Any column in a table can be established as the primary key, as long as it meets the following conditions:
• No two rows can have the same primary key value.
• Every row must have a primary key value. (Primary key columns may not allow NULL values.)
• Values in primary key columns should never be modified or updated.
• Primary key values should never be reused. (If a row is deleted from the table, its primary key may not be assigned to any new rows in the future.)
- Lesson 1. Understanding SQL
- from Sams Teach Yourself SQL in 10 Minutes, Fourth Edition
- Publisher: Sams
- Released: October 2012
primary key rules
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