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?

SQL Date and Time Functions Across Dialects

SQL Pagination Across Databases: LIMIT, TOP, and OFFSET-FETCH Compared

SQL NULL-Handling Functions Across Dialects: COALESCE, ISNULL, and IFNULL

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

SQL Date and Time Functions Across Dialects

SQL Pagination Across Databases: LIMIT, TOP, and OFFSET-FETCH Compared

SQL NULL-Handling Functions Across Dialects: COALESCE, ISNULL, and IFNULL

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

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