Foreign key

As explained in Build Python Web Apps with Streamlit

A foreign key constraint requires values in one table's column to correspond to existing values in a referenced table's key column.

A foreign key connects related tables by requiring a column’s value to match an existing value in another table’s key column. In a haiku app, each haiku can record its author using the username of an existing user. section 8.2.2 section 8.2.4

The referenced column is typically a primary key. Here, users.username uniquely identifies a user, while haikus.author points to it. This models the one-to-many relationship in which one user can be associated with multiple haikus. section 8.2.2

Connecting users and haikus

The schema declares the relationship with FOREIGN KEY (author) REFERENCES users(username). After inserting alice into users, rows in haikus can use alice as their author. The database also supplies haiku_id and created_at through their declared types and defaults. section 8.2.4

Figure 8.4 Database schema with a foreign key relationship between the users and haikus tables

Use the schema diagram to see users.username connected to haikus.author and to distinguish each table’s primary key. section 8.2.2

How Build Python Web Apps with Streamlit covers it

How deep it goes

Defines the relationship and shows its PostgreSQL declaration, but does not develop broader constraint-management or deletion behavior.

Examples

Uses users.username, haikus.author, the alice user, and inserted haiku rows to demonstrate the relationship.

What it assumes

Assumes tables, columns, and primary keys, while introducing how entities and one-to-many relationships become relational schema elements.

What you can do after

Apply it via PostgreSQL by declaring a FOREIGN KEY that references a primary-key column in another table.

How it connects

Connects primary keys, relational database schemas, ER diagrams, SQL, and one-to-many relationships.

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Also explained in

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Related concepts

• primary keyuses · A primary key is a designated column or attribute, or combination of them, that uniquely identifies each row or item in a table or data store.
• constraintkind of · A constraint is a condition or rule that restricts the permissible states, values, actions, or solutions in a system or problem.
• on delete cascadeconfigured by · ON DELETE CASCADE is a relational foreign-key action that automatically deletes referencing rows when their referenced row is deleted.
• referential integrityimplements · Referential integrity is the database property that references between related records correspond to valid existing keys or records.
• joinused by · A join is an operation that combines records or rows from multiple data sources according to specified matching conditions or relationships.
• schemapart of · A data schema is a formal specification of the structure, fields, types, relationships, and constraints of data.

This explainer was generated by Manning's knowledge graph from the text of Build Python Web Apps with Streamlit. The section links point to the parts of the book it draws on.