Primary Key

You might say

AI said to update this record by id. Where does that id come from?

The field in a table used to uniquely identify each rowWhen modifying records or linking across tables, you must rely on a uniquely valued primary key to pinpoint the target row. Names and emails can duplicate or change, making them unsafe as definitive anchors; basing updates and deletions on primary keys prevents accidentally touching adjacent rows.

Know first

Database

Primary KeyID

Which column uniquely identifies a row

Which column uniquely identifies a rowusers table · two rows share an email

id = 7identifies exactly one row

id = 8another row with the same email

name / emailcan repeat; cannot locate uniquely

EvidenceOnly the primary key column has distinct values

Updating by id affects exactly one row; updating by email changes both rows.

Why a primary key must be unique

Locate one row, not a batch: Updates and deletes by primary key affect exactly one row; locating by a repeatable field changes several rows.

Ordinary fields repeat: Identical names and emails happen all the time; a primary key's values cannot repeat and cannot be empty.

Tables link through it: When an orders table records which user an order belongs to, it stores the users table's primary key, not the name.

Quick checkChoose the best answer

You want to update someone's profile by email in a user table that has two identical emails. What is the problem?

Update any one of the rows; the impact is smallDelete all duplicate-email records first and keep oneEmail can repeat and cannot uniquely identify a row; locate by the primary key

You can say this to an AI Agent

Update this user record by primary key, not by email or name. Before updating, tell me how many rows match; if more than one, stop and explain before continuing. After the change, query the record again to confirm.

Learn next

1Database→2Field→3Database Write→

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