How to Fix Oracle Syntax Errors in PostgreSQL

How do I fix Oracle syntax errors when migrating SQL to PostgreSQL?

An Oracle syntax error appears when PostgreSQL encounters Oracle-specific SQL that does not follow PostgreSQL grammar.

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Description

What triggers an Oracle syntax error in PostgreSQL?

PostgreSQL raises the error when it meets Oracle-only keywords (e.g., DUAL), functions (NVL), or out-of-order clauses that its parser cannot interpret.

How do I identify the incompatible clause quickly?

Read the error text for the unexpected keyword and check the line number. Comment out suspect clauses, run again, and narrow the failure to the exact token.

Which Oracle constructs fail and what are the PostgreSQL equivalents?

DUAL table: remove it.
SYSDATE: replace with CURRENT_TIMESTAMP.
NVL: change to COALESCE.
CONNECT BY: rewrite as a recursive CTE.

How can I rewrite an Oracle query on ecommerce data?

Replace Oracle-only features with portable SQL. Example below converts an NVL and DUAL based query to valid PostgreSQL.

Original Oracle query

SELECT NVL(SUM(total_amount), 0) AS total FROM Orders, DUAL WHERE order_date > SYSDATE - 30;

PostgreSQL rewrite

SELECT COALESCE(SUM(total_amount), 0) AS total FROM Orders WHERE order_date > CURRENT_DATE - INTERVAL '30 days';

Best practices for avoiding Oracle syntax errors

Adopt ANSI-standard SQL, enable standard_conforming_strings, and lint code with tools like pg_verify before running migrations.

How do I automate massive script fixes?

Use Orafce for Oracle functions, or run find-and-replace scripts in CI to swap keywords across files.

Why How to Fix Oracle Syntax Errors in PostgreSQL is important

How to Fix Oracle Syntax Errors in PostgreSQL Example Usage


-- Oracle version (fails)
SELECT c.name,
       NVL(o.total_amount, 0) AS last_order
FROM Customers c
LEFT JOIN (
  SELECT customer_id, MAX(total_amount) AS total_amount
  FROM Orders
  GROUP BY customer_id
) o ON o.customer_id = c.id
WHERE ROWNUM <= 10;

-- PostgreSQL fix
SELECT c.name,
       COALESCE(o.total_amount, 0) AS last_order
FROM Customers c
LEFT JOIN (
  SELECT customer_id, MAX(total_amount) AS total_amount
  FROM Orders
  GROUP BY customer_id
) o ON o.customer_id = c.id
ORDER BY last_order DESC
LIMIT 10;

How to Fix Oracle Syntax Errors in PostgreSQL Syntax


-- Common Oracle-to-PostgreSQL rewrites
-- 1. NVL ➜ COALESCE
SELECT COALESCE(SUM(total_amount), 0) AS total_sum
FROM Orders;

-- 2. DUAL ➜ remove
SELECT 1;

-- 3. SYSDATE ➜ CURRENT_TIMESTAMP
SELECT CURRENT_TIMESTAMP;

-- 4. ROWNUM filtering ➜ LIMIT/OFFSET
SELECT * FROM Customers ORDER BY created_at LIMIT 10;

-- 5. CONNECT BY ➜ Recursive CTE
WITH RECURSIVE order_chain AS (
  SELECT id, customer_id, order_date, 1 AS lvl
  FROM Orders WHERE id = 1
  UNION ALL
  SELECT o.id, o.customer_id, o.order_date, oc.lvl + 1
  FROM Orders o
  JOIN order_chain oc ON o.customer_id = oc.customer_id
)
SELECT * FROM order_chain;

Common Mistakes

Using the DUAL table. Why wrong: PostgreSQL does not need a dummy table for single-row selects. Fix: Drop the FROM DUAL clause entirely.
Leaving NVL, SYSDATE, or ROWNUM unchanged. Why wrong: These functions and pseudo-columns are Oracle-only. Fix: Replace with COALESCE, CURRENT_TIMESTAMP, and LIMIT.