PostgreSQL raises error 42P09 (ambiguous_alias) when the same alias is used for more than one table, view, or subquery in a single SQL statement, making column references unclear.
PostgreSQL Error 42P09 (ambiguous_alias) occurs when a query assigns the same alias to multiple tables or subqueries. Make each alias unique or remove duplicates to resolve the conflict and run the statement successfully.
PostgreSQL Error 42P09
Error 42P09 appears when PostgreSQL detects that a query assigns the same alias to more than one table, view, or subquery.
The duplicate alias confuses the planner because it cannot decide which source a column reference should resolve to, so execution stops with an error.
Most often the same short alias like "t" or "a" is reused in a JOIN clause for two different relations.
The error also arises when a CTE or derived table reuses an alias already taken by another relation in the outer query.
Give every table, subquery, and CTE a unique alias in the statement.
Even a one-letter change removes the ambiguity.
Refactor shared snippets or ORMs to generate distinct aliases automatically, preventing the clash in dynamic SQL.
Duplicate alias in multi-table JOINs: rename one alias to a new identifier.
Alias collision between a CTE and an inner query: change either alias so they differ.
Adopt clear alias naming conventions such as prefixing with the original table name.
Validate generated SQL in staging using Galaxy or psql linters to catch duplicates early.
Error 42702 ambiguous_column_reference arises when two tables have columns with the same name and no table qualifier.
Add the table alias to fix.
Error 42P01 undefined_table is raised when a referenced table or alias does not exist. Verify spelling and schema search paths.
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Yes, aliases only need to be unique within a single SELECT scope. Separate subqueries can reuse the same alias safely.
No. Quoting preserves case but does not bypass the uniqueness rule. Each alias must still be distinct.
No. The check is part of PostgreSQL's parser and cannot be turned off. Unique aliases are required.
Galaxy flags duplicate aliases in real time and offers one-click renames, reducing the chance of running faulty SQL.
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