5.2 Defining constraints and relationships
3 min read•august 6, 2024
Constraints and relationships are the backbone of database design, ensuring and consistency. They define rules for data entry, establish connections between tables, and maintain the overall structure of your database.
Primary keys, unique constraints, and foreign keys work together to create a robust database schema. These tools help prevent data inconsistencies, enforce , and enable efficient data retrieval and manipulation across related tables.
Key Constraints
Ensuring Data Integrity with Primary Keys and Unique Constraints
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- constraint uniquely identifies each record in a table
- Consists of one or more columns that together form a unique combination for each row
- Prevents duplicate rows from being inserted into the table (students table with
as the primary key)student_id
- ensures that all values in a column or a group of columns are distinct
- Similar to PRIMARY KEY, but a table can have multiple UNIQUE constraints
- Allows NULL values, unlike PRIMARY KEY (email column in users table)
- enforces that a column must always contain a non-NULL value
- Prevents incomplete or missing data from being inserted into the column
- Commonly used for columns that are essential for the meaning and integrity of the data (name, date_of_birth)
Validating Data with CHECK Constraints
- defines a condition that must be satisfied for each row in a table
- Ensures that the values in a column meet specific criteria or fall within a certain range
- Helps maintain data consistency and prevents invalid data from being inserted (age >= 18, salary > 0)
- Can involve multiple columns and use complex expressions or functions (start_date < end_date, discount BETWEEN 0 AND 100)
- Provides an additional layer of data validation beyond data types and other constraints
Foreign Key Constraints
Establishing Relationships with Foreign Keys
- constraint establishes a link between two tables based on a column or a set of columns
- Refers to the PRIMARY KEY or a UNIQUE constraint in another table
- Ensures referential integrity by enforcing that the values in the foreign key column(s) must exist in the referenced table (order_details table with
as a foreign key referencing the orders table)order_id
- specifies the table and column(s) that the foreign key references
- Defines the parent table and the corresponding PRIMARY KEY or UNIQUE constraint
- Establishes the relationship between the child table (containing the foreign key) and the parent table (departments table referenced by
in the employees table)department_id
Maintaining Referential Integrity with ON DELETE and ON UPDATE
- clause specifies the action to be taken when a referenced row in the parent table is deleted
- : Automatically deletes the corresponding rows in the child table
- : Sets the foreign key column(s) in the child table to NULL
- : Prevents the deletion of the referenced row in the parent table if there are associated rows in the child table (default behavior)
- clause defines the action to be taken when a referenced column in the parent table is updated
- Similar options as ON DELETE (CASCADE, SET NULL, RESTRICT)
- Ensures data consistency and maintains referential integrity when changes occur in the parent table
- Referential integrity maintains the consistency and accuracy of data across related tables
- Prevents orphaned records in the child table that reference non-existing rows in the parent table
- Ensures that the relationships between tables remain valid and consistent (deleting a department cascades the deletion to associated employees)
Key Terms to Review (13)
Cascade: Cascade refers to a method of handling related data changes in a database, particularly when dealing with foreign key constraints. When a record in a parent table is deleted or updated, cascading actions automatically apply the same change to the related records in child tables. This ensures data integrity and maintains relationships between tables without requiring manual intervention.
Check constraint: A check constraint is a rule applied to a column in a database table that restricts the values that can be inserted or updated in that column based on a specific condition. It ensures data integrity by enforcing certain criteria, like a range of values or specific formats, which helps maintain the quality and accuracy of the data stored in a relational database. Check constraints play a significant role in defining how data relates to itself and other tables, providing a structured approach to maintaining valid data throughout its lifecycle.
Data Integrity: Data integrity refers to the accuracy, consistency, and reliability of data throughout its lifecycle. It ensures that data remains correct and trustworthy during various operations such as data entry, storage, retrieval, and manipulation. Data integrity is essential for maintaining quality and ensuring that information reflects the true state of the real-world entities it represents.
Foreign key: A foreign key is a field (or a collection of fields) in one table that uniquely identifies a row of another table, creating a link between the two tables. This concept is crucial for establishing relationships in relational databases, ensuring data integrity and enforcing referential constraints between related tables.
Not Null Constraint: A not null constraint is a rule in database management systems that ensures a column cannot have a null value, which means it must always contain a valid entry. This constraint is critical for maintaining data integrity, as it prevents incomplete records and ensures that essential fields are populated. It connects to various database features, including how relationships and constraints are mapped, how constraints are defined, and how data is manipulated through inserting, updating, or deleting.
On delete: 'On delete' is a referential action that specifies how to handle the deletion of a referenced row in a relational database. This term is crucial for maintaining data integrity within relationships between tables, particularly in defining what happens to child records when a parent record is deleted. It can enforce cascading deletions, restrict deletions, or set null values for the foreign key references, helping to ensure that the database remains consistent and free from orphaned records after a delete operation.
On update: The term 'on update' refers to a specific action defined in relational databases that dictates what happens to foreign key constraints when the primary key of the referenced table is updated. This action is crucial for maintaining referential integrity, ensuring that relationships between tables remain consistent even when data changes. It can be set to options like CASCADE, SET NULL, or NO ACTION, each determining how related records should react during an update operation.
Primary Key: A primary key is a unique identifier for each record in a database table, ensuring that no two rows can have the same key value. It plays a crucial role in maintaining the integrity of data by preventing duplicate entries and enabling efficient data retrieval.
References clause: The references clause is a part of a database schema definition that establishes a foreign key relationship between two tables. This clause ensures referential integrity by enforcing that the values in the foreign key column of one table must match the values in the primary key column of another table, thereby maintaining consistent data across related tables. By defining these relationships, the references clause helps to clarify how data in different tables interacts and relies on each other.
Referential Integrity: Referential integrity is a database concept that ensures relationships between tables remain consistent and valid. It requires that any foreign key value in one table must either match an existing primary key value in another table or be null, thereby preventing orphaned records and maintaining the accuracy of data across related tables.
Restrict: To restrict means to impose limitations on the data that can be entered, stored, or modified in a database. This concept is crucial for maintaining the integrity and consistency of the data by ensuring that certain conditions or constraints are met before operations can proceed. By restricting data entry and relationships, databases can enforce rules that uphold the validity of the information being managed.
Set null: Set null is a referential action that specifies what should happen to a foreign key in a database when the corresponding primary key it references is deleted or updated. This action allows the database to maintain referential integrity by ensuring that the foreign key is set to null instead of causing a violation of the relationship between tables. By setting the foreign key to null, it indicates that the relationship is no longer valid without deleting the entire record.
Unique Constraint: A unique constraint is a database rule that ensures all values in a specific column or a set of columns are distinct across all rows in a table. This is crucial for maintaining data integrity and helps prevent duplicate entries, which can lead to inconsistencies and errors within the database system.