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What are MySQL column and table constraints?

Constraints are user defined requirements that define what values are valid for a column or table. You can think of them as additional restrictions to narrow in on acceptable values more strictly than data types allow.

Constraints allow you to define qualities that all entries must have, with the server itself enforcing the restrictions upon data entry or update. As an example, it might not make sense for a column representing boiling point of various substances to be lower than its freezing point. A constraint can enforce this type of requirement, even though types would not be able to.

Where constraints are defined: column vs table constraints

MySQL allows you to create constraints associated with a specific column or with a table in general.

Almost all constraints can be used in both forms without modification:

ConstraintColumnTable
CHECKYesYes
NOT NULLYesNo*
UNIQUEYesYes
PRIMARY KEYYesYes
FOREIGN KEYNoYes

*: NOT NULL cannot be used as a table constraint. However, you can approximate the results by using IS NOT NULL as the statement within a CHECK table constraint.

Let's look at how column and table constraints differ.

Column constraints

Column constraints are constraints attached to a single column. They are used to determine whether a proposed value for a column is valid or not. Column constraints are evaluated after the input is validated against basic type requirements (like making sure a value is a whole number for int columns).

Column constraints are great for expressing requirements that are limited to a single field. They attach the constraint condition directly to the column involved. For instance, we could model the age restriction in a person table by adding a constraint after the column name and data type:

CREATE TABLE person (
. . .
age INT CHECK (age >= 0),
. . .
);

This snippet defines a person table with one of the columns being an int called age. The age must be greater than or equal to zero. Column constraints are easy to understand because they are added as additional requirements onto the column they affect.

Table constraints

The other type of constraint is called a table constraint. Table constraints can express almost any restrictions that a column constraint can, but can additionally express restrictions that involve more than one column. Instead of being attached to a specific column, table constraints are defined as a separate component of the table and can reference any of the table's columns.

The column constraint we saw earlier could be expressed as a table constraint like this:

CREATE TABLE person (
. . .
age INT,
. . .
CHECK (age >= 0)
);

The same basic syntax is used, but the constraint is listed separately. To take advantage of the ability for table constraints to introduce compound restrictions, we can use the logical AND operator to join multiple conditions from different columns.

For example, in a banking database, a table called qualified_borrowers might need to check whether individuals have an existing account and the ability to offer collateral in order to qualify for a loan. It might make sense to include both of these in the same check:

CREATE TABLE qualified_borrowers (
. . .
account_number INT,
acceptable_collateral BOOLEAN,
. . .
CHECK (account_number IS NOT NULL AND acceptable_collateral = true)
);

Here, we use the CHECK constraint again to check that the account_number is not null and that the loan officer has marked the client as having acceptable collateral by checking the acceptable_collateral column. A table constraint is necessary since multiple columns are being checked.

Now is a good time to mention that although we'll mainly be using the CREATE TABLE SQL command in these examples to create a new table, you can also add constraints to an existing table with ALTER TABLE. When using ALTER TABLE, new constraints cause the values currently in the table to be checked against the new constraint. If the values violate the constraint, the constraint cannot be added.

Creating names for constraints

Default constraint names

When you create constraints using the syntax above, MySQL automatically chooses a reasonable, but vague, name. In the case of the qualified_borrowers table above, MySQL would name the constraint qualified_borrowers_chk_1:

INSERT INTO qualified_borrowers VALUES (123, false);
ERROR 3819 (HY000): Check constraint 'qualified_borrowers_chk_1' is violated.

This name gives you information about the table and type of constraint when a constraint is violated. In cases where multiple constraints are present on a table, however, more descriptive names are helpful to help troubleshooting.

Custom constraint names

You can optionally specify the name for your constraints by preceding the constraint definition with the CONSTRAINT keyword followed by the name.

The basic syntax for adding a custom name is this:

CONSTRAINT <constraint_name> <constraint_type_and_details>

For example, if you wanted to name the constraint in the qualified_borrowers table loan_worthiness, you could instead define the table like this:

CREATE TABLE qualified_borrowers (
. . .
account_number INT,
acceptable_collateral BOOLEAN,
. . .
CONSTRAINT loan_worthiness CHECK (account_number IS NOT NULL AND acceptable_collateral = true)
);

Now, when we violate a constraint, we get our more descriptive label:

INSERT INTO qualified_borrowers VALUES (123, false);
ERROR 3819 (HY000): Check constraint 'loan_worthiness' is violated.

