Left vs Right Join: Difference and Comparison

The join generates a table that may be used alone or combined with one another. It is thus recommended to comprehend these joins and the distinctions between them to determine the specific join as well as whether right or left should be chosen in unique cases.

Key Takeaways

1. A left join includes all records from the left table and matched records from the right table, filling in NULL values for unmatched rows.
2. A right join includes all records from the right table and matched records from the left table, filling in NULL values for unmatched rows.
3. Left and right joins are mirrored images of each other and can be interchangeable depending on the table order in the query.

Left Join vs Right Join

A left join returns all the rows from the left table and matching rows from the right table, and if there are no matches in the right table, the result will contain null values. A right join, same with left join, but it returns all the rows from the right table and matching rows from the left table.

Even though there is no matching on the right side of the table, the SQL LEFT process incoming all entries from the left table. This implies that when the ON clause contains 0 (zero) entries on the right side of the table, the join would still return a record with NULL within every field from the main table.

The right join, on the other hand, finds that even in the left table, the SQL RIGHT receives and sends all entries from the right table.

This implies that a right join delivers all of the data from the right table, as well as any accepted values from the tables, or NULL if no fitting join predicate exists for this operation.

Comparison Table

Parameters of Comparison	Left Join	Right Join
Usage	It connects multiple tables and provides all records from the left table as well as matching rows as from right.	It joins multiple tables and delivers all data from the right table as well as matching rows first from the left table.
Query Execution	During the implementation of the query including the left join, all the entries of the tables are received first, and then the matching data of the right table column values are obtained using stacked loops, and the NULL value is replaced for other unmatched data.	During the implementation of the query including the right join, all data of the right table is obtained first, and afterwards the matched records of the left table column values are collected using stacked loops.
Result Set	Result set value will be NULL or zero if no matching data is found in the right table.	Result set value will be NULL or zero if no matching data is found in the left table.
Synonymous Term	LEFT OUTER JOIN	RIGHT OUTER JOIN
Syntax	SELECT column_name(s) FROM table1 LEFT JOIN table2 ON table1.column_name = table2.column_name;	SELECT column_name(s) FROM table1 RIGHT JOIN table2 ON table1.column_name = table2.column_name;

What is Left Join?

To merge data from two databases depending on a column containing matching values, we use JOIN in SQL. For instance, if you’ve had a table containing client data (client ID, first surname, identifier, etc.) and then another table containing order data for a particular time (transaction ID, time, client ID, product ID, etc.)

Let’s look at the syntax of the Left Join to understand its utility properly:

SELECT column_name(s)

FROM table1

LEFT JOIN table2

ON table1.column_name = table2.column_name

1. ‘SELECT’ – Begin by selecting the categories that we wish to view in the dataset.
2. ‘FROM’ – Enter the data of the tables, wherein we wish to maintain all of the information.
3. ‘LEFT JOIN’ – Enter the data of the second table that is mentioned.
4. ‘ON’ – Use this phrase to specify which fields will be utilized to connect the tables.

If there are multiple finds which are matching, the column in the left table is duplicated to include all data directly from the right table. If no match is found, it preserves the row as from the left table and then inserts NULL into the relevant columns of the right table.

What is Right Join?

To put it bluntly, the right join operation is very similar to the left join, just the opposite.

Right joins essentially are very similar to left joins in that they retrieve all rows out from the table specified in the RIGHT JOIN phrase but only corresponding rows from the table specified in the FROM clause.

The RIGHT JOIN is infrequently used since the contents of a RIGHT JOIN may be obtained by simply swapping the labels of the two connected tables in a LEFT JOIN.

It’s worth mentioning that LEFT JOIN and RIGHT JOIN may also be expressed as LEFT OUTER JOIN as well as RIGHT OUTER JOIN.

The syntax for Right Join is described further below. The operations of the right and left joins are identical, but in reverse order:

SELECT column_name(s)
FROM table1
RIGHT JOIN table2
ON table1.column_name = table2.column_name

Main Differences Between Left Join and Right Join

1. The left join phrase is more commonly used over the right join phrase.
2. The formula for left join is all the matching records + unmatched records of the left table whereas the formula for right join is all the matching records + unmatched records of the right table.

References

1. https://www.tutorialspoint.com/sql/sql-left-joins.htm#:~:text=The%20SQL%20LEFT%20JOIN%20returns,matches%20in%20the%20right%20table.&text=This%20means%20that%20a%20left,of%20no%20matching%20join%20predicate.
2. https://mode.com/sql-tutorial/sql-right-join/#:~:text=Right%20joins%20are%20similar%20to,names%20in%20a%20LEFT%20JOIN%20.

Last Updated : 11 June, 2023