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Everything You Need to Know About SQL Constraints: The What, Why, and How

intro

SQL constraints have a special place in the heart of your database — everyone defines them, but not everyone thinks about them. In this blog, we’re walking you through everything you need to know when defining, working with, or modifying constraints in your database.

Tools used in the tutorial
Tool Description Link
Dbvisualizer DBVISUALIZER
TOP RATED DATABASE MANAGEMENT TOOL AND SQL CLIENT
mariadb MARIADB
THE MARIADB DATABASE
MySQL logo MySQL
THE MYSQL DATABASE

During your career as a database administrator, you will certainly come across situations necessitating the specification of certain rules for data stored in a database. That’s where SQL constraints come into play. These have a key role in your database as they are used to specify rules for data in the tables of the database.

What are SQL Constraints?

Everyone has heard of constraints — they ensure that columns are or aren’t nullable, ensure that values in columns are unique and there are no duplicates, have a default value, or build upon indexes.

Any relational database management system will support many, if not all, of these SQL constraints:

  • NULL|NOT NULL — a clause identifying a nullable or not nullable column. Find out more in our guide on how to work with NULL values.
  • DEFAULT — a clause specifying a default value in a column. If values are not specified, the default value is used.
  • INDEX — identifies that a column is using an index.
  • PRIMARY KEY — a clause that makes a column act as an automatically incrementing integer in a table.
  • CHECK — a clause that enables us to implement a function-based check on a table or a column.

These SQL constraints will help you ensure that your data is presented correctly. There’s a reason some of you expect NULL to be returned when no rows in your database are found, some of you want your database to return "Empty" instead of "" when no values exist in a row or check if a column is using an index when a certain query is being run by using EXPLAIN.

Think of it this way: constraints in SQL help us put data in order and understand it once it’s presented to the end-user — developers know that NULL holds weight and is an industry-standard way to denote that a value in a column is missing and should be used by default together with clauses like IS [NOT] NULL. That’s why such clauses exist in the first place: they’re used to easily relay trivial information that otherwise would be hard to find.

SQL Constraints in the Real World

Many of us don’t think about SQL constraints like something out of the ordinary and that’s because of a good reason — they’re everywhere. Think of the last time you created a table in a DBMS — what constraints have you used?

I’ll provide an example based on MyBB 1.8.38 (a forum content management software.) Below, you see two tables denoting whether users in the forum are banned and what their reputation looks like:

Tables mybb_banned and mybb_reputation in DbVisualizer
Tables mybb_banned and mybb_reputation in DbVisualizer

Here’s another example — this time in a formatted manner (thanks DbVisualizer!) and depicting the user table in MyBB 1.8.38:

The User Table in MyBB 1.8.38 in a Formatted Manner by DbVisualizer
The User Table in MyBB 1.8.38 in a Formatted Manner by DbVisualizer

So many columns! And all of those columns have constraints on them — weird, no? Don’t be weirded out — constraints in SQL are literally everywhere and if you would inspect a couple of tables you have in your database, I’m certain the results would be the same.

How many constraints can you spot? I’ll outline four:

  1. NOT NULL
  2. DEFAULT
  3. KEY
  4. PRIMARY KEY

One or more of these four constraints can be spotted everywhere across these tables: we define automatically incrementing values and assign a primary key to a column, use default values, or add indexes (keys) to a table.

SQL constraints are seemingly few and far between, but they’re everywhere and a table you created seldom goes far without them. They’re the backbone of everything we do, but to properly work with them, we must adhere to best practices and other things.

Best Practices When Using SQL Constraints

By now, you know — there are five most widely used constraints in SQL. Cool!

Regardless, constraints are defined for a reason in a database. These reasons differ from developer to developer and from DBA to DBA, but most of them are based on rules:

  1. Make columns nullable: When we expect them to not have a value or have values that wouldn’t make any sense. Think of NULL as “I’m not sure” or “I don’t know”: if you don’t know what value to expect but need to query the column for analytics or other purposes, NULL is the place to turn. The same goes for NOT NULL SQL constraint values, just the logic is the opposite: we use NOT NULL constraints once we’re 100% sure that our column will have a value.
  2. Make columns have default values: When we need them to have a value, and it makes sense for them to be set to something that’s easily queryable. (e.g., if we have a user table with an age column, setting its default value to “25” could mean the person is 25 of age or older if the age is not specified. If an age is specified, a different age is used.)
  3. We index columns to speed up read operations at the expense of slowing down INSERTs, UPDATEs, and DELETEs: The KEY constraint is a synonym for an index. A database index can have a specific type depending on what database management system you find yourself using and can be useful in a variety of different circumstances. They can be used as a pointer to data or used for your database to read data from them directly without accessing the disk. Regardless, they’re extremely good at two things: occupying space and improving the performance of SELECT queries.
  4. We add PRIMARY KEY constraints to columns to make them able to be automatically incremented: These days, almost every table has some sort of an id column and that’s not without a reason: with a PRIMARY KEY constraint, we make that column increment by 1 every time we insert a NULL value inside of it. In MySQL, the PRIMARY KEY is to be used together with an AUTO_INCREMENT clause while other database management systems may have different approaches. Investigate and find a solution suitable for yourself.

Follow these four rules and you will be on your way to database perfection. Of course, always keep in mind that constraints aren’t a panacea and their application and use cases are likely to differ from DBMS to DBMS so it’s wise to refer to the documentation of your database management system of choice. Also remember that SQL constraints like the ones described above should be looked at as “helpers” to your database and not as a replacement for any software appliance, SQL client, or other software solution you may find yourself using.

DbVisualizer’s Role in Keeping Databases Safe

As we’ve already mentioned above, constraints in SQL are only one part of your database equation. That’s not without a reason — they aren’t likely to replace your tables, databases, or data within them, but they will certainly make sure that data inside of these databases or tables adheres to certain structures or standards.

Once you find yourself searching for ways to adhere to structures or standards in the database world, many of you would find SQL clients to be of use. One of those SQL clients is DbVisualizer — being the database client with the highest user satisfaction in the market, it can certainly alleviate your database burdens and make your life easier.

DbVisualizer’s extensive support for the most popular databases makes it a go-to tool for many database administrators and developers across the globe: and its internal features such as the ability to automatically complete queries and build them when necessary, create visual explain plans for more efficient queries, create databases (if a user has enough privileges), format queries to make them look nice and others will certainly make your life as a DBA easier.

An Unformatted SQL Query Creating a Table in MyBB
An Unformatted SQL Query Creating a Table in MyBB

DbVisualizer is used by some of the best engineers across the globe working at a variety of companies Netflix to Tesla and that’s not without a reason: it acts as a virtual DBA working behind the scenes to make your databases sing.

That’s not the best part yet — it comes with a free 21-day trial to help you unleash the database glory. It doesn’t cost that much either — it’d certainly cost much more to maintain a full-time DBA crew than to hire DbVisualizer, so make sure to try DbVisualizer today, and until next time.

Summary

SQL constraints are database components that ensure data integrity. These components are vital for every developer and are used very widely, no matter what database management system is in use and that’s because they have a very important task to accomplish.

For users of most database management systems, there are 4-5 constraints that you will find yourself working with and these are NULL or NOT NULL denoting whether the value in a column is nullable or not, DEFAULT denoting a default value in a column, KEY denoting an index (a key is also a synonym to an index), and PRIMARY KEY allowing integer-based values in our columns to increment automatically.

Make use of constraints in SQL and don’t forget to unleash the power of database clients for more data perfection.

FAQ

What is an SQL constraint?

An SQL constraint is a database component that is used to guarantee data integrity and set expected values. Some constraints (indexes) are used to make read queries faster, too.

When and how to use SQL constraints?

SQL constraints should be used once we’re defining or modifying our table structure, see examples above.

Where can I learn more about databases?

To learn more about databases and the components within them, make sure to follow our blog and follow us on social media as well as have a gander through a YouTube channel called Database Dive: they share database-related news in a video format.

Why should I use DbVisualizer?

Consider using DbVisualizer because it is the best-rated SQL client on the market and it can help you or your team alleviate database issues by acting as a virtual DBA. DbVisualizer also offers a 21-day money-back guarantee, so you have nothing to lose. Download DbVisualizer today!

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About the author
LukasVileikisPhoto
Lukas Vileikis
Lukas Vileikis is an ethical hacker and a frequent conference speaker. He runs one of the biggest & fastest data breach search engines in the world - BreachDirectory.com, frequently speaks at conferences and blogs in multiple places including his blog over at lukasvileikis.com.
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