You can name column constraints in the same way:

CREATE TABLE teenagers (
. . .
age INT CONSTRAINT is_teenager CHECK (age >= 13 AND age <= 19),
. . .
);

MySQL's list of available constraints

Now that we've covered some of the basics of how constraints work, we can take a deeper look at what constraints are available and how they may be used.

Check constraints

Check constraints are a general purpose constraint that allows you to specify an expression involving column or table values that evaluates to a boolean.

You've already seen a few examples of check constraints earlier. Check constraints begin with the keyword CHECK and then provide an expression enclosed in parentheses. For column constraints, this is placed after the data type declaration. For table constraints, these can be placed anywhere after the columns that they interact with are defined.

For example, we can create a film_nominations table that contains films that have been nominated and are eligible for a feature length award for 2019:

CREATE TABLE film_nominations (
title VARCHAR(250),
director VARCHAR(250),
release_date DATE CHECK ('2019-01-01' <= release_date AND release_date <= '2019-12-31'),
length INT,
votes INT,
CHECK (votes >= 10 AND length >= 40)
);

We have one column check restraint that checks that the release_date is within 2019. Afterwards, we have a table check constraint ensuring that the film has received enough votes to be nominated and that the length qualifies it for the "feature length" category.

When evaluating check constraints, acceptable values evaluate as being true. If the new record's values satisfy all type requirements and constraints, the record will be added to the table:

INSERT INTO film_nominations VALUES (
'A great film',
'Talented director',
'2019-07-16',
117,
45
);
Query OK, 1 row affected (0.01 sec)

Values that evaluate to false produce an error indicating that the constraint was not satisfied:

INSERT INTO film_nominations VALUES (
'A poor film',
'Misguided director',
'2019-10-24',
128,
1
);
ERROR 3819 (HY000): Check constraint 'film_nominations_chk_2' is violated.

In this case, the film has satisfied every condition except for the number of votes required. MySQL rejects the submission since it does not pass the final table check constraint.

Not null constraints

The NOT NULL constraint is much more focused. It guarantees that values within a column are not null. While this is a simple constraint, it is used very frequently.

How to add not null constraints in MySQL

To mark a column as requiring a non-null value, add NOT NULL after the type declaration:

CREATE TABLE national_capitals (
country VARCHAR(250) NOT NULL,
capital VARCHAR(250) NOT NULL
);

In the above example, we have a simple two column table mapping countries to their national capitals. Since both of these are required fields that would not make sense to leave blank, we add the NOT NULL constraint.

Inserting a null value now results in an error:

INSERT INTO national_capitals VALUES (
NULL,
'London',
);
ERROR 1048 (23000): Column 'country' cannot be null

The NOT NULL constraint functions only as a column constraint (it cannot be used as a table constraint). However, you can easily work around this by using IS NOT NULL within a table CHECK constraint.

For example, this offers equivalent guarantees using a table constraint:

CREATE TABLE national_capitals (
country VARCHAR(250),
capital VARCHAR(250),
CHECK (country IS NOT NULL AND capital IS NOT NULL)
);
RELATED ON PRISMA.IO

When working with Prisma Client, you can control whether each field is optional or mandatory to get equivalent functionality to the NOT NULL constraint in PostgreSQL.

Unique constraints

The UNIQUE constraint tells MySQL that each value within a column must not be repeated. This is useful in many different scenarios where having the same value in multiple records should be impossible.

For example, columns that deals with IDs of any kind should, by definition, have unique values. A social security number, a student or customer ID, or a product UPC (barcode number) would be useless if they were not able to differentiate between specific people or items.

A UNIQUE constraint can be specified at the column level:

CREATE TABLE supplies (
supply_id INT UNIQUE,
name VARCHAR(250),
inventory INT
);

They can also be specified as table constraints:

CREATE TABLE supplies (
supply_id INT,
name VARCHAR(250),
inventory INT,
UNIQUE (supply_id)
);

One of the advantages of using UNIQUE table constraints is that it allows you to perform uniqueness checks on a combination of columns. This works by specifying two or more columns that MySQL should evaluate together. The values in individual columns may repeat but the combination of values specified must be unique.

