Inner Join vs. Outer Join: What's the Difference?
Edited by Aimie Carlson || By Janet White || Published on February 17, 2024
Inner join returns rows with matching values in both tables, while outer join returns all rows from one table and matched rows from the other.
Key Differences
An inner join in SQL combines rows from two or more tables based on a related column between them, returning rows where the join condition is met. Conversely, an outer join returns all rows from one table and the matched rows from the other table(s), which can be categorized as left, right, or full outer join depending on the inclusion of rows from the left, right, or both tables, respectively.
Inner joins are used when the requirement is to retrieve only the matching data from the tables; it excludes rows where there is no match. On the other hand, an outer join is used when it's necessary to include all rows from one or both tables, regardless of whether there's a match, filling in with NULLs where data is absent.
The inner join results in a set that includes rows that have matching values in both tables. This type of join is often used for extracting intersecting data. In contrast, an outer join extends the result of the inner join by including non-matching rows, thus providing a more comprehensive dataset that includes unmatched as well as matched rows.
Inner joins typically result in a smaller result set since they exclude non-matching rows. In contrast, outer joins can result in a larger result set as they include all rows from one or both tables, which is useful for finding unmatched records along with the matched ones.
The choice between inner and outer joins depends on the specific requirements of the query. An inner join is used for strict matching criteria, while an outer join is suitable for cases where retaining all rows from one or both tables is important, even if they don't have corresponding matches in the other table.
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Comparison Chart
Matching Criteria
Returns rows with matching values in both tables
Returns all rows from one table and matched rows from the other
Result Set Size
Typically smaller, as it only includes matching rows
Potentially larger, includes both matching and non-matching rows
Use Case
Used when only intersecting data is needed
Used when all data from one or both tables is needed
Null Handling
Excludes rows with null matches
Includes rows with null matches, filling them with NULLs
Types
Only one type
Three types: Left, Right, Full Outer Join
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Inner Join and Outer Join Definitions
Inner Join
Inner join combines rows from multiple tables based on a related column, showing only matching rows.
SELECT * FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID;
Outer Join
Outer join is useful for finding unmatched records in one or both tables.
SELECT Customers.Name, Orders.OrderID FROM Customers FULL OUTER JOIN Orders ON Customers.CustomerID = Orders.CustomerID;
Inner Join
Inner join filters out rows that do not have matching values in both tables.
SELECT * FROM Products INNER JOIN Orders ON Products.ProductID = Orders.ProductID;
Outer Join
Outer join fills missing matches with NULL values in the result set.
SELECT Employees.Name, Departments.Name FROM Employees LEFT OUTER JOIN Departments ON Employees.DeptID = Departments.DeptID;
Inner Join
Inner join returns a result set with rows that have common values in both joined tables.
SELECT * FROM Employees INNER JOIN Departments ON Employees.DeptID = Departments.DeptID;
Outer Join
Outer join can be left, right, or full, depending on which table's rows are to be included.
SELECT * FROM Orders RIGHT OUTER JOIN Customers ON Orders.CustomerID = Customers.CustomerID;
Inner Join
Inner join creates a new result table by combining column values of two tables.
SELECT Customer.Name, Order.Date FROM Customer INNER JOIN Order ON Customer.ID = Order.CustomerID;
Outer Join
Outer join ensures that all rows from at least one of the joined tables are returned.
SELECT * FROM Products FULL OUTER JOIN Orders ON Products.ProductID = Orders.ProductID;
Inner Join
Inner join is used to retrieve intersecting data from related tables.
SELECT Employees.Name, Departments.Name FROM Employees INNER JOIN Departments ON Employees.DeptID = Departments.DeptID;
Outer Join
Outer join combines rows from tables even if there are no matching values in one of them.
SELECT * FROM Employees LEFT OUTER JOIN Departments ON Employees.DeptID = Departments.DeptID;
FAQs
Is outer join useful for finding missing data?
Yes, outer joins help identify unmatched data in one or both tables.
What is an outer join?
An outer join is a SQL operation that returns all rows from one or both tables, matched or unmatched.
When should you use an inner join?
Use an inner join when you need to retrieve rows that have matching values in both joined tables.
What's a practical use of an inner join?
An inner join is used to combine related data from different tables, like orders and customers.
What is an inner join?
An inner join is a SQL operation that returns rows when there's a match in both joined tables.
What are the types of outer joins?
There are three types: Left Outer Join, Right Outer Join, and Full Outer Join.
Can inner join result in missing data?
Yes, if there are no matches in the joined columns, inner join can exclude relevant data.
Does an inner join include NULL values?
No, inner joins exclude rows with NULL values in the matched columns.
How do inner joins affect query performance?
Inner joins can be more performant as they return a smaller result set.
Do outer joins impact database performance?
Outer joins can be less performant as they potentially return larger result sets.
Can an outer join create duplicate rows?
Yes, especially in full outer joins, duplicate rows can occur.
Is SQL knowledge required to understand joins?
Basic SQL knowledge is helpful for understanding and using joins effectively.
How does an outer join handle NULL values?
An outer join includes NULL values for unmatched columns in the result set.
What's a key difference between inner and outer join?
Inner join returns only matching rows, while outer join includes non-matching rows as well.
When is a left outer join used?
A left outer join is used when you need all rows from the left table, matched or not.
Can an outer join be used to combine three tables?
Yes, outer joins can combine multiple tables, though the logic can get complex.
What is a natural join?
A natural join is a type of inner join where the join is made on all columns with the same name.
Are inner joins reversible?
Yes, swapping tables in an inner join doesn't change the result.
What happens if join conditions are omitted in an inner join?
Omitting join conditions in an inner join may result in a Cartesian product, creating a large result set.
How do you choose between inner and outer join?
The choice depends on whether you need all records from one or both tables (outer) or only matching records (inner).
About Author
Written by
Janet WhiteJanet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Aimie CarlsonAimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.
