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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>5.4. Constraints</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="ddl-generated-columns.html" title="5.3. Generated Columns" /><link rel="next" href="ddl-system-columns.html" title="5.5. System Columns" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">5.4. Constraints</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ddl-generated-columns.html" title="5.3. Generated Columns">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><th width="60%" align="center">Chapter 5. Data Definition</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ddl-system-columns.html" title="5.5. System Columns">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="DDL-CONSTRAINTS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">5.4. Constraints</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-CHECK-CONSTRAINTS">5.4.1. Check Constraints</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#id-1.5.4.6.6">5.4.2. Not-Null Constraints</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-UNIQUE-CONSTRAINTS">5.4.3. Unique Constraints</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-PRIMARY-KEYS">5.4.4. Primary Keys</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-FK">5.4.5. Foreign Keys</a></span></dt><dt><span class="sect2"><a href="ddl-constraints.html#DDL-CONSTRAINTS-EXCLUSION">5.4.6. Exclusion Constraints</a></span></dt></dl></div><a id="id-1.5.4.6.2" class="indexterm"></a><p>
Data types are a way to limit the kind of data that can be stored
in a table. For many applications, however, the constraint they
provide is too coarse. For example, a column containing a product
price should probably only accept positive values. But there is no
standard data type that accepts only positive numbers. Another issue is
that you might want to constrain column data with respect to other
columns or rows. For example, in a table containing product
information, there should be only one row for each product number.
</p><p>
To that end, SQL allows you to define constraints on columns and
tables. Constraints give you as much control over the data in your
tables as you wish. If a user attempts to store data in a column
that would violate a constraint, an error is raised. This applies
even if the value came from the default value definition.
</p><div class="sect2" id="DDL-CONSTRAINTS-CHECK-CONSTRAINTS"><div class="titlepage"><div><div><h3 class="title">5.4.1. Check Constraints</h3></div></div></div><a id="id-1.5.4.6.5.2" class="indexterm"></a><a id="id-1.5.4.6.5.3" class="indexterm"></a><p>
A check constraint is the most generic constraint type. It allows
you to specify that the value in a certain column must satisfy a
Boolean (truth-value) expression. For instance, to require positive
product prices, you could use:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer,
name text,
price numeric <span class="emphasis"><strong>CHECK (price > 0)</strong></span>
);
</pre><p>
</p><p>
As you see, the constraint definition comes after the data type,
just like default value definitions. Default values and
constraints can be listed in any order. A check constraint
consists of the key word <code class="literal">CHECK</code> followed by an
expression in parentheses. The check constraint expression should
involve the column thus constrained, otherwise the constraint
would not make too much sense.
</p><a id="id-1.5.4.6.5.6" class="indexterm"></a><p>
You can also give the constraint a separate name. This clarifies
error messages and allows you to refer to the constraint when you
need to change it. The syntax is:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer,
name text,
price numeric <span class="emphasis"><strong>CONSTRAINT positive_price</strong></span> CHECK (price > 0)
);
</pre><p>
So, to specify a named constraint, use the key word
<code class="literal">CONSTRAINT</code> followed by an identifier followed
by the constraint definition. (If you don't specify a constraint
name in this way, the system chooses a name for you.)
</p><p>
A check constraint can also refer to several columns. Say you
store a regular price and a discounted price, and you want to
ensure that the discounted price is lower than the regular price:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer,
name text,
price numeric CHECK (price > 0),
discounted_price numeric CHECK (discounted_price > 0),
<span class="emphasis"><strong>CHECK (price > discounted_price)</strong></span>
);
</pre><p>
</p><p>
The first two constraints should look familiar. The third one
uses a new syntax. It is not attached to a particular column,
instead it appears as a separate item in the comma-separated
column list. Column definitions and these constraint
definitions can be listed in mixed order.
</p><p>
We say that the first two constraints are column constraints, whereas the
third one is a table constraint because it is written separately
from any one column definition. Column constraints can also be
written as table constraints, while the reverse is not necessarily
possible, since a column constraint is supposed to refer to only the
column it is attached to. (<span class="productname">PostgreSQL</span> doesn't
enforce that rule, but you should follow it if you want your table
definitions to work with other database systems.) The above example could
also be written as:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer,
name text,
price numeric,
CHECK (price > 0),
discounted_price numeric,
CHECK (discounted_price > 0),
CHECK (price > discounted_price)
);
</pre><p>
or even:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer,
name text,
price numeric CHECK (price > 0),
discounted_price numeric,
CHECK (discounted_price > 0 AND price > discounted_price)
);
</pre><p>
It's a matter of taste.
