SELECT statement must have the same number of columns, and the data types of corresponding columns must be compatible.

For example, if you have two tables, "table1" and "table2", and you want to combine the results of two SELECT statements from these tables, you can write a query like this:

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

This query will return a result set that combines the results of the two SELECT statements, removing any duplicate rows.

It is important to note that the UNION operator automatically removes duplicates from the result set. If you want to include duplicate rows in the result set, you can use the UNION ALL operator instead.

How can I handle NULL values when using UNION in Oracle SQL?

You can handle NULL values when using UNION in Oracle SQL by using COALESCE or NVL functions to replace NULL values with some default value before performing the UNION operation. This can help you avoid errors or unexpected outcomes when combining results from different queries that may contain NULL values.

For example, you can use the COALESCE function like this:

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SELECT column1, column2
FROM table1

UNION

SELECT COALESCE(column3, 'N/A'), column4
FROM table2

In this example, if column3 from table2 contains NULL values, they will be replaced with 'N/A' before the UNION operation is performed. You can customize the default value to suit your specific requirements.

What is the difference between UNION and UNION ALL in Oracle SQL?

UNION and UNION ALL are both used to combine the result sets of two or more SELECT statements in Oracle SQL, but there is a key difference between the two:

1. UNION:

• The UNION operator combines and returns the result sets of two or more SELECT statements without duplicate rows.
• It removes duplicate rows from the combined result set.
• The order of the rows in the final result set may vary, as Oracle may need to perform sorting to remove duplicates.

Example: SELECT column1 FROM table1 UNION SELECT column1 FROM table2;

1. UNION ALL:

• The UNION ALL operator combines and returns the result sets of two or more SELECT statements including duplicates.
• It does not remove duplicate rows from the combined result set.
• Since duplicates are not removed, UNION ALL has better performance compared to UNION.

Example: SELECT column1 FROM table1 UNION ALL SELECT column1 FROM table2;

In summary, use UNION if you want to exclude duplicate rows from the combined result set, and use UNION ALL if you want to include duplicate rows.

What are the advantages of using UNION in Oracle SQL?

1. Combining results from multiple queries: UNION can be used to combine the results of multiple queries into a single result set. This can be useful when you need to aggregate data from different sources or filter results based on different conditions.
2. Simplifying queries: UNION can simplify complex queries by allowing you to break them down into smaller, more manageable parts. This can make queries easier to read and maintain.
3. Avoiding duplicates: UNION automatically removes duplicate rows from the result set, which can help ensure that your data is clean and accurate.
4. Flexibility: UNION allows you to combine data from different tables or even different databases, giving you the flexibility to pull in data from various sources to meet your specific requirements.
5. Performance: When used correctly, UNION can improve query performance by breaking down a complex query into smaller, more optimized parts. This can help reduce the time it takes to retrieve and process the data.

Can I use UNION to perform set operations in Oracle SQL?

Yes, you can use the UNION operator in Oracle SQL to perform set operations. The UNION operator is used to combine the result sets of two or more SELECT statements into a single result set. It is used to retrieve all unique records from multiple queries and remove any duplicates.