How to Fix Common Errors in PostgreSQL

How do I quickly fix common PostgreSQL error messages?

Identifies typical PostgreSQL error messages, explains why they occur, and shows quick fixes.

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Description

What are the most frequent PostgreSQL errors?

Most developers hit syntax error at or near, duplicate key value violates unique constraint, null value in column violates not-null constraint, and relation does not exist. Understanding the cause lets you correct queries fast.

How do I read a “syntax error at or near” message?

PostgreSQL pinpoints the token that broke parsing. Check spelling, misplaced commas, missing parentheses, or using a keyword as an identifier.Reformatting the statement often reveals the problem spot.

Example fix

SELECT id name FROM Customers; fails. Add the missing comma: SELECT id, name FROM Customers;.

Why do unique-constraint errors occur?

When an INSERT or UPDATE tries to write a duplicate value into a column or index marked UNIQUE, PostgreSQL aborts the statement. Ensure incoming data is clean or use ON CONFLICT to handle duplicates.

Example fix

INSERT INTO Customers(id,name,email) VALUES(1,'Ana','ana@x.com') repeats id 1.Solve with ON CONFLICT (id) DO UPDATE SET email=EXCLUDED.email;.

How do I avoid “relation does not exist”?

The error means the referenced table, view, or sequence name is wrong or not visible in the current search_path. Quote mixed-case names or schema-qualify objects.

Example fix

SELECT * FROM "OrderItems"; if the table was created with capital letters, or SELECT * FROM ecommerce.OrderItems; if it lives in another schema.

What causes NOT NULL violations?

Trying to insert NULL into a column defined with NOT NULL triggers this error.Provide a value or change the column default.

Example fix

INSERT INTO Orders(customer_id,order_date,total_amount) VALUES(NULL,CURRENT_DATE,50) fails. Supply a valid customer_id or relax the constraint.

Can I catch errors inside functions?

Yes. PL/pgSQL offers EXCEPTION blocks to capture and react to errors without aborting the transaction.

Handler example

BEGIN INSERT INTO Orders ...;EXCEPTION WHEN unique_violation THEN RAISE NOTICE 'Order already exists.';END;

Why How to Fix Common Errors in PostgreSQL is important

How to Fix Common Errors in PostgreSQL Example Usage


-- Safely upsert a customer and handle duplicates
INSERT INTO Customers(id, name, email, created_at)
VALUES (10, 'Lara', 'lara@mail.com', NOW())
ON CONFLICT (id)
DO UPDATE SET name = EXCLUDED.name, email = EXCLUDED.email
RETURNING *;

How to Fix Common Errors in PostgreSQL Syntax


-- Basic EXCEPTION handling in PL/pgSQL
DO $$
BEGIN
    -- risky operation
    INSERT INTO Orders (id, customer_id, order_date, total_amount)
    VALUES (1001, 5, CURRENT_DATE, 120);
EXCEPTION
    WHEN unique_violation THEN  -- duplicate key
        RAISE NOTICE 'Order 1001 already exists';
    WHEN foreign_key_violation THEN  -- bad customer_id
        RAISE NOTICE 'Invalid customer_id';
    WHEN others THEN  -- fallback handler
        RAISE NOTICE 'Unexpected error: %', SQLERRM;
END;
$$;

Common Mistakes

Ignoring the exact wording of the error. PostgreSQL tells you the failing token or constraint. Always read the full message before debugging.
Quoting identifiers inconsistently. Creating tables with mixed-case names then querying them unquoted leads to "relation does not exist". Always use lower-case names or quote consistently.