Outer joins are an important concept to understand when it comes to the field of relational databases and SQL. An outer join is used to combine two tables, even when one or more values in one of the tables has no corresponding value in the other table. This type of join is used to complete the information between two tables and fill any gaps that may be present.
In order to better understand outer joins, it’s useful to familiarize yourself with the two basic types of joins: the inner join and the outer join. An inner join combines two tables based on a shared value or set of values. This type of join will only return records from both tables where there is a match. Conversely, an outer join combines two tables, even if there is no matching value between them.
When it comes to outer joins in MySQL, there are three types available to use: left outer join, right outer join, and full outer join. The name of the join type represents which of two tables the join applies to. A left outer join will only include records from the “left” table, even if there is no matching record in the “right” table. A right outer join operates in reverse, including all records from the “right” table, even if there is no match in the “left” table. A full outer join combines both sets, including all records from both tables, even if there is no matching record in either table.
To use an outer join in MySQL, you can use the basic SELECT statement syntax, but with the addition of the OUTER JOIN keyword. For example, the following query would be a left outer join, returning all records from the left table (table 1) and any matching records from the right table (table 2):
SELECT *
FROM table1
LEFT OUTER JOIN table2
ON table1.ID = table2.ID;
A right outer join would be the same, except it would return all records from the right table and any matching records from the left table:
SELECT *
FROM table1
RIGHT OUTER JOIN table2
ON table1.ID = table2.ID;
Finally, a full outer join combines both sets and returns all records from both tables, whether or not there is a match in either table:
SELECT *
FROM table1
FULL OUTER JOIN table2
ON table1.ID = table2.ID;
Using outer joins can be a useful way to complete and fill in potential data gaps between two tables in a MySQL database. They are an important tool to know when working with relational databases and SQL.
