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Introduction
SQL, or Structured Query Language, whatever suits your palette, is a powerful programming language used to manage and manipulate relational databases. One of the most important concepts in SQL is joining tables. This allows developers to combine data from multiple tables into a single query result.
In this article, we will explore the difference between inner join and outer join, and we’ll understand it through a table.
What is a Join in SQL?
Read the articleJoins in DBMS as a prerequisite for this article. Here, we will cover it briefly. See, In RDBMS, the end goal is that data should be present only once. Before we explore the differences between inner and outside joins, let's try to understand what a SQL join is. A join is a SQL operation that joins data from two or more tables using a common column or collection of columns.
The output is a single table containing all the data from both tables.
For example, we have two tables: "orders" and "customers". The "orders" table stores information about customer orders, whereas the "customers" table stores information on the customers themselves. We can combine these tables using a join operation to generate a single database containing information about orders and customers.
What is an Inner Join?
An inner join is a type of join that returns only the rows from both tables that have matching attributes in the specified columns. In other words, an inner join only returns the rows with a match between the columns in both tables.
Let's continue with our example of the "orders" and "customers" tables. Inner join on these tables using the "customer_id" column. The resulting output will only contain rows with a match between the "customer_id" column in both tables. Any rows without a match will not be included in the output.
What is an Outer Join?
An outer join returns all rows from one table and the matched rows from the other table. If there is no match, the output will contain NULL values for the columns from the table that did not have a match.
There are three types of outer joins:
Left Outer Join: A left outer join returns all rows from the left table and the rows that match from the right table.
Right Outer Join: A right outer join returns all rows from the right table as well as the rows that match from the left table.
Full Outer Join: In SQL, a full outer join returns all rows from both tables, including those that do not have a match in the other table.
While deciding between an inner join and an outer join, consider factors like the number of tables being joined, the size of the tables, the presence of null values in the data, the intended output of the query, and the operation's performance. When combining two tables and no null values are present, an inner join is used.
An outer join is used when joining several tables, requiring all rows from a single table or incorporating null values in the results. Lastly, the proper type of join is determined by the query's unique requirements and the structure of the tables involved.
Here’s a table to highlight the difference between inner join and outer join:
Factors
Inner Join
Left Outer Join
Right Outer Join
Full Outer Join
Number of tables
Generally used for joining two tables
Used for joining more than two tables
Used for joining more than two tables
Used for joining more than two tables
Size of tables
Works well for small tables
It can be used for large tables
It can be used for large tables
It can be used for large tables
Presence of nulls
Does not include rows with nulls in the result
Includes all rows from the left table
Includes all rows from the right table
Includes all rows from both tables
Desired output
Returns only the matching rows
Includes all rows from the left table
Includes all rows from the right table
Includes all rows from both tables and matching
Performance
Generally faster as it only returns matches
It may be slower due to including all rows from one
It may be slower due to including all rows from one
Outer join can be classified into 3 main categories: Left Outer Join, Right Outer Join, and Full Outer Join. While deciding between them, consider factors like the number and size of the tables, the presence of null values in the data, etc.
Where full outer join is useful?
Full outer join is particularly useful when you want to compare two tables while merging related data, handling missing or incomplete data, also while working with historical data in big data warehouses.
What is the importance of Joins in DBMS?
Joins help eliminate data redundancy, retrieve complex data, Optimise query performance, Simplify data analysis, and join multiple tables also enable data mining which is useful in marketing and finance.
Conclusion
This article covered the difference between inner join and outer join and some prerequisites like the joins in SQL and inner and outer joins briefly.
After reading this blog, you will have an understanding of the difference between inner join and outer join.
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