What is a SQL constraint?
SQL constraints are rules used to ensure the integrity and accuracy of database data. Common types include: 1.NOT NULL ensure that the column is not empty; 2.UNIQUE ensures that the column value is unique; 3.PRIMARY KEY is a non-empty and unique row identifier; 4. FOREIGN KEY associates the data between tables; 5. CHECK applies conditions to column data; 6. DEFAULT sets the default value of the column. These constraints can be defined with CREATE TABLE when creating a table, or modified with ALTER TABLE afterwards. For example, NOT NULL is suitable for required fields such as mailboxes. CHECK can prevent the inventory quantity from being negative. FOREIGN KEY can avoid orphan records. Constraints can be applied directly through CREATE TABLE. For example, setting id as primary key name name, non-empty email, unique age must not be less than 18. ALTER TABLE can be added later. If you set country default to USA, different database syntax may be slightly different.
A SQL constraint is a rule applied to a column or table in a database to enforce data integrity and accuracy. These constraints ensure that the data entering or existing in the database adheres to specific conditions, preventing invalid or inconsistent information from being stored. They play a cruel role in maintaining the reliability of data across applications and systems.
Common Types of SQL Constraints
There are several commonly used SQL constraints, each serving a unique purpose when defining how data should behave within a database:
- NOT NULL – Ensures that a column cannot have a
NULLvalue. - UNIQUE – Makes sure all values in a column are distinct.
- PRIMARY KEY – Combines
NOT NULLandUNIQUE, uniquely identifying each row in a table. - FOREIGN KEY – Links data between tables by ensuring that values in one column match values in another table's primary key.
- CHECK – Enforces certain conditions on the data in a column (eg, age must be at least 18).
- DEFAULT – Sets a default value for a column if no value is provided during insertion.
These constraints can be defined either when creating a table using CREATE TABLE or added later using ALTER TABLE .
When and Why You'd Use Constraints
Constraints are most useful when you need to maintain consistency and avoid bad data. For example:
- A
NOT NULLconstraint makes sense for columns like email or phone number in a user table—fields that shouldn't be left blank. - If you're tracking product inventory, a
CHECKconstraint could prevent negative numbers from being entered into a "quantity" column. - Using a
FOREIGN KEYhelps avoid orphaned records; for instance, an order in an orders table should always link to a valid customer in the customers table.
Without these rules, your database could end up with missing, duplicated, or mismatched data, which leads to errors in reporting, application logic failures, and more maintenance headaches.
How to Apply Constraints in Practice
Applying constraints is straightforward. Here's a basic example using CREATE TABLE :
CREATE TABLE Users (
id INT PRIMARY KEY,
name VARCHAR(50) NOT NULL,
email VARCHAR(100) UNIQUE,
age INT CHECK (age >= 18)
);In this case:
- The
idcolumn serves as the primary key. -
namecan't be empty. - Each
emailmust be unique across rows. -
agemust be at least 18.
If you want to add or remove constraints later, you can use ALTER TABLE . For example, adding a new DEFAULT constraint:
ALTER TABLE Users ALTER COLUMN country SET DEFAULT 'USA';
Keep in mind that different databases (like MySQL, PostgreSQL, SQL Server) might have slight syntax differences, so always check the documentation for the system you're using.
Basically that's it.
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