</p><p>
Names can be assigned to table constraints in the same way as
column constraints:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer,
name text,
price numeric,
CHECK (price > 0),
discounted_price numeric,
CHECK (discounted_price > 0),
<span class="emphasis"><strong>CONSTRAINT valid_discount</strong></span> CHECK (price > discounted_price)
);
</pre><p>
</p><a id="id-1.5.4.6.5.12" class="indexterm"></a><p>
It should be noted that a check constraint is satisfied if the
check expression evaluates to true or the null value. Since most
expressions will evaluate to the null value if any operand is null,
they will not prevent null values in the constrained columns. To
ensure that a column does not contain null values, the not-null
constraint described in the next section can be used.
</p><div class="note"><h3 class="title">Note</h3><p>
<span class="productname">PostgreSQL</span> does not support
<code class="literal">CHECK</code> constraints that reference table data other than
the new or updated row being checked. While a <code class="literal">CHECK</code>
constraint that violates this rule may appear to work in simple
tests, it cannot guarantee that the database will not reach a state
in which the constraint condition is false (due to subsequent changes
of the other row(s) involved). This would cause a database dump and
reload to fail. The reload could fail even when the complete
database state is consistent with the constraint, due to rows not
being loaded in an order that will satisfy the constraint. If
possible, use <code class="literal">UNIQUE</code>, <code class="literal">EXCLUDE</code>,
or <code class="literal">FOREIGN KEY</code> constraints to express
cross-row and cross-table restrictions.
</p><p>
If what you desire is a one-time check against other rows at row
insertion, rather than a continuously-maintained consistency
guarantee, a custom <a class="link" href="triggers.html" title="Chapter 38. Triggers">trigger</a> can be used
to implement that. (This approach avoids the dump/reload problem because
<span class="application">pg_dump</span> does not reinstall triggers until after
reloading data, so that the check will not be enforced during a
dump/reload.)
</p></div><div class="note"><h3 class="title">Note</h3><p>
<span class="productname">PostgreSQL</span> assumes that
<code class="literal">CHECK</code> constraints' conditions are immutable, that
is, they will always give the same result for the same input row.
This assumption is what justifies examining <code class="literal">CHECK</code>
constraints only when rows are inserted or updated, and not at other
times. (The warning above about not referencing other table data is
really a special case of this restriction.)
</p><p>
An example of a common way to break this assumption is to reference a
user-defined function in a <code class="literal">CHECK</code> expression, and
then change the behavior of that
function. <span class="productname">PostgreSQL</span> does not disallow
that, but it will not notice if there are rows in the table that now
violate the <code class="literal">CHECK</code> constraint. That would cause a
subsequent database dump and reload to fail.
The recommended way to handle such a change is to drop the constraint
(using <code class="command">ALTER TABLE</code>), adjust the function definition,
and re-add the constraint, thereby rechecking it against all table rows.
</p></div></div><div class="sect2" id="id-1.5.4.6.6"><div class="titlepage"><div><div><h3 class="title">5.4.2. Not-Null Constraints</h3></div></div></div><a id="id-1.5.4.6.6.2" class="indexterm"></a><a id="id-1.5.4.6.6.3" class="indexterm"></a><p>
A not-null constraint simply specifies that a column must not
assume the null value. A syntax example:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer <span class="emphasis"><strong>NOT NULL</strong></span>,
name text <span class="emphasis"><strong>NOT NULL</strong></span>,
price numeric
);
</pre><p>
</p><p>
A not-null constraint is always written as a column constraint. A
not-null constraint is functionally equivalent to creating a check
constraint <code class="literal">CHECK (<em class="replaceable"><code>column_name</code></em>
IS NOT NULL)</code>, but in
<span class="productname">PostgreSQL</span> creating an explicit
not-null constraint is more efficient. The drawback is that you
cannot give explicit names to not-null constraints created this
way.
</p><p>
Of course, a column can have more than one constraint. Just write
the constraints one after another:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer NOT NULL,
name text NOT NULL,
price numeric NOT NULL CHECK (price > 0)
);
</pre><p>
The order doesn't matter. It does not necessarily determine in which
order the constraints are checked.
</p><p>
The <code class="literal">NOT NULL</code> constraint has an inverse: the
<code class="literal">NULL</code> constraint. This does not mean that the
column must be null, which would surely be useless. Instead, this
simply selects the default behavior that the column might be null.
