How Does Oracle's ( ) Operator Create Outer Joins, and Why Should You Avoid It?

Oracle's ( ) Operator: A Legacy Approach to Outer Joins

Older SQL code often uses Oracle's ( ) operator, a syntax unfamiliar to many developers. This operator was vital for creating OUTER JOINs before the ANSI-89 standard, which lacked explicit OUTER JOIN support.

The ( ) operator essentially creates a LEFT OUTER JOIN. It allows rows from one table (let's say table 'b') to join with rows from another table ('a') based on a specified ID field. If a row in 'b' doesn't have a matching row in 'a', the corresponding columns from 'b' will contain NULL values in the result set.

Removing the ( ) would change the join to an INNER JOIN, returning only rows with matching IDs in both tables. Thus, the ( ) is crucial when you need all rows from one table, accepting potential NULLs for unmatched rows from the other.

Despite its functionality, Oracle strongly discourages using the ( ) operator. It's non-standard (not ANSI compliant), hindering portability to other database systems. Furthermore, it's subject to unique rules and limitations not found in OUTER JOINs created using standard FROM clause syntax.

For better compatibility, performance, and adherence to modern best practices, Oracle recommends this standard OUTER JOIN syntax:

SELECT ...
FROM a
LEFT JOIN b ON b.id = a.id

This achieves the same outcome as the ( ) operator while leveraging the benefits of ANSI-92 syntax and Oracle's optimized query execution plans.

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