The error appears when a column with the same name is defined twice in a statement or already exists in the table, blocking DDL or query execution.
Duplicate column name error means your SQL statement defines the same column twice. Remove or rename the second occurrence, or use column aliases, to execute successfully.
MySQL: ERROR 1060 (42S21): Duplicate column name 'col_name'; PostgreSQL: column "col_name" specified more than once; SQL Server: Cannot create duplicate column name 'col_name'.
The “Duplicate column name” error occurs when an SQL statement defines or references a column name that already exists in the target scope.
It stops table alterations, CREATE TABLE commands, or SELECT queries with conflicting column names.
Because databases require unique column identifiers per table or result set, the engine throws this error to prevent ambiguous references that could corrupt data or mislead queries.
Adding a column with ALTER TABLE that already exists triggers the error immediately.
The server checks the system catalog and finds a duplicate, then aborts the statement.
Creating a new table with two columns of the same name or importing a schema file that lists a column twice raises the same violation.
SELECT statements that join tables without aliases can return two columns with identical names in the output, causing duplication in client libraries and raising an error in some drivers.
First, verify the table definition with DESCRIBE or information_schema to see if the column already exists.
If yes, skip the ADD COLUMN or rename it.
For SELECT queries, prefix columns with table aliases or use AS to rename duplicates, ensuring each output column label is unique.
Schema migrations often re-run and attempt to add a column twice.
Guard scripts with IF NOT EXISTS (MySQL 8.0+) or conditional queries to avoid duplicates.
Data import tools generating CREATE TABLE may duplicate a column accidentally; manually inspect generated DDL or use Galaxy’s schema diff to catch issues.
Maintain a single source of truth for schemas, apply migrations sequentially, and version-control them to prevent re-execution.
Leverage Galaxy’s AI copilot to lint SQL in real time; it highlights duplicate column definitions before execution, reducing runtime failures.
Errors such as “Duplicate key name,” “Column does not exist,” and “Ambiguous column name” stem from schema or naming conflicts.
Solutions involve syncing metadata and using clear aliases.
MySQL and SQLite support IF NOT EXISTS for ADD COLUMN, while PostgreSQL requires a conditional DO block and SQL Server uses dynamic SQL.
Yes. Dropping removes the data. Instead, rename the column to preserve history.
Galaxy’s AI copilot analyzes live schema metadata, flags duplicate column definitions, and suggests aliases before execution.
Some drivers allow duplicate labels but it leads to ambiguous access. Always alias to unique names.
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