10 - Using Keys and Key Types in SQL — Module Topics

Introduction to Keys in SQL

• What Is a Key in a Relational Database? — A key is one or more columns in a database table used to uniquely identify rows or establish relationships between tables.
• Why Keys Are Essential for Data Organization — Keys provide the structural backbone of a relational database by enforcing order, uniqueness, and traceability across datasets.
• Keys and Data Integrity — One of the primary roles of keys is to enforce data integrity, ensuring that the information stored in a database remains accurate and consistent.
• Overview of Key Types in SQL — SQL databases recognize several distinct key types, each serving a specific purpose in data identification and relationship management.
• Keys as Constraints in SQL — In SQL, keys are implemented as constraints — rules applied to table columns that the database engine enforces automatically.

Primary Keys

• What Is a Primary Key? — A primary key is a column or set of columns that uniquely identifies each record in a database table.
• Why Primary Keys Matter — Primary keys are fundamental to relational database design because they guarantee data integrity and provide a reliable way to reference individual records.
• Defining a Primary Key at Table Creation — A primary key constraint can be declared directly when creating a table using the CREATE TABLE statement in SQL.
• Adding a Primary Key to an Existing Table — If a table was created without a primary key, you can add one later using the ALTER TABLE statement.
• Enforcing the Primary Key Constraint — Once defined, the database engine automatically enforces the primary key constraint on every INSERT and UPDATE operation.
• Dropping a Primary Key Constraint — Primary key constraints can be removed from a table when the schema needs to be restructured, using the ALTER TABLE statement.
• Best Practices for Choosing a Primary Key — Selecting the right column or columns as a primary key is a critical design decision that affects performance, maintainability, and data integrity.

Foreign Keys and Referential Integrity

• What Is a Foreign Key? — A foreign key is a column or set of columns in one table that references the primary key of another table, establishing a logical link between the two.
• Defining a Foreign Key Constraint in SQL — Foreign key constraints are declared using the FOREIGN KEY keyword either inline during column definition or as a table-level constraint in a CREATE TABLE or ALTER TABLE statement.
• Understanding Referential Integrity — Referential integrity is the database principle that ensures every foreign key value in a child table corresponds to an existing value in the parent table.
• Referential Actions: ON DELETE and ON UPDATE — SQL allows you to define referential actions that specify what happens to child rows when a referenced parent row is deleted or updated.
• Foreign Keys and NULL Values — A foreign key column can be allowed to hold NULL values, which indicates that the relationship is optional rather than mandatory for that row.
• Foreign Keys vs. Primary Keys: Roles in Relationships — Primary keys uniquely identify rows within their own table, while foreign keys create cross-table relationships by referencing those primary keys.

Candidate Keys

• What Is a Candidate Key? — A candidate key is any column or combination of columns in a table that can uniquely identify every row and therefore qualifies to serve as the primary key.
• Properties That Define a Candidate Key — For a column or set of columns to qualify as a candidate key, it must satisfy two core properties: uniqueness and irreducibility.
• Identifying Candidate Keys in a Table — Finding candidate keys requires analyzing the data and business rules to determine which columns or column combinations reliably and uniquely identify each row.
• Candidate Keys vs. Primary Keys — While all primary keys are candidate keys, not all candidate keys become the primary key — the designer selects the most suitable one for that role.
• Enforcing Candidate Keys with UNIQUE Constraints — In SQL, candidate keys that are not selected as the primary key should still be enforced using UNIQUE constraints to maintain their identifying integrity.
• Role of Candidate Keys in Database Design — Understanding candidate keys is fundamental to good relational database design because they reveal all the natural ways rows can be uniquely identified.

Composite Keys

• What Is a Composite Key? — A composite key is a primary key or unique constraint that consists of two or more columns working together to uniquely identify a row in a table.
• When to Use a Composite Key — Composite keys are appropriate when the data model naturally requires more than one attribute to produce a unique identifier for each record.
• Defining a Composite Primary Key in SQL — A composite primary key is declared using the PRIMARY KEY constraint at the table level, listing all participating columns inside parentheses.
• Defining a Composite Unique Constraint — Beyond primary keys, composite uniqueness can also be enforced with a UNIQUE constraint spanning multiple columns, allowing the table to have a separate surrogate primary key.
• Referencing a Composite Key with Foreign Keys — When a composite primary key is referenced by another table, the foreign key declaration must include all the same columns in matching order.
• Advantages and Limitations of Composite Keys — Composite keys offer meaningful, naturally derived identifiers but also introduce complexity in querying, indexing, and referencing.

Surrogate Keys

• What Is a Surrogate Key? — A surrogate key is a system-generated identifier assigned to each row in a table, with no inherent business meaning.
• Surrogate Keys vs. Natural Keys — Understanding the difference between surrogate and natural keys helps designers choose the right approach for a given table.
• Advantages of Using Surrogate Keys — Surrogate keys offer several practical benefits that make them a preferred choice in modern relational database design.
• Common Surrogate Key Implementations in SQL — SQL databases provide built-in mechanisms to automatically generate surrogate key values when new rows are inserted.
• When to Use Surrogate Keys — Surrogate keys are especially valuable in specific design scenarios where natural keys present challenges.
• Surrogate Keys and Referential Integrity — Surrogate keys work seamlessly with foreign key constraints to maintain referential integrity across related tables.

Enforcing Key Constraints for Data Integrity

• Understanding Why Key Constraints Matter — Key constraints are rules enforced by the database engine to ensure that data remains accurate, consistent, and free from duplication or orphaned records.
• Defining and Enforcing Primary Key Constraints — A primary key constraint guarantees that every row in a table has a unique, non-null identifier, forming the foundation of data integrity for that table.
• Applying Foreign Key Constraints for Referential Integrity — Foreign key constraints enforce referential integrity by ensuring that values in a child table's column always correspond to existing values in the referenced parent table.
• Using UNIQUE Constraints to Enforce Candidate Keys — Candidate keys are columns or column combinations that could serve as a primary key; those not chosen as the primary key should be enforced with UNIQUE constraints.
• Implementing Composite and Surrogate Keys Strategically — Choosing between composite keys and surrogate keys requires balancing naturalness of identification against simplicity and stability of the key column.
• Managing Constraints: Adding, Disabling, and Dropping — Database administrators often need to modify existing constraints during schema evolution, bulk data loads, or migrations without permanently removing data integrity rules.
• Best Practices for Consistent Constraint Enforcement Across a Schema — Applying constraints consistently and systematically across all tables in a schema ensures holistic data integrity rather than isolated guarantees on individual tables.