What is a Full Outer Join In SQL?

What Is A Full Outer Join In SQL?

A Full Outer Join in SQL is like combining two lists and ensuring you see everything from both, even if something doesn't match exactly. You get all the items from both lists, and when something's missing in one, it shows up as a space.

In other words, it includes all records from both the left and right tables, and where there is no match, the columns from the side without a matching row will contain NULL values. This join type is useful when you want to retrieve all records from both tables, regardless of whether there is a match or not.

Syntax

The syntax typically involves specifying the tables to be joined and the joining condition using the ON keyword. For example:

SELECT * 

FROM table1

FULL OUTER JOIN table2 ON table1.column = table2.column;

Why Use Full Outer Join In SQL?

A Full Outer Join in SQL is employed when you want to retrieve all records from two tables, regardless of whether there is a match between the joined columns. This type of join is useful in various scenarios:

Inclusive Data Retrieval:

• When you want to include all rows from both tables in the result set.
• It ensures that no data is excluded, providing a comprehensive view of the data from both tables.

Handling Incomplete Data:

• In situations where the two tables may not have a perfect one-to-one match, a Full Outer Join allows you to capture all records and identify where matches occur while accommodating cases where no match is found.

Merging Datasets:

• When working with data from different sources or systems, a Full Outer Join can be useful for merging datasets with potentially incomplete or unmatched records.

Analyzing Data Discrepancies:

• It aids in identifying discrepancies or missing information in the data by highlighting instances where records in one table do not have corresponding matches in the other table.

Handling NULL Values:

• Full Outer Join returns NULL values for columns where there is no match in either the left or right table. This can be valuable when dealing with data where certain attributes may not be present in every record.

Comparing Data Sets:

• When comparing two datasets, a Full Outer Join helps in understanding the differences between them by presenting both matching and non-matching records.

Complex Reporting Scenarios:

• In complex reporting scenarios where a comprehensive set of data is needed, a Full Outer Join allows you to bring together information from multiple tables without excluding any records.

Full Outer Join Example 

Example 1: Employees And Departments

Suppose you have two tables, Employees and Departments. You want to retrieve a list of all employees and their corresponding departments, including those not currently assigned to any department and departments with no assigned employees.

This query will return all records from both the Employees and Departments tables, matching them where possible and including NULL values where there is no match.

Example 2: Sales Data

Consider two tables, Sales2019 and Sales2020, containing sales data for different years. You want to compare the sales for each product across both years, including products that were sold in only one of the years.

This query will provide a comprehensive view of sales data for each product, including matches where products were sold in both years and NULL values for products that were only sold in one of the years.

Conclusion

A Full Outer Join in SQL is a versatile method that ensures inclusivity in data retrieval by combining records from two tables, both matching and non-matching, facilitating comprehensive data analysis.

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FAQs

1. When Should I Use A Full Outer Join In SQL?

Use a Full Outer Join when you want to retrieve all records from two tables, including matching and non-matching rows. 

2. Can I Use Full Outer Join With More Than Two Tables?

In standard SQL, directly performing a Full Outer Join with more than two tables is not supported. However, you can achieve this by chaining multiple Full Outer Joins, combining pairs of tables at a time.

3. What's The Difference Between Full Outer Join And Inner Join?

An Inner Join returns only the matching rows between two tables based on the specified condition, excluding non-matching rows. In contrast, a Full Outer Join returns all rows from both tables, filling in NULL values for columns where there is no match.

4. Are Full Outer Joins Efficient For Large Datasets?

Full Outer Joins can be less efficient for large datasets than Inner Joins, as they combine all records from both tables. 

5. Can I Perform Calculations After A Full Outer Join?

Yes, you can perform calculations after a Full Outer Join. Once the Full Outer Join is executed and the result set is obtained, you can use the SELECT statement to perform calculations, aggregate functions, or any other operations on the combined data.