How does Oracle SQL differ from ANSI SQL in terms of syntax and functionality?

There are key differences in syntax and functionality between Oracle SQL and ANSI SQL, which are mainly reflected in the use of ||, external connections using ( ) operators, DECODE functions instead of CASE expressions, and unique date and time functions. Specifically, it includes: 1. Oracle uses || for string splicing, which complies with the ANSI standard but is different from MySQL or SQL Server; 2. Oracle traditionally uses the ( ) operator to implement external connections, while ANSI uses LEFT/RIGHT/FULL JOIN; 3. Oracle's DECODE function provides functions similar to simple CASE but has different structures; 4. Oracle provides proprietary date functions such as SYSDATE, ADD_MONTHS, etc. rather than relying entirely on ANSI standard functions. Mastering these differences can help across database development and maintenance.

Oracle SQL and ANSI SQL share a lot of similarities, but they also have key differences in syntax and functionality that can trip up developers, especially when moving between databases or writing portable code. If you're used to Oracle's way of doing things, some ANSI SQL constructs might feel unfamiliar — and vice versa.

String Concatenation Uses || in Oracle

In Oracle SQL, concatenating strings is done using the double pipe symbol ( || ). This is different from some other databases like MySQL, which uses the CONCAT() function, or SQL Server, which uses the plus sign ( ). ANSI SQL, however, does include the || operator as part of its standard, so Oracle is actually aligned with the standard here.

Example:

 SELECT first_name || ' ' || last_name AS full_name FROM employees;

This may seem minor, but if you're writing SQL that needs to work across multiple platforms without rewriting, this difference matters. You might need to use CONCAT() or adjust your approach depending on the database engine.

Oracle Uses ( ) for Outer Joins

One of the more noticeable syntax differences is how Oracle handles outer joins. While ANSI SQL uses LEFT JOIN , RIGHT JOIN , and FULL JOIN keywords, Oracle traditionally used the ( ) operator placed on the side of the table that might not have matching rows.

Example (Oracle syntax):

 SELECT * FROM employees e, departments d
WHERE e.department_id = d.department_id( );

This is equivalent to a LEFT JOIN in ANSI SQL:

 SELECT * FROM employees e LEFT JOIN departments d
ON e.department_id = d.department_id;

While newer versions of Oracle support ANSI join syntax, you'll still see legacy code using the ( ) operator. It's important to recognize both styles, especially when maintaining older systems.

Handling NULL Values in DECODE vs CASE

Oracle provides the DECODE function, which is unique to Oracle and not part of ANSI SQL. It works similarly to a simple CASE expression but has a different structure.

Example using DECODE :

 SELECT DECODE(status, 'A', 'Active', 'I', 'Inactive', 'Unknown') AS status_desc FROM users;

The equivalent ANSI SQL would use a CASE statement:

 SELECT CASE status
         WHEN 'A' THEN 'Active'
         WHEN 'I' THEN 'Inactive'
         ELSE 'Unknown'
       END AS status_desc
FROM users;

Although CASE is more flexible and portable, DECODE is still widely used in Oracle environments due to its brevity for simple conditions checks.

Oracle Has Unique Date and Timestamp Functions

Oracle offers several built-in date functions that different from ANSI SQL standards. For example:

• SYSDATE gives the current date and time (similar to CURRENT_DATE or NOW() in other databases).
• ADD_MONTHS(date, n) adds a number of months to a date.
• NEXT_DAY(date, 'DAY') finds the next occurrence of a specific day.

ANSI SQL tends to rely on standardized functions like CURRENT_DATE , EXTRACT() , and vendor-neutral date math, while Oracle often provides its own flavor.

If you're writing cross-database queries, you'll want to stick with ANSI functions or wrap Oracle-specific ones in compatibility layers.

There are a few other smaller differences too — like Oracle's use of the DUAL table for expressions that don't reference actual tables, or its handling of subqueries in the FROM clause without requiring an alias (which ANSI SQL typically does).

Overall, Oracle SQL supports most ANSI SQL features but adds its own twists in syntax and built-in functions. Once you're aware of these quirks, switching between them becomes much smoother.

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