For a sound relational schema, give each row a reliable identifier, enforce other uniqueness rules separately, connect tables with foreign keys, and use constraints to reject invalid data. The SQL examples below use PostgreSQL syntax and document behavior described in PostgreSQL 18 unless noted; check your own database engine and version before relying on engine-specific details.
What is a primary key?
A primary key is the column or group of columns that identifies each row in a table. In PostgreSQL, its values must be unique and non-null, and a table can have at most one primary-key constraint. A key can consist of multiple columns. See the PostgreSQL 18 constraints documentation.
CREATE TABLE customers (
customer_id bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
email text NOT NULL UNIQUE
);Here, customer_id is the table’s designated row identifier. The separate UNIQUE constraint protects email addresses from duplication; it does not make email the primary key. This separation is useful when an account also has a business identifier, such as an externally assigned code, that must not repeat.
When should I use a composite key?
Use a composite key when the rule says a combination of values identifies a row or must not repeat. For example, a membership table might allow a person to join a group only once:
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CREATE TABLE group_memberships (
group_id bigint NOT NULL,
person_id bigint NOT NULL,
joined_at timestamptz NOT NULL DEFAULT now(),
PRIMARY KEY (group_id, person_id)
);The pair is unique even though neither column has to be unique on its own. If other tables or application code benefit from a compact, stable single-column identifier, you can instead define that identifier as the primary key and enforce the pair with a composite unique constraint:
CREATE TABLE group_memberships (
membership_id bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
group_id bigint NOT NULL,
person_id bigint NOT NULL,
joined_at timestamptz NOT NULL DEFAULT now(),
UNIQUE (group_id, person_id)
);Choose based on the data rule and how the row will be referenced; a composite key is neither automatically superior nor inferior to a single-column key. PostgreSQL supports both multi-column primary keys and multi-column unique constraints.
What does a foreign key do?
A foreign key requires values in one table to match an eligible key in another table, protecting referential integrity. In PostgreSQL, the referenced columns must be covered by a primary key, a unique constraint, or a non-partial unique index. For example:
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CREATE TABLE orders (
order_id bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
customer_id bigint NOT NULL REFERENCES customers(customer_id)
);Because customer_id is NOT NULL, every order must refer to a customer, and the foreign key rejects an order whose customer does not exist. If the relationship is optional, allow the referencing column to be null and omit NOT NULL. PostgreSQL’s foreign-key tutorial demonstrates that a reference to a nonexistent parent row is rejected.
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Composite foreign keys and nulls
A foreign key may reference a group of columns. Under PostgreSQL’s default MATCH SIMPLE behavior, a referencing row does not need a match if any referencing column is null. MATCH FULL permits that exception only when all referencing columns are null; a mix of null and non-null values does not qualify. Make the columns NOT NULL when partial or absent references should not be allowed.
CREATE TABLE shipment_items (
shipment_id bigint NOT NULL,
item_number integer NOT NULL,
product_id bigint NOT NULL REFERENCES products(product_id),
PRIMARY KEY (shipment_id, item_number)
);How do I model relationships?
Start by deciding whether a relationship is mandatory or optional for each row, then choose where the foreign key belongs. A foreign key on the many-side is the common relational pattern for a one-to-many relationship: each order refers to one customer, while multiple orders can refer to that customer. The orders.customer_id example above expresses that structure; making it NOT NULL makes the association mandatory for each order.
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For a many-to-many relationship, represent the association as its own table with foreign keys to both related tables. A composite primary key or unique constraint on the two foreign-key columns prevents the same pair from appearing twice, as shown by group_memberships. If the association has its own attributes—such as a join date or role—they belong on that table.
For a one-to-one relationship, a foreign key alone does not ensure that each referenced row is used only once. Add a UNIQUE constraint to the referencing column (or make it the referencing table’s primary key) if the schema must prevent multiple rows from pointing to the same parent. Whether the reference is also mandatory depends on whether that row is required to have a parent.
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Choose a foreign-key action according to the relationship’s meaning and data-retention needs; do not apply cascading deletion as a default. PostgreSQL supports actions including CASCADE, SET NULL, SET DEFAULT, RESTRICT, and NO ACTION. The constraint documentation describes their behavior and timing.
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| Action | Effect | Use when |
|---|---|---|
CASCADE |
Deletes dependent rows when the referenced row is deleted; an update can likewise propagate a changed referenced key. | The dependent row should share the referenced row’s lifecycle. |
SET NULL |
Sets the referencing columns to null when the referenced row is deleted or updated. | The relationship is optional and the referencing columns permit nulls. |
SET DEFAULT |
Sets the referencing columns to their defaults when the referenced row is deleted or updated. | The default is meaningful and satisfies the foreign-key requirement. |
RESTRICT |
Blocks the referenced delete or update when it would affect the constraint. | The operation must not proceed while dependent references exist. |
NO ACTION |
Checks the constraint at its applicable check time; it is PostgreSQL’s default action. | You want the default check behavior rather than automatic changes to dependent rows. |
PostgreSQL distinguishes RESTRICT from NO ACTION in timing: NO ACTION can permit a statement’s resulting state to pass when the check is deferred, while RESTRICT prevents the operation immediately. The exact choice matters most when constraints are deferred or updates are involved.
CREATE TABLE order_lines (
order_id bigint NOT NULL REFERENCES orders(order_id) ON DELETE CASCADE,
line_number integer NOT NULL,
product_id bigint NOT NULL REFERENCES products(product_id),
PRIMARY KEY (order_id, line_number)
);This example makes order lines follow an order’s lifecycle. It does not cascade product deletion, because preserving the order line’s product reference may be important; the default restrictive behavior prevents deleting a referenced product until the reference is addressed.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which constraints belong in the schema?
Constraints make the database reject invalid writes no matter which application or process attempts them. Use the constraint that matches the rule:
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NOT NULL: a value is required.UNIQUE: a value, or combination of values, must not be duplicated.FOREIGN KEY: a reference must point to an eligible row in another table.CHECK: each inserted or updated row must satisfy a predicate.
For example, to prevent negative line-item quantities:
CREATE TABLE inventory_adjustments (
adjustment_id bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
quantity_change integer NOT NULL CHECK (quantity_change <> 0)
);Keep CHECK rules row-level
In PostgreSQL, a CHECK constraint is appropriate for a condition evaluated on the row being inserted or updated. Do not rely on it to enforce a rule involving other rows or tables: PostgreSQL warns that such a check cannot guarantee consistency when those other rows change. Use a suitable unique, exclusion, or foreign-key constraint for cross-row integrity where one fits. See the PostgreSQL 17 constraints documentation.
Do foreign keys create indexes?
In PostgreSQL, primary keys and unique constraints create unique B-tree indexes. PostgreSQL does not automatically create an index on the referencing foreign-key columns. An index on those columns can help joins and filters, and can make checks associated with deleting or updating a referenced row more efficient, but it also takes storage and adds work to writes.
Add such an index when table size, query patterns, parent-row maintenance, or observed query plans justify it—not automatically on every foreign key. For a frequently queried order list by customer, for example, an index may be useful:
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