Understanding the Similarities Between Left Join and Left Outer Join in SQL

In SQL, the terms ldquo;left joinrdquo; and ldquo;left outer joinrdquo; refer to the same operation. Both terms describe a type of join that returns all records from the left table (the first table listed) and the matched records from the right table (the second table listed). If there is no match, the result will contain NULL values for columns from the right table. This article dives into the nuances of these terms, providing clear insights into how they function and when to use one over the other.

Key Points:

Left Join: This is a shorthand for left outer join. It retrieves all records from the left table and the matched records from the right table. Left Outer Join: This is the full term explicitly stating the type of join being performed.

Example: A Comprehensive Illustration

Consider you have two tables, A and B, structured as follows:

Table A Table B ID Name ID Age ------ ----- ------ ----- 1 Alice 1 25 2 Bob 2 30 3 Charlie

A left join or left outer join query might look like this:

SELECT * FROM ALEFT JOIN B ON   

The resulting output will be:

ID Name Age 1 Alice 25 2 Bob 30 3 Charlie

In this result, Charlie from Table A has no corresponding record in Table B, so the Age column returns NULL.

No Difference: A Closer Look

The terms ldquo;left joinrdquo; and ldquo;left outer joinrdquo; are essentially the same. The reason it is called an ldquo;outer joinrdquo; as opposed to an ldquo;inner joinrdquo; is that an inner join only returns matched records and excludes all unmatched records from both the left and the right.

An inner join can neither have a left or right side, whereas an outer join returns either the matched items, the matched record section of the join will be the same for any type of join. Furthermore, an outer join can return unmatched items from the left side (left outer join) or the right side (right outer join). In the case where the left unmatched and the right unmatched are returned with the matched records, it's called a full outer join or just a full join.

In the case of a full join, the result set will have a uniform matched set, and all unmatched records will appear to be randomly staggered depending on the order if no 'order by' is included in the query. The nature of this result is such that where records are unmatched, it appears to have matched with rows where all the columns are null. This is the only way to isolate the subsets of unmatched records in the left, right, or both sides for a full join.

Using left join, right join, and full join, as opposed to the inclusion of the 'outer' keyword, is both verbally and syntactically acceptable in the context of SQL and RDBMS.