Inner Join Vs Outer Join: Examples with SQL Queries

Inner Join Vs Outer Join: Examples with SQL Queries
7 min read

Before we discuss the differences between inner join of outer join in SQL queries, it is important to know what these actually are. Basically, a join is a method of combining two or more tables into a single one. A joined table is a special type of relation that contains one or more primary keys. These keys can be either primary keys or column aliases. Inner join vs outer joins is a bit confusing question as DBMS Vs RDBMS, but I will provide all the details with the help of diagrams, tables, and queries.

An inner join is generally used to join multiple rows of two different tables together with a common key between them, with no explicit or implicit columns. When there’s a matching key between two tables, where the inner join joins the two tables by inserting the key value as an extra into each table, it is known as an outer join. It can be more than just that, two or more tables or several more. If the size of the database is significant, using a single-table outer join can make the performance of the queries much better.

Why do We need SQL Joins?

In order to understand, let’s consider an example of two database tables having the price and quantity of the product given above. So, if you want to combine or link both tables for valuable information, then what would you do? First, you will figure out the common things between these tables, which is the product name, and you will get the following result after applying the SQL joins.

You can easily combine multiple tables having common data as you have just seen the results of database tables after joining.

SQL Joins: What are the key differences?

We can better understand SQL inner join vs Outer join with the help of informative diagrams. Inner joins mean getting the common fields or entries of multiple database tables into a single table. However, Outer join focuses on the dissimilar as well as common data from tables. There are four types of SQL joins; inner, left, right, and full outer joins that are used in any situation.


As I have discussed earlier, SQL inner join is used to get common or matching rows from multiple database tables. Let’s understand the example of Table 1 and Table 2 given above; there are two things (Kiwis and Onions) that are common for both tables in the product name. Keep in mind, Tomatoes and Broccoli can’t be included in the Inner join table because we are using inner join that returns common or matching values.

Inner Join SQL Query Example:

 SELECT Table1.*, Table2.Quantity
 FROM Table1
 ON Table1.Product = Table2.Product 


As you can view in the example given above, the left outer join returns data from the left table and only corresponding data from the right table as an output. In order to simplify, I have highlighted the parts of Table 1 and Table 2 that are included in the new table.

Left Outer Join SQL Query Example:

SELECT Table1.*, Table2.Quantity
FROM Table1
ON Table1.Product = Table2.Product 


Right outer join is used to show all the data from the right table even there is no matching or common data from the left table. You can view the highlighted parts in Table 1 and Table 2 that are included in the right join.

Right Outer Join SQL Query Example:

 SELECT Table1.*, Table2.Quantity
 FROM Table1
 ON Table1.Product = Table2.Product 


As the name indicates, full outer join returns all data from both database tables, and it looks like we are using both Left and Right outer joins at the same time. In the case of our example, both tables were not the same length tables, so you can find two nulls which are for Broccoli and Tomatoes, in the result table.

Full Outer Join SQL Query Example:

 SELECT Table1.*, Table2.Quantity
 FROM Table1
 ON Table1.Product = Table2.Product  

Inner Join Vs Outer Join: Performance & Reliability

Now, when it comes to the performance of a two-table inner join versus an outer join, a query on a single database can take two to three times longer if there are many more tables involved. If the database consists of twenty tables, it can take forty-three seconds for a query to return the first row after a join operation and another fifty seconds for the last row if the join did not fail. This can add up quickly, especially when there are more joins. This is also because two tables must be joined in order to get the information about the records within them.

Let’s look at how the inner join compares with the right to join in terms of reliability. A well-written inner join can efficiently return the required information even for complex joins involving hundreds of tables. It is a fine tradeoff. For instance, if one knows that one table contains data that does not belong to any other tables, he/she can use the outer join to access the secondary data without any problem.

However, if the database has a lot of data that is not related to any of the tables, the inner join will be slower than the natural join for accessing those tables. The inner join first checks if all the relevant data is available in the primary tables and then uses the secondary data only when the primary one is inaccessible or too much data is needed from another source. This process is called searching for matching tuples. So, the inner join can access the secondary data faster than the natural join would. But remember that more data will mean larger searching tuples, and the slower the processing speed becomes, the larger the amount of information that needs to be accessed. It is the main reason why people need high speed Dedicated SEO Servers for speed up their website data read and write time.

Inner Join Vs Outer Join: Comparison Table

Inner JoinOuter Join
Only matching records or values from both the database tables are taken where there is a common ID.All the records from the database tables are taken, and a common ID is not required at all.
All rows will have any value because these are taken from the table with the same entries.In outer join, some entries may return a null value.
There are no alternatives or variants of inner join.Outer join can be left, right, or full join.
The optimizer can be used as it gives many valuable options.Optimizer use in the outer join is very limited because the joins take all the entries, and its usage is not practicable.
The results provided by the inner join have the least entries and will be able to update the user requirements.As you know, outer join has different options such as left, right, and full join, so its usage must be monitored and checked thoroughly.
You can observe the lack of performance because SQL inner join is slower.Outer joins especially left outer joins, are faster and better performance in most cases.
The satisfaction of the inner join condition is mandatory.There are no conditions that we have to meet in the outer join query necessarily.
There is no output of those entries having no matching with the entries of another table.If entries of one table do not have any matching with another table, then you will get null values.
It is a straightforward process in which the matching values should not be considered for left or right tables.In outer join, when the entries match, it will check either condition is given for left or right tables, and entry is considered accordingly except full outer join.
If a related data entry is needed as per the customer requirement or database design, then the inner join is recommended.If a related data entry is not required as per the customer requirement or database design, then the outer join is more suitable.

Final Words

In addition, both inner and outer joins can take longer to finish when there are more joins, but the performance level can vary depending on the size of the database. Also, both inner and outer joins can incur temporary data costs, but the former is more expensive because it creates more opportunities for data loss. An outer join can result in a temporary decrease in database traffic, but it will quickly regain its position once the data has been restored. However, the inner join will remain open even after a certain amount of time due to constant partial table joins. Outer joins also slow down the application performance because of the constant application reads.

So which of these two approaches is better? To find out, you will need to consider the performance characteristics of your database. Remember that both of these types of joins can improve database performance, but the inner one will perform better when the number of tuples is large, and the database is designed to support them. If you have a two-tier architecture or you have linked database features, the two-table join will work better for you.

One thought on

Inner Join Vs Outer Join: Examples with SQL Queries

  • altaf

    thanks for this great amount of information

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