46 lines
1.4 KiB
Markdown
46 lines
1.4 KiB
Markdown
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---
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title: SQL Primary Key Constraint
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---
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## Primary Key Constraint
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### Introduction
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A primary key is a column or a set of columns that uniquely identifies each row in a table.
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It's called a "constraint" because it causes the system to restrict the data allowed in these column(s). In this case....
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* to contain data (NOT NULL)
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* be UNIQUE from all other rows in the table.
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* Each table can have only ONE primary key
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Primary keys are mostly used to maintain the data integrity of each row.
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It also allows the system and applications to be sure they are reading, updating and joining the data correctly.
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### Example with create table
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Here is a create table command that will also create a primary key using two fields.
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```sql
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CREATE TABLE priKeyExample(
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rcdKey_id_a INT NOT NULL,
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rcdKeySeq_id INT NOT NULL,
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someData varchar(256) NOT NULL,
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PRIMARY KEY(rcdKey_id_a,rcdKeySeq_id));
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```
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### Example with alter table
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The existing one must be deleted first
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```sql
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DROP INDEX `primary` ON priKeyExample;
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```
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Now we'll add the new one.
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```sql
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ALTER TABLE priKeyExample
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ADD CONSTRAINT myPriKey PRIMARY KEY(rcdKey_id_a,rcdKeySeq_id);
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```
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As with all of these SQL things there is MUCH MORE to them than what's in this introductory guide.
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I hope this at least gives you enough to get started.
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Please see the manual for your database manager and have fun trying different options yourself.
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