---
title: Constraints
description: NOT NULL, UNIQUE, DEFAULT, and CHECK constraints that keep data valid.
sidebar:
  order: 6
---

Constraints are rules the database enforces on a column's values, so invalid
data can never be stored in the first place. There are four:

| Constraint | Rule |
|---|---|
| `not null` | The column must always have a value. |
| `unique` | No two rows may share a value (empty values aside). |
| `default <value>` | The value to use when an `insert` omits the column. |
| `check (<expr>)` | Every value must satisfy a condition you write. |

## Adding constraints

You can add a constraint to an existing column at any time. In the
[example library](/getting-started/example-library/), a book's publication year
should be positive, and a sensible fallback is useful when it is unknown:

```rdbms
add constraint default 2000 to books.published
add constraint check (published > 1400) to books.published
```

Constraints show up in the column's **Constraints** cell in `show table`:

```rdbms
show table books
```

```
books
┌───────────┬────────┬──────────────────────────────────────────┐
│ Name      │ Type   │ Constraints                              │
├───────────┼────────┼──────────────────────────────────────────┤
│ book_id   │ serial │ PK                                       │
│ title     │ text   │ NOT NULL                                 │
│ author_id │ int    │                                          │
│ published │ int    │ DEFAULT 2000, CHECK ("published" > 1400) │
│ isbn      │ text   │ UNIQUE                                   │
└───────────┴────────┴──────────────────────────────────────────┘
```

Here `title` is `NOT NULL` and `isbn` is `UNIQUE` — both were declared when the
table was built (see below). `PK` marks the primary key, which is fixed at
creation time ([Tables](/reference/tables/)).

## Declaring constraints when you build the table

Constraints can also be part of a column's spec from the start. In advanced
mode they sit inline in `CREATE TABLE`:

```sql
create table books (
  book_id serial primary key,
  title text not null,
  author_id int,
  published int default 2000 check (published > 1400),
  isbn text unique
)
```

## Adding a constraint to existing data

When you add a constraint to a column that already holds rows, the playground
checks the current data first. If any row would break the new rule — a `null`
where you are adding `not null`, a duplicate where you are adding `unique`, or a
value that fails a `check` — the change is **refused** and the offending rows
are listed, so you can fix the data and try again. Nothing is changed until the
constraint can hold for every row.

## Removing a constraint

Name the kind to drop it (there is at most one of each kind per column):

```rdbms
drop constraint check from books.published
drop constraint default from books.published
```

## In advanced mode

A table-level `check` or `unique` can be added and dropped with `ALTER TABLE`;
a named constraint can then be dropped by that name:

```sql
alter table books add constraint published_range check (published > 1400)
alter table books add unique (title, author_id)
alter table books drop constraint published_range
```

## Syntax

```rdbms-syntax
add constraint not null to <Table>.<col>
add constraint unique to <Table>.<col>
add constraint default <value> to <Table>.<col>
add constraint check (<expr>) to <Table>.<col>
drop constraint (not null | unique | default | check) from <Table>.<col>
```

See also [Columns](/reference/columns/), [Types](/reference/types/), and
[Inserting & editing data](/reference/inserting-and-editing-data/).