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Databases & SQL vocabulary · Keys and Constraints

Unique constraint

Meaning in databases & SQL, with examples

nounCommon term

In databases & SQL, unique constraint means: A rule that stops two rows from having the same value in a column or combination of columns, for example two users with the same email address.

In Turkish: benzersizlik kısıtı, tekillik kısıtı, unique kısıtı

How it is used

Unlike a primary key, a table can have many unique constraints, and most databases let unique columns contain NULLs. Behind the scenes the database builds a unique index. Trying to insert a duplicate gives a 'duplicate key' or 'unique violation' error.

Example sentences

  1. Add a unique constraint on email so the same person can't sign up twice.

  2. The import stopped at row 812 because of a unique constraint violation.

Common collocations

  • add a unique constraint
  • unique index
  • unique violation
  • duplicate key error
  • constraintA rule set on a table or column that the database enforces automatically, rejecting any data that would break it.
  • primary keyA column, or a set of columns, whose value uniquely identifies each row in a table. It cannot be empty and no two rows can share it.
  • indexAn extra data structure built on one or more columns that lets the database find rows quickly without reading the whole table, like the index at the back of a book.
  • upsertTo insert a row if it does not exist yet, or update it if it already does, in a single operation. The word combines 'update' and 'insert'.

This word in other fields

The same English word can have a different meaning in another field:

More terms: Keys and Constraints

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