What is functional dependency?

Functional dependency describes the concept that all other columns in a table must depend completely on the primary key column.
For example, the values in the CompanyName column and ContactName column depend completely on the value in the CustomerID column. In other words, if we know CustomerID, then we can tell the customer's company name and contact name. This reads like CustomerID determines CompanyName and ContactName.

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  The effect of achieving functional dependency is that each related set of data is put into its own table. In other words, each table only represents one subject.

It should be noted that functional dependencies are not limited to depending on a single column. We can have a combination of several columns determine other columns in the table.