The <code class="literal">NULL</code> constraint is not present in the SQL
standard and should not be used in portable applications. (It was
only added to <span class="productname">PostgreSQL</span> to be
compatible with some other database systems.) Some users, however,
like it because it makes it easy to toggle the constraint in a
script file. For example, you could start with:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer NULL,
name text NULL,
price numeric NULL
);
</pre><p>
and then insert the <code class="literal">NOT</code> key word where desired.
</p><div class="tip"><h3 class="title">Tip</h3><p>
In most database designs the majority of columns should be marked
not null.
</p></div></div><div class="sect2" id="DDL-CONSTRAINTS-UNIQUE-CONSTRAINTS"><div class="titlepage"><div><div><h3 class="title">5.4.3. Unique Constraints</h3></div></div></div><a id="id-1.5.4.6.7.2" class="indexterm"></a><a id="id-1.5.4.6.7.3" class="indexterm"></a><p>
Unique constraints ensure that the data contained in a column, or a
group of columns, is unique among all the rows in the
table. The syntax is:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer <span class="emphasis"><strong>UNIQUE</strong></span>,
name text,
price numeric
);
</pre><p>
when written as a column constraint, and:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer,
name text,
price numeric,
<span class="emphasis"><strong>UNIQUE (product_no)</strong></span>
);
</pre><p>
when written as a table constraint.
</p><p>
To define a unique constraint for a group of columns, write it as a
table constraint with the column names separated by commas:
</p><pre class="programlisting">
CREATE TABLE example (
a integer,
b integer,
c integer,
<span class="emphasis"><strong>UNIQUE (a, c)</strong></span>
);
</pre><p>
This specifies that the combination of values in the indicated columns
is unique across the whole table, though any one of the columns
need not be (and ordinarily isn't) unique.
</p><p>
You can assign your own name for a unique constraint, in the usual way:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer <span class="emphasis"><strong>CONSTRAINT must_be_different</strong></span> UNIQUE,
name text,
price numeric
);
</pre><p>
</p><p>
Adding a unique constraint will automatically create a unique B-tree
index on the column or group of columns listed in the constraint.
A uniqueness restriction covering only some rows cannot be written as
a unique constraint, but it is possible to enforce such a restriction by
creating a unique <a class="link" href="indexes-partial.html" title="11.8. Partial Indexes">partial index</a>.
</p><a id="id-1.5.4.6.7.8" class="indexterm"></a><p>
In general, a unique constraint is violated if there is more than
one row in the table where the values of all of the
columns included in the constraint are equal.
However, two null values are never considered equal in this
comparison. That means even in the presence of a
unique constraint it is possible to store duplicate
rows that contain a null value in at least one of the constrained
columns. This behavior conforms to the SQL standard, but we have
heard that other SQL databases might not follow this rule. So be
careful when developing applications that are intended to be
portable.
</p></div><div class="sect2" id="DDL-CONSTRAINTS-PRIMARY-KEYS"><div class="titlepage"><div><div><h3 class="title">5.4.4. Primary Keys</h3></div></div></div><a id="id-1.5.4.6.8.2" class="indexterm"></a><a id="id-1.5.4.6.8.3" class="indexterm"></a><p>
A primary key constraint indicates that a column, or group of columns,
can be used as a unique identifier for rows in the table. This
requires that the values be both unique and not null. So, the following
two table definitions accept the same data:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer UNIQUE NOT NULL,
name text,
price numeric
);
</pre><p>
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer <span class="emphasis"><strong>PRIMARY KEY</strong></span>,
name text,
price numeric
);
</pre><p>
</p><p>
Primary keys can span more than one column; the syntax
is similar to unique constraints:
</p><pre class="programlisting">
CREATE TABLE example (
a integer,
b integer,
c integer,
<span class="emphasis"><strong>PRIMARY KEY (a, c)</strong></span>
);
</pre><p>
</p><p>
Adding a primary key will automatically create a unique B-tree index
on the column or group of columns listed in the primary key, and will
force the column(s) to be marked <code class="literal">NOT NULL</code>.
</p><p>
A table can have at most one primary key. (There can be any number
of unique and not-null constraints, which are functionally almost the
same thing, but only one can be identified as the primary key.)
Relational database theory
dictates that every table must have a primary key. This rule is
not enforced by <span class="productname">PostgreSQL</span>, but it is
usually best to follow it.
