According to the rules of 1NF, what does it mean for attributes to be single-valued?

In the context of the First Normal Form (1NF) in database normalization, an attribute being single-valued means that each attribute of a table has a single, atomic or indivisible value.

Here are the steps to understand this:

1. The First Normal Form (1NF) is a property of a relation in a relational database. A relation is in first normal form if and only if the domain of each attribute contains only atomic (indivisible) values, and the value of each attribute contains only a single value from that domain.

2. The requirement for an attribute to be single-valued in 1NF means that it cannot hold multiple values. For example, a column in a table cannot hold an array or a list of values. Each cell in the table should hold a single value.

3. This rule helps to ensure data integrity and to simplify the design of a database. It makes it easier to search, sort, and update data in a database.

4. For example, consider a table of customers where one of the columns is "Phone Number". According to 1NF, each customer can have only one phone number in this column. If a customer has more than one phone number, this would be represented with multiple rows in the table (one for each phone number), not with multiple phone numbers in a single cell.

5. Therefore, the concept of attributes being single-valued in 1NF is fundamental to the structure and function of a relational database.