Are you confident about SQL Constraints? If not, don’t you worry? Here is everything you should know about SQL Constraints. Structured Query Language (SQL) is a programming language that rules and manages structured data in a database system. 

SQL Constraints are a set of rules in the database. These rules determine if you can include or remove data from the database system. Furthermore, SQL data are structured in a table and SQL Constraints are used to ensure the validity and accuracy of the data presented in the table. 

Once you set the constraints, any applied operation in the table is meant to be followed or the operation will be aborted. It means constraints are specifically required to set a rule that can be implemented throughout a database to maintain uniformity. 

SQL Constraints

A quick orientation before digging into SQL Constraints:

One of the popular Data Base Management Systems (DBMS) is a relational database. In this database, information is structured either in facts or in dimensions. Data that can be expressed in the form of mathematical equations can be considered as facts and the rest data are dimensions. You can gain expertise in this field with the help of SQL Certification Courses in India.

Another key point is different data are collected in different tables. In a business, one can find the relation between two types of tables and in this way, relational database forms. Additionally, by applying constraints in the database, one can prevent entering corrupt data, enforce uniqueness, and document important data.

What are The Different Types of SQL Constraints:

There are two types of SQL Constraints as follows:

  1. Column SQL Constraints.
  2. Table SQL Constraints.

As those names are referring, column constraints are only applied to specific columns of a table. On the other hand, table constraints can be applied to the whole table.

SQL Constraints

Types of Column SQL Constraints are: 

  1. NOT NULL Constraint
  2. UNIQUE Constraint
  3. DEFAULT Constraint
  4. CHECK Constraint
  5. PRIMARY KEY Constraint
  6. FOREIGN KEY Constraint

Types of Table SQL Constraints are:

  1. UNIQUE Constraint
  2. CHECK Constraint
  3. PRIMARY KEY Constraint
  4. FOREIGN KEY Constraint
  5. INDEX Constraint

Above all, some popular constraints are NOT NULL, UNIQUE, CHECK, PRIMARY KEY, FOREIGN KEY, and INDEX. Those who want to gain a strong grip on SQL and related fields can explore different SQL Developer Curriculum. Let’s learn about these constraints one by one in brief. Don’t you worry, you’ve got you. 

NOT NULL SQL Constraint: 

This SQL Constraint ensures that any row for a particular column has no null or blank value. Hence, this is very important in those cases where the column contains values like salary distribution, user ID, and sale amount.

On applying: 

Specifying columns with NOT NULL in programming, some or each row in the columns has a blank value.

On declining:

 If you deny the NOT NULL constraint during the table creation, no rows can remain void.

UNIQUE SQL Constraint: 

This SQL Constraint confirms that each row has a different value in the column or the table. The presented values in the rows should be different from each other. For example, you can imagine a column that contains the employee ID. 

Remember, a table can contain various column that requires UNIQUE constraints.

On applying: 

Values can’t be repeated throughout the column or the table (since this SQL Constraint is useful for both table and column).

On declining:

Values can repeat ( you may think about the employee name column in this case).

By the same token Become an SQL Expert by learning from the industry experts.


The 1st SQL constraints are a combination of UNIQUE and NOT NULL constraints. The latter ensures the protection of the links between tables from getting impeded.

 On applying PRIMARY KEY Constraint:

The rows of the column in a table have to be UNIQUE and can NOT be NULL.

On declining PRIMARY KEY SQL Constraint:

This rule can prevent the creation of blank cells. On removing this SQL Constraint the table or the column does not need to be unique or blank.

On applying FOREIGN KEY SQL Constraint: 

Links connect between tables or columns to ensure alignment of the data. This SQL Constraint helps to protect this link.

On declining FOREIGN KEY SQL Constraint:

The purpose of this SQL Constraint is not established during the creation of tables. So, one can expect huge anomalies in data.

CHECK SQL Constraints:

To protect correct business logic throughout the columns, the cells of a table must contain CHECK constraints. Your manager doesn’t need to see the Sales amount in the place of the Name of Business Development Executive column. So, one must enforce a SQL FOREIGN constraint. Since that protects data from getting inserted in the wrong column. 

On applying:

Once you set secure specific columns from getting accepted wrong data on the application of this SQL constraint.

On declining: 

Generally, it will be difficult to maintain uniformity throughout the column if we do not apply CHECK constraints.

DEFAULT SQL Constraint: 

This SQL constraint is essential for cases where you intentionally keep some cells blank by default. So, this is the opposite operation of the “NOT NULL” constraint.

On applying:

After you set this SQL Constraint, a default value is set if no specified value is applied in the cell.

On declining:

Lastly, on removing this SQL Constraint a blank cell contains a null value which may be misleading in some scenarios.

INDEX SQL Constraint: 

As the name suggests, this SQL Constraint helps to create indexes on a table in a relational database. Also, the INDEX Constraint helps to retrieve data from a long series of numbers, whereas SQL may take time to complete the process. 

On applying:

This SQL Constraint makes the data retrieval process easier.

On declining: 

Without applying this SQL Constraint, one can still retrieve data but fail to see indexes.

Ready for the next step in your career? 

A leading Ed-tech company, Henry Harvin Education will help you get a SQL course certification. By the time you start your journey, you are discovering one of the biggest E-learning Stages in the entire world. Meanwhile, you will find out, that Henry Harvin offers 1200+ courses across 37 classes. It has more than 600+ representatives and it operates within 13+ urban places, which participate in work with Henry Harvin. 

Another thing to remember is that we indeed are a reputable brand. We have 900+ corporate partners and also, 210+ college partners. Herny Harvin offers 7000+ live classes every month. So, what are you waiting for?

Recommended Reads

  1. Row Number Function in SQL: A Complete Guide
  2. Top 50 MySQL Interview Questions and Answers
  3. What is SQL? Learn SQL Basics, SQL Full Form
  4. 10 Best Online SQL Courses in 2024
  5. Difference between SQL and Microsoft SQL Server


In a fast-paced world, choosing a career that will help you land a job might be frightening. But,  we’ve got you. Henry Harvin SQL Developer Course is easy to understand and we also help you with your placement. In brief, start your journey today to learn SQL Constraints and be a certified SQL Developer to excel in your career.


Q1. How do I prepare for an SQL interview?

Ans.  To give your best for SQL, you may need to focus on several topics like the following:

Learn about DBMS.

Different constraints.

What is JOIN in SQL? 

and also, how do you elaborate JOIN in SQL?

Hence, go through the above questions to ace an SQL interview.

Q2. What is Data Integrity?

Ans. Let’s say, you have a table with a few students’ LAST_NAMES. Provide a way in SQL to find and replace “m” with “M” present in every name.

Ans. Enter the code as follows: SELECT REPLACE (LAST_NAME, ‘m’, ‘M’) From Student;

Q3. Are SQL Developer and Data Analysts the same job?

Ans. No. SQL Developer and Data Analysts have different job profiles. So, they both work in data management, but their responsibilities lie in different areas.

Q4. Do I need to know Python to learn SQL?

 Ans. Both Python and SQL are important for your data analytics career. So, they are equally important. The best advice to learn one by one is to master both of them.

Q5. Is Python harder than SQL?

Ans. Though it is very subjective to answer, most professionals find Python more difficult than SQL.

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