How to diagnose and fix the 42701 duplicate_column error code in Postgres. 

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The 42701 error code in PostgreSQL refers to a duplicate_column error. This error occurs when you attempt to create a new column in a table that already has a column with the same name, or when you specify the same column name more than once within a single SQL statement.

To diagnose and fix this issue, you should:

  1. Identify the SQL statement that is causing the error.
  2. Check the table definition or the SQL statement for any duplicate column names.
  3. Rename the column or remove the duplication from your SQL statement.

Here are some examples and sample code to explain and cover the possibilities:

Example 1: Creating a Table with Duplicate Column Names

If you try to create a table with columns that have the same name:

CREATE TABLE employee (
    id INT,
    name TEXT,
    name TEXT
);

This will result in a 42701 error because the column name name is duplicated.

To fix this, you should ensure each column has a unique name:

CREATE TABLE employee (
    id INT,
    first_name TEXT,
    last_name TEXT
);

Example 2: Adding a New Column to an Existing Table

If you attempt to add a new column to a table that already has a column with the same name:

ALTER TABLE employee ADD COLUMN name TEXT;

Assuming the employee table already has a name column, this will cause a 42701 error.

To resolve this, you should choose a different name for the new column:

ALTER TABLE employee ADD COLUMN middle_name TEXT;

Example 3: Using a Duplicate Column Name in a View or Subquery

When creating a view or a complex query with joins, you might inadvertently create duplicate column names:

CREATE VIEW employee_view AS
SELECT e1.id, e1.name, e2.name
FROM employee e1
JOIN employee_info e2 ON e1.id = e2.employee_id;

If both employee and employee_info tables have a name column, this will result in a 42701 error.

To fix it, you should alias at least one of the columns:

CREATE VIEW employee_view AS
SELECT e1.id, e1.name AS employee_name, e2.name AS info_name
FROM employee e1
JOIN employee_info e2 ON e1.id = e2.employee_id;

General Tips

  • Always check your table structure for existing column names before adding new ones.
  • Use descriptive and unique column names to avoid confusion and potential errors.
  • When joining tables or creating views, use aliases to differentiate between columns with the same name.
  • Consult the PostgreSQL documentation for error codes and their descriptions for further clarity.

By following these steps and ensuring that each column in your table has a unique name, you will be able to diagnose and resolve the 42701 duplicate column error in PostgreSQL.

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