SQL: Constraints

Instructor: Kaitlin Oglesby

Kaitlin has a BA in political science and experience teaching.

Do you want to make sure that the data entered into your SQL database meets certain criteria? This lesson on constraints will help make sure that each record has the required minimums to be recorded.

Constraining Data

Chances are that you're not playing with SQL for fun. Instead, you really want the work that you put into your database to shine through and be exactly what you want in the end. However, we all make mistakes sometimes. Wouldn't it be nice if there were a way to have SQL check your work as you were adding it, at least making sure that the information that you enter into a field is relevant? Better yet, since SQL is designed to be used by a large number of different people, wouldn't it be nice if there was a way to put a simple constraint on the type of data that can be entered in each field?

Luckily, just such a mechanism exists. Through using constraints, you can make sure that data entered in a field meets a minimum requirement to go there. From making sure that there is a value to making sure that it meets certain criteria, constraints help make sure your data is as accurate as possible.

Where Does the Constraint Go?

When creating a table, you put the constraint in right after the column and data type. Often, you'll see them set apart in brackets, but because SQL is so neutral about empty space and case, you probably won't be surprised to hear that those aren't necessary. However, to make things easier, let's set my constraints out in brackets, and encourage you to do the same - it just makes debugging much easier.

As mentioned, you list the constraint after the data type. For example, if we have the data column 'name' and want to make sure that an entry is put in for each name that has a length of up to 15 characters, you'd type it into SQL like this:


name VARCHAR(15) [NOT NULL],


Types of Constraints

Let's look at a few more constraints that you can use. Let's say that you were putting in employee ID numbers. Obviously, you want to be sure that those employee ID numbers are unique. Otherwise, you may confuse someone's paychecks! We'll use a column called 'id' and a data type of CHAR(6), since each ID number should be exactly six digits long. Finally, we'll add the constraint UNIQUE at the end, making sure to put it in brackets. Here it is in action:


id CHAR(6)[UNIQUE],```


Make sure that you are including the comma unless it is the very last column.

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