How Do Oracle's ' ' Operator and WHERE Clause Create Outer Joins?

Outer Joins in Oracle WHERE Clauses: The " " Operator

In Oracle databases, the " " operator can be used within a WHERE clause to specify an outer join between two tables. Outer joins allow the retrieval of records that match in both tables, as well as those that exist in only one of the tables.

Syntax

The syntax for using the " " operator in a WHERE clause is as follows:

WHERE Table1.Column = Table2.Column(+)

Functionality

When the " " operator appears on the left side of the "=" sign, it indicates a LEFT OUTER JOIN. This means that all records from Table1 will be returned, regardless of whether or not they have matching records in Table2. Any unmatched records in Table2 will be returned with null values for the joined columns.

Conversely, when the " " operator appears on the right side of the "=" sign, it indicates a RIGHT OUTER JOIN. In this case, all records from Table2 will be returned, even if they don't have matching records in Table1. Matched records from Table1 will be included in the results, while unmatched records will have null values.

Example

Consider the following query:

SELECT
    Table1.Category1,
    Table1.Category2,
    count(*) as Total,
    count(Tab2.Stat) AS Stat
FROM Table1, Table2
WHERE (Table1.PrimaryKey = Table2.ForeignKey(+))
GROUP BY Table1.Category1, Table1.Category2

This query performs a LEFT OUTER JOIN between Table1 and Table2 based on the PrimaryKey and ForeignKey columns. All records from Table1 will be included in the results, even if they don't have a matching record in Table2. For unmatched records in Table2, the Stat column will be null.

Recommendation

While the " " operator can be used for creating outer joins in Oracle, it is generally considered outdated and not as readable as modern join syntax. It is recommended to use LEFT JOIN or RIGHT JOIN instead, as they provide more explicit and intuitive join semantics.

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