"relation already exists" / "Table already exists"

A CREATE TABLE ran against a name the database already has. Usually a re-run script, sometimes a genuine naming collision — IF NOT EXISTS fixes the first, not the second.

• SQLite: table customers already exists
• PostgreSQL: ERROR: relation "customers" already exists
• MySQL: Table 'customers' already exists
• SQL Server: There is already an object named 'customers' in the database.

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Two different situations, one error

A setup script or migration got run twice. By far the most common cause — a CREATE TABLE that ran successfully once shouldn't run again, but something re-executed it: a re-deployed migration, a retried setup step, a script run manually a second time by mistake. The fix is making the script safe to re-run, not chasing the error each time:

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IF NOT EXISTS makes the statement a no-op when the table is already there instead of erroring — every engine on this page supports it with this exact syntax.

Two different things actually needed two different names, and they collided by coincidence — a genuinely new table happened to be named the same as an existing one for something unrelated. IF NOT EXISTS is the wrong fix here: it would silently skip creating your new table and leave the old one in place under a name you now think belongs to something else. Rename the new table instead of suppressing the error.

Telling the two apart

Before reaching for IF NOT EXISTS as a reflex, check what the existing table actually contains — if a table you expected to be creating fresh already has unexpected data or a different shape, that's the second case, not the first:

-- PostgreSQL / MySQL
SELECT column_name, data_type FROM information_schema.columns
WHERE table_name = 'customers';

-- SQLite
PRAGMA table_info(customers);

The DROP-then-CREATE trap

A common but dangerous "fix" for this error in a script is DROP TABLE customers; CREATE TABLE customers (...); — this makes the script re-runnable, but it also silently deletes everything in the table on every re-run. If the table is supposed to accumulate real data between runs, that's not idempotency, it's data loss with a green checkmark. CREATE TABLE IF NOT EXISTS doesn't have this problem, because it leaves an existing table — and its data — untouched.