Databases · Junior · Question 6 of 75

What is the difference between `UNIQUE` and `PRIMARY KEY`?

1UNIQUE works only on text columns
2There is no difference
✓Correct answer: A table has one primary key, which can't be NULL; it can have many UNIQUE constraints, and in PostgreSQL they allow NULLs

Explanation

By default PostgreSQL treats NULLs as distinct, so a UNIQUE column can contain several NULLs. UNIQUE NULLS NOT DISTINCT changes that.

Further reading

PostgreSQL: Constraints ↗

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