</p><p>
Primary keys are useful both for
documentation purposes and for client applications. For example,
a GUI application that allows modifying row values probably needs
to know the primary key of a table to be able to identify rows
uniquely. There are also various ways in which the database system
makes use of a primary key if one has been declared; for example,
the primary key defines the default target column(s) for foreign keys
referencing its table.
</p></div><div class="sect2" id="DDL-CONSTRAINTS-FK"><div class="titlepage"><div><div><h3 class="title">5.4.5. Foreign Keys</h3></div></div></div><a id="id-1.5.4.6.9.2" class="indexterm"></a><a id="id-1.5.4.6.9.3" class="indexterm"></a><a id="id-1.5.4.6.9.4" class="indexterm"></a><p>
A foreign key constraint specifies that the values in a column (or
a group of columns) must match the values appearing in some row
of another table.
We say this maintains the <em class="firstterm">referential
integrity</em> between two related tables.
</p><p>
Say you have the product table that we have used several times already:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer PRIMARY KEY,
name text,
price numeric
);
</pre><p>
Let's also assume you have a table storing orders of those
products. We want to ensure that the orders table only contains
orders of products that actually exist. So we define a foreign
key constraint in the orders table that references the products
table:
</p><pre class="programlisting">
CREATE TABLE orders (
order_id integer PRIMARY KEY,
product_no integer <span class="emphasis"><strong>REFERENCES products (product_no)</strong></span>,
quantity integer
);
</pre><p>
Now it is impossible to create orders with non-NULL
<code class="structfield">product_no</code> entries that do not appear in the
products table.
</p><p>
We say that in this situation the orders table is the
<em class="firstterm">referencing</em> table and the products table is
the <em class="firstterm">referenced</em> table. Similarly, there are
referencing and referenced columns.
</p><p>
You can also shorten the above command to:
</p><pre class="programlisting">
CREATE TABLE orders (
order_id integer PRIMARY KEY,
product_no integer <span class="emphasis"><strong>REFERENCES products</strong></span>,
quantity integer
);
</pre><p>
because in absence of a column list the primary key of the
referenced table is used as the referenced column(s).
</p><p>
You can assign your own name for a foreign key constraint,
in the usual way.
</p><p>
A foreign key can also constrain and reference a group of columns.
As usual, it then needs to be written in table constraint form.
Here is a contrived syntax example:
</p><pre class="programlisting">
CREATE TABLE t1 (
a integer PRIMARY KEY,
b integer,
c integer,
<span class="emphasis"><strong>FOREIGN KEY (b, c) REFERENCES other_table (c1, c2)</strong></span>
);
</pre><p>
Of course, the number and type of the constrained columns need to
match the number and type of the referenced columns.
</p><a id="id-1.5.4.6.9.11" class="indexterm"></a><p>
Sometimes it is useful for the <span class="quote">“<span class="quote">other table</span>”</span> of a
foreign key constraint to be the same table; this is called
a <em class="firstterm">self-referential</em> foreign key. For
example, if you want rows of a table to represent nodes of a tree
structure, you could write
</p><pre class="programlisting">
CREATE TABLE tree (
node_id integer PRIMARY KEY,
parent_id integer REFERENCES tree,
name text,
...
);
</pre><p>
A top-level node would have NULL <code class="structfield">parent_id</code>,
but non-NULL <code class="structfield">parent_id</code> entries would be
constrained to reference valid rows of the table.
</p><p>
A table can have more than one foreign key constraint. This is
used to implement many-to-many relationships between tables. Say
you have tables about products and orders, but now you want to
allow one order to contain possibly many products (which the
structure above did not allow). You could use this table structure:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer PRIMARY KEY,
name text,
price numeric
);
CREATE TABLE orders (
order_id integer PRIMARY KEY,
shipping_address text,
...
);
CREATE TABLE order_items (
product_no integer REFERENCES products,
order_id integer REFERENCES orders,
quantity integer,
PRIMARY KEY (product_no, order_id)
);
</pre><p>
Notice that the primary key overlaps with the foreign keys in
the last table.
</p><a id="id-1.5.4.6.9.14" class="indexterm"></a><a id="id-1.5.4.6.9.15" class="indexterm"></a><p>
We know that the foreign keys disallow creation of orders that
do not relate to any products. But what if a product is removed
after an order is created that references it? SQL allows you to
handle that as well. Intuitively, we have a few options:
</p><div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: disc; "><li class="listitem"><p>Disallow deleting a referenced product</p></li><li class="listitem"><p>Delete the orders as well</p></li><li class="listitem"><p>Something else?</p></li></ul></div><p>
</p><p>
To illustrate this, let's implement the following policy on the
many-to-many relationship example above: when someone wants to
remove a product that is still referenced by an order (via
<code class="literal">order_items</code>), we disallow it. If someone
removes an order, the order items are removed as well:
</p><pre class="programlisting">
CREATE TABLE products (
product_no integer PRIMARY KEY,
name text,
price numeric
);
CREATE TABLE orders (
order_id integer PRIMARY KEY,
shipping_address text,
...