As an example, let's look back at the national_capitals table we used before:

CREATE TABLE national_capitals (
country VARCHAR(250) NOT NULL,
capital VARCHAR(250) NOT NULL,
);

If we wanted to make sure that we don't add multiple records for the same pair, we could add UNIQUE constraints to the columns here:

CREATE TABLE national_capitals (
country VARCHAR(250) NOT NULL UNIQUE,
capital VARCHAR(250) NOT NULL UNIQUE
);

This would ensure that both the countries and capitals are only present once in each table. However, some countries have multiple capitals. This would mean we may have multiple entries with the same country value. These wouldn't work with the current design:

INSERT INTO national_capitals VALUES (
'Bolivia',
'Sucre'
);
INSERT INTO national_capitals VALUES (
'Bolivia',
'La Paz'
);
ERROR 1062 (23000): Duplicate entry 'Bolivia' for key 'national_capitals.country'

If we still want to make sure we don't end up with duplicate entries while allowing for repeated values in individual columns, a unique check on the combination of country and capital would suffice:

CREATE TABLE national_capitals (
country VARCHAR(250),
capital VARCHAR(250),
UNIQUE (country, capital)
);

Now, we can add both of Bolivia's capitals to the table without an error:

INSERT INTO national_capitals VALUES (
'Bolivia',
'Sucre'
);
INSERT INTO national_capitals VALUES (
'Bolivia',
'La Paz'
);
Query OK, 1 row affected (0.00 sec)
Query OK, 1 row affected (0.00 sec)

However, attempting to add the same combination twice is still caught by the constraint:

INSERT INTO national_capitals VALUES (
'Bolivia',
'Sucre'
);
INSERT INTO national_capitals VALUES (
'Bolivia',
'Sucre'
);
Query OK, 1 row affected (0.00 sec)
ERROR 1062 (23000): Duplicate entry 'Bolivia-Sucre' for key 'national_capitals.country'

Primary key constraints

The PRIMARY KEY constraint serves a special purpose. It indicates that the column can be used to uniquely identify a record within the table. This means that it must be reliably unique and that every record must have a value in that column.

Primary keys are recommended for every table but not required, and every table may only have one primary key. Primary keys are mainly used to identify, retrieve, modify, or delete individual records within a table. They allow users and administrators to target the operation using an identifier that is guaranteed by MySQL to match exactly one record.

Let's use the supplies table we saw before as an example:

CREATE TABLE supplies (
supply_id INT UNIQUE,
name VARCHAR(250),
inventory INT
);

Here we've identified that the supply_id should be unique. If we wanted to use this column as our primary key (guaranteeing uniqueness and a non-null value), we could simply change the UNIQUE constraint to PRIMARY KEY:

CREATE TABLE supplies (
supply_id INT PRIMARY KEY,
name VARCHAR(250),
inventory INT
);

This way, if we needed to update the inventory amounts for a specific supply, we could target it using the primary key:

INSERT INTO supplies VALUES (
38,
'nails',
5
);
UPDATE supplies set inventory = 10 WHERE supply_id = 38;
Query OK, 1 row affected (0.00 sec)
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0

While many tables use a single column as the primary key, it is also possible to create a primary key using a set of columns, as a table constraint.

The national_capitals table is a good candidate to demonstrate this. If we wanted to create a primary key using the existing columns, we could replace the UNIQUE table constraint with PRIMARY KEY:

CREATE TABLE national_capitals (
country VARCHAR(250),
capital VARCHAR(250),
PRIMARY KEY (country, capital)
);

Foreign key constraints

Foreign keys are columns within one table that reference column values within another table. This is desirable and often necessary in a variety of scenarios where tables contain related data. This ability for the database to easily connect and reference data stored in separate tables is one of the primary features of relational databases.

For example, you may have a orders table to track individual orders and a customers table to track contact info and information about your customers. It makes sense to put this information separately since customers may have many orders. However, it also makes sense to be able to easily link the records in these two tables to allow more complex operations.

How to create foreign key constraints in MySQL

Let's start by trying to model the customers table:

CREATE TABLE customers (
customer_id SERIAL PRIMARY KEY,
first_name VARCHAR(250),
last_name VARCHAR(250),
phone_number BIGINT,
);

This table is pretty simple. It includes columns to store the customer's first name, last name, and phone number. It also specifies an ID column that uses the PRIMARY KEY constraint. The serial alias is used to automatically generate the next ID in the sequence if an ID is not specified.

