Dbvisualizer unique constraint
2: Row IdentificationĪnother example of when to use a UNIQUE constraint is the unique identification of each row. Nowadays, you could also think of email addresses or phone numbers as being unique to each person, but this depends on the exact business requirements for your application. However, a social security number value must be unique to each person. However, these values are not unique to each person there can be two persons with the same name or the same date of birth. Each person has a name, age, and a date of birth. We use a UNIQUE constraint when the column must contain a unique value for each row. When to Use a Unique Constraint (with Examples) Example 1: Unique Values Duplicate entry '8765234545677890123465' for key bankaccounts.AccountNumber'.Ĭonstraints are like security guards – they make sure that all the non-compliant rows do not get inserted into the table. INSERT INTO BankAccounts VALUES ('8765234545677890123465', '233') Īfter the execution of the above statement, we get an error message:Įrror Code: 1062. To try violating this constraint, we’ll insert another row into the BankAccounts table with an AccountNumber column value ( 8765234545677890123465) that’s already in the table: The AccountNumber column implements a UNIQUE constraint. It assigns an account number to a person one person can own one or more bank accounts, but one bank account cannot belong to more than one person. We can do that by simply inserting two rows with the same value for a column implementing the UNIQUE constraint.
You might be wondering what happens when we try to violate a UNIQUE constraint. The SSN column implements the UNIQUE constraint, i.e. Let’s see an example using the Person table with the columns Name, Age, and SSN.
Why Use a Unique Constraint?īy using a UNIQUE constraint on a particular column, we ensure that this column cannot contain two of the same values. By doing so, we ensure that no employees in the same company have the same employee number. EmployeeIdĮach employee must have a unique number hence, the EmployeeNumber column implements the UNIQUE constraint. Let’s see an example using the Employees table. A UNIQUE constraint dictates that a column must not store duplicate values. it prevents the user from entering the wrong type of data). A constraint is a rule that makes a column fulfill a particular condition (i.e. The UNIQUE constraint is one of many SQL constraints that can be defined for a column. Let’s get started! SQL UNIQUE Constraint Basics If you are interested in learning about other SQL constraints, check out our course on SQL Constraints and the article What is a SQL Constraint?. In the last section, I’ll touch on multicolumn UNIQUE constraints.
DBVISUALIZER UNIQUE CONSTRAINT HOW TO
We’ll discuss how to define the UNIQUE constraint during table creation, how to add it to an existing table, and how to remove it. Then I’ll present some real-world uses of the UNIQUE constraint. Next, we’ll try to violate that constraint by adding duplicate values to a column that’s declared unique. In this article, we’ll first review the basics of a UNIQUE constraint. By using a UNIQUE constraint on a column, we define that this column cannot store duplicate values. There can be only one social security number (SSN) per person and each one must be unique. The idea of a unique constraint is not unique to SQL it’s a familiar concept in real life, although we probably call it something else! For example, think of ID numbers. In this article, we’ll discuss how, when, and why to implement it. To ensure the uniqueness of data in a database, we use the SQL UNIQUE constraint.