);
CREATE TABLE order_items (
product_no integer REFERENCES products <span class="emphasis"><strong>ON DELETE RESTRICT</strong></span>,
order_id integer REFERENCES orders <span class="emphasis"><strong>ON DELETE CASCADE</strong></span>,
quantity integer,
PRIMARY KEY (product_no, order_id)
);
</pre><p>
</p><p>
Restricting and cascading deletes are the two most common options.
<code class="literal">RESTRICT</code> prevents deletion of a
referenced row. <code class="literal">NO ACTION</code> means that if any
referencing rows still exist when the constraint is checked, an error
is raised; this is the default behavior if you do not specify anything.
(The essential difference between these two choices is that
<code class="literal">NO ACTION</code> allows the check to be deferred until
later in the transaction, whereas <code class="literal">RESTRICT</code> does not.)
<code class="literal">CASCADE</code> specifies that when a referenced row is deleted,
row(s) referencing it should be automatically deleted as well.
There are two other options:
<code class="literal">SET NULL</code> and <code class="literal">SET DEFAULT</code>.
These cause the referencing column(s) in the referencing row(s)
to be set to nulls or their default
values, respectively, when the referenced row is deleted.
Note that these do not excuse you from observing any constraints.
For example, if an action specifies <code class="literal">SET DEFAULT</code>
but the default value would not satisfy the foreign key constraint, the
operation will fail.
</p><p>
Analogous to <code class="literal">ON DELETE</code> there is also
<code class="literal">ON UPDATE</code> which is invoked when a referenced
column is changed (updated). The possible actions are the same.
In this case, <code class="literal">CASCADE</code> means that the updated values of the
referenced column(s) should be copied into the referencing row(s).
</p><p>
Normally, a referencing row need not satisfy the foreign key constraint
if any of its referencing columns are null. If <code class="literal">MATCH FULL</code>
is added to the foreign key declaration, a referencing row escapes
satisfying the constraint only if all its referencing columns are null
(so a mix of null and non-null values is guaranteed to fail a
<code class="literal">MATCH FULL</code> constraint). If you don't want referencing rows
to be able to avoid satisfying the foreign key constraint, declare the
referencing column(s) as <code class="literal">NOT NULL</code>.
</p><p>
A foreign key must reference columns that either are a primary key or
form a unique constraint. This means that the referenced columns always
have an index (the one underlying the primary key or unique constraint);
so checks on whether a referencing row has a match will be efficient.
Since a <code class="command">DELETE</code> of a row from the referenced table
or an <code class="command">UPDATE</code> of a referenced column will require
a scan of the referencing table for rows matching the old value, it
is often a good idea to index the referencing columns too. Because this
is not always needed, and there are many choices available on how
to index, declaration of a foreign key constraint does not
automatically create an index on the referencing columns.
</p><p>
More information about updating and deleting data is in <a class="xref" href="dml.html" title="Chapter 6. Data Manipulation">Chapter 6</a>. Also see the description of foreign key constraint
syntax in the reference documentation for
<a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a>.
</p></div><div class="sect2" id="DDL-CONSTRAINTS-EXCLUSION"><div class="titlepage"><div><div><h3 class="title">5.4.6. Exclusion Constraints</h3></div></div></div><a id="id-1.5.4.6.10.2" class="indexterm"></a><a id="id-1.5.4.6.10.3" class="indexterm"></a><p>
Exclusion constraints ensure that if any two rows are compared on
the specified columns or expressions using the specified operators,
at least one of these operator comparisons will return false or null.
The syntax is:
</p><pre class="programlisting">
CREATE TABLE circles (
c circle,
EXCLUDE USING gist (c WITH &&)
);
</pre><p>
</p><p>
See also <a class="link" href="sql-createtable.html#SQL-CREATETABLE-EXCLUDE"><code class="command">CREATE
TABLE ... CONSTRAINT ... EXCLUDE</code></a> for details.
</p><p>
Adding an exclusion constraint will automatically create an index
of the type specified in the constraint declaration.
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