What is a primary Key?

What a primary key is depends, somewhat, on the database.  However, in its simplest form a primary key:

• Is a field (Column) or combination of Fields (columns) which uniquely identifies every row.
• Is an index in database systems which use indexes for optimization
• Is a type of table constraint
• Is applied with a data definition language (DDL) alter command
• And, depending on the data model can, define parent-Child relationship between tables

Related References

• Data Modeling – Fact Table Effective Practices
• Data Modeling – Dimension Table Effective Practices
• Netezza / PureData Table Documentation Practices
• Database Table Field Ordering Effective Practices
• What is DDL?
• Netezza/ PureData – how to add a primary key