A table is in first normal form (1NF) if and only if all columns contain only atomic values, that is, each column can have only one value for each row in the table.
Relational database tables, such as the Sales table, have only atomic values for each row and for each column. Such tables are considered to be in first normal form, the most basic level of normalized tables.
To better understand the definition for 1NF, it helps to know the difference between a domain, an attribute, and a column. A domain is the set of all possible values for a particular type of attribute, but may be used for more than one attribute. For example, the domain of people’s names is the underlying set of all possible names that could be used for either customer-name or salesperson-name in the database table. Each column in a relational table represents a single attribute, but in some cases more than one column may refer to different attributes from the same domain. When this occurs, the table is still in 1NF because the values in the table are still atomic. In fact, standard SQL assumes only atomic values and a relational table is by default in 1NF.