Primary Keys
- A primary key is a set of columns that uniquely specify a row in a table
- This is how we tell one row from another
- It’s also why every row in a table is unique
- We’ll discuss primary keys next week
03aSchemas
Database Management
12 May 2026
Database Schemas
- We have said that a relational database is structured. How?
- A schema is a description of the structure of a database
- A schema must touch on all the structures in the database
- Tables
- Columns
- Data Types
- Primary Keys
- Foreign Keys
- Constraints
- Indexes
Tables
We’ve already talked about tables and can think of a table as a collection of entity instances, where each entity instance is described by a row
Columns
We can think of a column (also known as a field) as an attribute of an entity
Data Types
- Every piece of data has a type, such as text, number, or date
- We’ll discuss data types in the next video
Foreign Keys
- When data in one table refers to data in another table, there has to be a way to specify a row in the other table
- The foreign key is that way
- We’ll discuss foreign keys next week
Constraints
- Constraints specify rules for columns
- Such as “the value must be unique” or “the value can not be NULL” or “the value must be positive”
- The dbms will raise an error if you try to enter data that violates a constraint
Indexes
- We use indexes without needing to understand them except for the following
- An index applies to a column
- An index greatly speeds up searching for a cell in a column
Self description
- We have said that a database is self-describing
- This means that the schema is a set of tables associated with the rest of the database
- So a database has tables that describe things in the real world plus tables that contain metadata, that is, data about the data in the database
END
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