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Description

This error occurs when you attempt to define a table, view, or other relational object with two or more columns sharing the exact same name. PostgreSQL requires all column names within a given scope to be unique to avoid ambiguity and ensure data integrity.

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Error Message

duplicate column

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Known Causes

3 known causes

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Redundant Column Definition

Attempting to create a table or alter an existing one by specifying the same column name multiple times in the CREATE TABLE or ALTER TABLE ADD COLUMN statement.

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Conflicting Column Names in Views/Subqueries

When creating a view or using a subquery where two or more selected columns, possibly from different tables, resolve to the same alias or original name without explicit aliasing.

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Inherited Column Name Conflict

In PostgreSQL table inheritance, a child table inherits columns from its parent. This error can occur if you try to define a column in the child table with the same name as an inherited column.

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Solutions

3 solutions available

1. Identify and Remove Duplicate Column Name easy

Locate the table with the duplicate column definition and remove the redundant definition.

1

Examine the `CREATE TABLE` or `ALTER TABLE` statement that is failing. Look for instances where the same column name is defined more than once.

2

Correct the statement by removing the duplicate column definition. Ensure each column has a unique name within the table.

--- Example of an incorrect statement ---
CREATE TABLE users (
    user_id SERIAL PRIMARY KEY,
    username VARCHAR(50),
    email VARCHAR(100),
    email VARCHAR(100) -- Duplicate column definition
);

--- Corrected statement ---
CREATE TABLE users (
    user_id SERIAL PRIMARY KEY,
    username VARCHAR(50),
    email VARCHAR(100)
);

3

If the table already exists and you are trying to add a column, use `ALTER TABLE` and ensure the column name you are adding does not already exist. If it does, you might need to rename the existing column or choose a different name.

--- Example of an incorrect ALTER TABLE ---
ALTER TABLE products
ADD COLUMN price DECIMAL(10, 2),
ADD COLUMN price DECIMAL(10, 2); -- Duplicate column definition

--- Corrected ALTER TABLE ---
ALTER TABLE products
ADD COLUMN price DECIMAL(10, 2);

2. Review Schema Migrations for Duplicates medium

If using a schema migration tool, check recent migration scripts for accidental duplicate column definitions.

1

Identify the specific migration script that is causing the error. Migration tools often provide context for the failing operation.

2

Open the relevant migration file (e.g., `.sql`, `.rb`, `.py` depending on your tool) and carefully review the `CREATE TABLE` or `ALTER TABLE` statements.

3

Look for any instances where the same column name is being defined or added multiple times within the same table operation.

4

Edit the migration script to remove the duplicate column definition. Ensure that the intended schema changes are preserved.

5

Re-run the migration after correcting the script. If the migration is already applied partially, you might need to manually resolve the state of the table before reapplying or skipping the problematic migration.

3. Inspect Existing Table Structure advanced

Query the PostgreSQL catalog to find tables with duplicate column definitions and then correct them.

1

Connect to your PostgreSQL database using `psql` or another client.

2

Query the `information_schema.columns` view to identify tables with duplicate column names. This query groups by table and column name and counts occurrences.

SELECT table_schema, table_name, column_name, COUNT(*) FROM information_schema.columns GROUP BY table_schema, table_name, column_name HAVING COUNT(*) > 1 ORDER BY table_schema, table_name, column_name;

3

For each identified duplicate, you will need to decide which definition to keep and which to remove. You can then use `ALTER TABLE` to drop the redundant column.

--- Example: Dropping a duplicate column ---
-- First, identify the specific column to drop. You might need to inspect the table schema further if the definitions are identical or if the duplicate was added with different constraints.
-- Assuming you want to drop the second 'email' column from the 'users' table in the 'public' schema:
ALTER TABLE public.users
DROP COLUMN email;

-- If the duplicate column has a different name, you'll need to specify that name.
-- For instance, if the duplicate was named 'email_backup':
-- ALTER TABLE public.users
-- DROP COLUMN email_backup;

4

If you are creating a new table and encounter this error, the issue is in your `CREATE TABLE` statement itself. Go back to Solution 1. This query is primarily for identifying issues in existing schemas.

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Related Errors

5 related errors

PostgreSQL Error 42701: Duplicate Column Definition