18 lines
1,014 B
Markdown
18 lines
1,014 B
Markdown
---
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categories:
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- Databases
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tags: [relational_databases]
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---
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# Primary key
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> Every table in a relational database should have a **primary key**. A primary key is one **field that uniquely identifies each record**.
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This is essential for carrying out operations across database tables and for creating and deleting database entires. It is also a safeguard: it means you can always identify a record by itself and don't have to rely on generic queries to identify it.
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Sometimes you will have a dedicated field such as `UNIQUE_ID` for the primary key. Other times you can use an existing field to fulfil that function. In both cases the following constraints **must be met:**
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1. No two records can have the **same** primary key data
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1. The primary key value should **never be reused**. Thus, if a record is deleted from the table, it should not be re-allocated to a new record.
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1. A primary key value **must not be modified** once it has been created
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1. A primary key **must have a value**; it cannot be `null`
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