Constraints in SQL

Constraints In SQL: A Detailed Overview For Beginners

SQL stands for “Structured Query Language”, When a person starts learning SQL, there are various terms that they need to know about to better grasp all about SQL. As we all know, to learn something, it is very important that we go with all the basic details. If you are going to learn SQL, you should understand all the concepts to get a better understanding of SQL.

There are various important topics in a SQL course that you know. One of them is Constraints in SQL. We will clarify the word “Constraints in SQL” in this blog. We’ll do our best to cover every term that might be relevant to constraints in SQL. So let’s begin by discussing what are constraints in SQL before moving on to SQL constraints.

Definition of Constraint

Constraints are specified guidelines and limitations applied to one or more columns on the allowed values to maintain the consistency, accuracy, and dependability of the data in those columns. In other words, the data will be successfully inserted if it complies with the constraint criteria. The insert operation will fail if the inserted data violates the defined constraint. 

“Basically, we are trying to say that constraint is nothing but an operation that we put on a column to get some desired values. Let us consider an example.”

We put a constraint on a column that the selected column will only contain a numerical value. Now, what constraint will do is it will not allow you to put a character on that column. You can write any numerical value but not character. 

What are constraints in SQL?

In SQL Server, constraints can be expressed individually at the table or column level, where they are specified as part of the column definition and only apply to that particular column. In this instance, more than one column in the provided table will be subject to the constraint rules. The constraint can be applied using the ALTER TABLE T-SQL command after the table has already been formed or during the table, creation using the CREATE TABLE T-SQL command. 

In this blog, we’ll go over the six basic constraints frequently applied in SQL Server. These constraints in SQL are:- 


“NOT NULL” SQL Constraints

The columns have a default value of NULL. In SQL, a NOT NULL constraint is used to prevent the insertion of NULL values into the specified column by treating them as invalid input. Given that the column will always have data, you should give it a valid SQL NOT NULL value in the INSERT or UPDATE commands. Simply NOT NULL Constraint ensures that a column cannot have a NULL value.

For Example

In the image given below, we have created a SQL table in which we are going to take students’ data, i.e., their first name, middle name, and last name but notice one thing that we put “NOT NULL” constraint on Column having a first name and last name. So these columns can’t be null and should contain a value.


“UNIQUE” Constraints in SQL

In SQL, the UNIQUE constraint is used to prevent duplicate values from being placed into a particular column or group of columns that are not a part of the PRIMARY KEY but are instead participating in the UNIQUE constraint. 

To put it another way, the index that is automatically created when a UNIQUE constraint is defined will ensure that no two rows in that table can have the same value for the index’s participating columns, with the ability to insert only one distinct NULL value to these columns, if the column allows NULL.


In the image, you can see that we have put a constraint “UNIQUE,” So the values in this will be unique. This constraint will not allow you to put duplicate values. 

PRIMARY KEY In SQL Constraints

The primary purpose of the PRIMARY KEY is to maintain the table’s entity integrity. Each row in the table must be a distinct entity, ensured by entity integrity. The index automatically constructed when defining the SQL PRIMARY KEY will enforce data uniqueness and allow quick access to the table’s contents. 

The UNIQUE and SQL NOT NULL requirements are combined into the SQL PRIMARY KEY constraint, which states that a participating column or collection of columns cannot accept a NULL value. Keep in mind that only one PRIMARY KEY can be defined per table, and it is advised to use tiny or INT columns in the PRIMARY KEY. The combination values of all PRIMARY KEY columns must be unique even if the PRIMARY KEY is defined in multiple columns. You can insert duplicate values separately into each column, however.

The PRIMARY KEY constraint differs from the UNIQUE constraint in that you can only declare one SQL PRIMARY KEY for each table, whereas you can construct numerous UNIQUE constraints in a table. The UNIQUE constraint permits one NULL value, whereas the PRIMARY KEY does not, which is another difference.


Here, a row’s unique identification is represented by the value of the student id column. Similarly, it must be UNIQUE and cannot store NULL values.

FOREIGN KEY In SQL Constraints

Referential integrity is executed via a foreign key. Each row in another table can be uniquely identified by a field known as a foreign key. Using this constraint, you can designate a field as a foreign key. When two tables are present, and one references the other, i.e. when the identical column is present in both tables and serves as the primary key for one table, In another table, that specific column will serve as a foreign key. Students used to search for what is a foreign key in SQL Let’s Take an Example.

“CHECK” SQL Constraints

All the records in a particular column must stick to a specified measure, and this is where the CHECK constraint comes in. This constraint is typically used to enforce business logic on values in a column, preventing inaccurate data entry.

Let’s imagine, for instance, that the company has a regulation that sales made to a particular vendor, XYZ, shall not be recorded in the “Sales” table. We place a CHECK constraint on the “Vendor Name” column, which will reject any operation that attempts to insert the value “XYZ” in the “Vendor Name” column to guarantee that sales to this vendor are not recorded in our table.

For Example

The user can enter any value in a table with an age column. The user will also type in any invalid value, even negative numbers; however, if the user placed a check constraint with the criteria “age more than 18” in the age column. Due to the check constraint on the age column, “even if a user tries to insert an incorrect value—such as zero or any other value lower than 18—the age column will not accept that value. It will not enable the user to insert it.”

“DEFAULT”  SQL Constraints

SQL Server provides a column’s default value using the DEFAULT constraint. SQL Server uses its default assigned value if we do not supply a value in the INSERT statement for the column with the DEFAULT constraint. Consider the scenario where all customer orders are recorded in an order database. You can use the GETDATE() function to collect the order date without providing a specific value.




A relational database’s INDEX constraint builds indexes on a table. A relational database’s tables can become extensive and include many rows; obtaining data using SQL can occasionally be highly time-consuming under these conditions. The performance of data retrieval queries can be significantly enhanced by constructing an index. The SQL engine employs indexes, which are invisible to users, to speed up searches and queries.

SQL Constraint benefits

In the following situations, SQL Server’s constraints can be useful:

  • Application of business logic.
  • Referential integrity enforcement.
  • Avoiding the incorrect data from being stored in SQL Server tables.
  • Ensuring that column data is unique.
  • The query optimizer is aware of unique data and validates values collections, improving query performance.
  • Preventing NULL values from being stored in SQL tables.
  • Writing codes to prevent NULL while the application is displaying data.

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Foreign key vs Primary key

A relational database table’s foreign key is a column or set of columns that create a connection between the data in two tables. It designates a record in the relational database table in a unique way. A primary key is required to guarantee that the data in a given column is unique.

Can a table have 2 Primary Keys?

There can only be one primary key for each table. When you create a table, Access can automatically construct a primary key field for you, or you can select the fields you want to use instead.

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