Database Constraints and Unique Keys

Many-to-many data loading requires parsing JSON and distributing the data across two entity tables and one junction table.

Programming

From One JSON Record to Three Tables

A many-to-many data-loading task begins with one structured JSON record but ends with data distributed across three database tables. The record contributes information to one entity table, information to a second entity table, and a relationship row to a junction table. The main challenge is preserving the connections between those rows without creating duplicate entities or duplicate relationships.

How does one JSON record become rows in two entity tables and one relationship table?

Reading Entities and Relationships

When parsing a JSON record, separate the information into two categories. Entity information describes the records that belong in the two entity tables. Relationship information describes how those two entities are connected. The entity values are inserted or located first because the junction table needs their primary keys. The relationship is then represented by storing those two keys together in the junction table.

Which fields represent entities, and which field describes the relationship between them?

Separating a Generated Record

A JSON record contains an entity A value, an entity B value, and information that the two entities are related. Determine what each part contributes to the database.

Identify entity A: The entity A value belongs in the first entity table, where it must have or receive a primary key.

Identify entity B: The entity B value belongs in the second entity table, where it must have or receive a primary key.

Identify the relationship: The relationship does not become a duplicate copy of either entity. It becomes a junction-table row that links the two primary keys.

The JSON record supplies two entity values and one relationship. The relationship row can be created only after both entity primary keys are available.

Ensuring Entities Exist

For each entity, use INSERT OR IGNORE and then SELECT. INSERT OR IGNORE handles both cases: the entity may be new, or it may already exist. After that operation, SELECT retrieves the primary key for the entity that will be used in later work. This pattern avoids errors from attempting to insert an existing record while still giving the loader the key it needs.

What happens when an entity already exists, and how is its primary key retrieved?

Retrieve the primary key immediately after INSERT OR IGNORE. Do not postpone the lookup until after processing other records, because the junction-table operation depends on having the correct key for each entity.

What do you think happens?

An entity already exists when INSERT OR IGNORE is applied. What should happen next?

Stop because the entity cannot be used again
Run SELECT to retrieve the existing entity's primary key
Create a second entity row with a new relationship
Insert the relationship before finding either key

Reveal answer

Answer: Run SELECT to retrieve the existing entity's primary key

The INSERT OR IGNORE and SELECT pattern handles both new and existing records. SELECT supplies the primary key needed for subsequent operations.

Linking Keys in the Junction

Once the two entity primary keys have been retrieved, place them together in the junction table. One junction-table column stores the primary key from the first entity table, and another junction-table column stores the primary key from the second entity table. This pair represents the relationship extracted from the JSON record.

How do the primary keys from two entity tables become foreign-key columns in the junction table?

Building One Relationship Row

A parsed JSON record identifies entity A and entity B. Each entity has been processed with INSERT OR IGNORE followed by SELECT, and both primary keys are now available.

Keep the first key: Use the primary key retrieved for entity A as the first key value for the junction row.

Keep the second key: Use the primary key retrieved for entity B as the second key value for the junction row.

Insert the pair: Store the two keys together in the junction table so the relationship from the JSON record is represented in the database.

The junction row links the two existing entity records through their primary keys.

Rejecting Duplicate Relationships

A unique constraint on the junction table prevents the same relationship from being stored repeatedly. The relevant identity is the pair of entity keys: when that pair is already represented, another attempt describes the same relationship rather than a new one. INSERT OR REPLACE is used for junction-table inserts to prevent duplicate relationships while maintaining idempotency.

How does a unique constraint detect that the same pair of entities is already linked?

What changes in the junction table when an attempted insert conflicts with an existing unique key?

Loading Sequence Checklist

Parse the JSON record and separate the two entity values from the relationship information.
Apply INSERT OR IGNORE for the first entity.
Immediately use SELECT to retrieve the first entity's primary key.
Apply INSERT OR IGNORE for the second entity.
Immediately use SELECT to retrieve the second entity's primary key.
Use the two retrieved keys to identify the relationship in the junction table.
Apply INSERT OR REPLACE to the junction table so the unique constraint prevents a duplicate relationship.

Inserting directly into the junction table before retrieving entity keys
The junction row must link the primary keys from both entity tables.
Fix: Process each entity with INSERT OR IGNORE followed immediately by SELECT, then create the relationship row.
Treating an existing entity as an error
The pattern is designed to handle both new and existing records.
Fix: Run SELECT after INSERT OR IGNORE and use the existing record's primary key.
Inserting the same relationship without a duplicate-prevention strategy
Repeated processing can attempt to represent the same relationship again.
Fix: Use a unique constraint for the relationship key pair and INSERT OR REPLACE for junction-table inserts.

Practice the Loading Logic

MEDIUM

A JSON record identifies entity A and entity B, and the relationship between them has already been seen once. Describe the correct processing order. Include where INSERT OR IGNORE, SELECT, and INSERT OR REPLACE belong, and explain why the second relationship attempt does not create a duplicate junction-table entry.

Hints

The two entity keys must be available before the junction-table operation.
SELECT follows INSERT OR IGNORE for each entity.
The unique constraint applies to the pair of entity keys in the junction table.

Your attempt (not saved)

Checking the Complete Pattern

A record contains two entities and their relationship. The first entity is new, the second already exists, and the relationship has been loaded previously.

Process the first entity: Use INSERT OR IGNORE, then SELECT its primary key after the operation.

Process the second entity: Use INSERT OR IGNORE even though the entity already exists, then SELECT its existing primary key.

Prepare the relationship: Combine the two retrieved primary keys as the junction-table key pair.

Apply the junction operation: Use INSERT OR REPLACE. The unique constraint recognizes that the pair already represents a relationship, so the relationship is not duplicated.

The entity tables contain the required records, both primary keys are available, and the junction table continues to represent the relationship without a duplicate entry.

Reliable Many-to-Many Loading

Parse each JSON record into two entity values and one relationship.
Use INSERT OR IGNORE followed immediately by SELECT for each entity so both new and existing records yield usable primary keys.
Build the junction-table row from the two retrieved entity primary keys.
Use a unique constraint and INSERT OR REPLACE to prevent duplicate relationships and support idempotent loading.

Key Takeaways

Many-to-many JSON data is distributed across two entity tables and one junction table.
INSERT OR IGNORE followed by SELECT ensures that an entity exists and provides its primary key whether the entity is new or already present.
The junction table links the two entities by storing their primary keys together.
A unique constraint identifies an existing relationship, while INSERT OR REPLACE prevents duplicate junction-table entries.