What is a SQL upsert?

An upsert is a single statement that inserts a new row if it doesn't exist yet, or updates the existing row if it does — combining an INSERT and an UPDATE into one operation instead of a separate SELECT-then-branch.

Does upsert require a unique constraint?

Yes. Every dialect's upsert syntax needs a primary key, unique constraint, or unique index on the column(s) used to detect a conflict — without one, the database has nothing to match against.

Is MERGE the same as UPSERT?

MERGE is a superset of upsert. Where ON CONFLICT and ON DUPLICATE KEY UPDATE only handle insert-or-update, MERGE can also delete unmatched rows in the same statement, which is why SQL Server and BigQuery use it as their upsert mechanism.

Does MySQL support ON CONFLICT?

No — MySQL uses its own INSERT ... ON DUPLICATE KEY UPDATE syntax instead. ON CONFLICT is PostgreSQL and SQLite syntax; writing it against MySQL raises a syntax error.

Which dialects can upsert without any special keyword?

None of the five — PostgreSQL, MySQL, SQLite, SQL Server, and BigQuery all require dialect-specific syntax (ON CONFLICT, ON DUPLICATE KEY UPDATE, or MERGE); there's no ANSI-standard keyword that works unmodified everywhere.

SQL Dialects

SQL UPSERT Across Dialects: ON CONFLICT, ON DUPLICATE KEY, and MERGE

By Sharon Ben-Moshe · August 8, 2026 · 3 min read

Sharon Ben-Moshe is the founder of sqlfmt, a browser-based SQL formatter and validator with misspelled-keyword suggestions across PostgreSQL, MySQL, SQLite, SQL Server, and BigQuery.

Every major SQL dialect can insert-or-update in one statement, but the syntax is different everywhere — ON CONFLICT, ON DUPLICATE KEY UPDATE, INSERT OR REPLACE, and MERGE all solve the same problem in incompatible ways.

Quick answer

An upsert inserts a new row or updates an existing one in a single statement, avoiding a separate exists-check. PostgreSQL and SQLite use INSERT ... ON CONFLICT, MySQL uses INSERT ... ON DUPLICATE KEY UPDATE, and SQL Server and BigQuery use the ANSI MERGE statement — the same operation, five different keywords.

SQL UPSERT Across Dialects: ON CONFLICT, ON DUPLICATE KEY, and MERGE

Quick answer

FAQs

What is a SQL upsert?

Does upsert require a unique constraint?

Is MERGE the same as UPSERT?

Does MySQL support ON CONFLICT?

Which dialects can upsert without any special keyword?

The Comprehensive Guide to SQL Data Types: Storage, Precision, and Cross-Dialect Portability

The Definitive Guide to SQL Window Functions: Syntax, Framing, and Real-World Recipes

The Ultimate Guide to SQL JOINs: Visualizing Every Join Type, Cardinality, and Performance

Key takeaways

PostgreSQL: INSERT ... ON CONFLICT

SQLite: INSERT ... ON CONFLICT (and INSERT OR REPLACE)

MySQL: INSERT ... ON DUPLICATE KEY UPDATE

SQL Server: MERGE

BigQuery: MERGE

Comparison at a glance

A MERGE concurrency caveat worth knowing

Quick answer

FAQs

What is a SQL upsert?

Does upsert require a unique constraint?

Is MERGE the same as UPSERT?

Does MySQL support ON CONFLICT?

Which dialects can upsert without any special keyword?

Keep reading

The Comprehensive Guide to SQL Data Types: Storage, Precision, and Cross-Dialect Portability

The Definitive Guide to SQL Window Functions: Syntax, Framing, and Real-World Recipes

The Ultimate Guide to SQL JOINs: Visualizing Every Join Type, Cardinality, and Performance

Key takeaways

PostgreSQL: INSERT ... ON CONFLICT

SQLite: INSERT ... ON CONFLICT (and INSERT OR REPLACE)

MySQL: INSERT ... ON DUPLICATE KEY UPDATE

SQL Server: MERGE

BigQuery: MERGE

Comparison at a glance

A MERGE concurrency caveat worth knowing

Related reading