For the orders table, we want to be able to specify information about individual orders. One essential piece of data is what customer placed the order. We can use a foreign key to link the order to the customer without duplicating information. We do this with the FOREIGN KEY constraint, which defines a foreign key relationship to a column in another table:

CREATE TABLE orders (
order_id SERIAL PRIMARY KEY,
order_date DATE,
customer BIGINT UNSIGNED,
FOREIGN KEY (customer) REFERENCES customers(customer_id)
);

Here, we are indicating that the customer column in the orders table has a foreign key relationship with the customer_id column in the customers table.

We have to ensure that the type of the foreign key column is compatible with the type used in the foreign table. The customer_id column in the customers table uses the SERIAL alias, which stands for BIGINT UNSIGNED NOT NULL AUTO_INCREMENT UNIQUE, so we can use BIGINT UNSIGNED as our data type for the customer column in the orders table to match.

If we try to insert a value into the orders table that doesn't reference a valid customer, MySQL will reject it:

INSERT INTO orders VALUES (
100,
'2019-11-19',
300
);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`prisma`.`orders`, CONSTRAINT `orders_ibfk_1` FOREIGN KEY (`customer`) REFERENCES `customers` (`customer_id`))

If we add the customer first, our order will then be accepted by the system:

INSERT INTO customers VALUES (
300,
'Jill',
'Smith',
5551235677
);
INSERT INTO orders VALUES (
100,
'2019-11-19',
300
);
Query OK, 1 row affected (0.00 sec)
Query OK, 1 row affected (0.00 sec)

While the primary key is a great candidate for foreign keys because it guarantees to match only one record, you can also use other columns as long as they're unique.

You can also use sets of columns that are guaranteed unique:

CREATE TABLE example (
. . .
FOREIGN KEY (column1, column2) REFERENCES other_table (column1, column2)
);
RELATED ON PRISMA.IO

We cover how to define relations in the Prisma schema in our documentation.

Deciding what to do with foreign keys when deleting or updating

One consideration you'll need to think about when defining foreign key constraints is what to do when a referenced table value is deleted or updated.

As an example, let's look at the customers and orders tables again. We need to specify how we want the system to respond when we delete a customer from the customers table when the customer has an associated order in the orders table.

We can choose between the following options:

  • RESTRICT: Choosing to restrict deletions means that MySQL will refuse to delete the customer record if it's referenced by a record in the orders table. To delete a customer, you will first have to remove any associated records from the orders table. Only then will you be able to remove the value from the customer table. This is the default action.
  • CASCADE: Selecting the cascade option means that when we delete the customer record, the records that reference it in the orders table are also deleted. This is useful in many cases but must be used with care to avoid deleting data by mistake.
  • NO ACTION: Although some other database systems allow you to defer checks with the NO ACTION option, in MySQL, this is equivalent to RESTRICT. The system will reject the update or deletion request.
  • SET NULL: This option tells MySQL to set the referencing columns to NULL when the referenced records are removed. So if we delete a customer from the customers table, the customer column in the orders table will be set to NULL.
  • SET DEFAULT: Although some other database systems allow you to set a column to a default value in case of a reference deletion or update, MySQL does not actually allow this action and will not let you define tables using this option.

These actions can be specified when defining a foreign key constraint by adding ON DELETE followed by the action. So if we want to remove associated orders from our system when a customer is deleted, we could specify that like this:

CREATE TABLE orders (
order_id SERIAL PRIMARY KEY,
order_date DATE,
customer INT,
FOREIGN KEY (customer) REFERENCES customers(customer_id) ON DELETE CASCADE
);

These type of actions can also be applied when updating a referenced column instead of deleting one by using ON UPDATE instead of ON DELETE.

Conclusion

In this guide, we covered what constraints are and how they can help you control the data that is entered into your MySQL tables. We discussed the difference between column and table constraints and the increased flexibility offered by using the table format. We then went over what constraints MySQL supports and how to use them in your tables.

Constraints help you define the exact requirements of your table columns and as such, they are indispensable in many scenarios. Understanding the way various constraints work and what scenarios they help you prevent will go a long way into ensuring that you data conforms to the standards you require. Once defined, MySQL can help you enforce constraints automatically to prevent problems before they occur.