Discussion
In a relationship between tables, what is the field in a table where each value appear only once called?
Answer: B
The field is the primary key.
A primary key uniquely identifies each row of a table, so every value in it appears only once and it cannot be empty. Relationships between tables are built on it: another table stores the same value to point back to the row.
That pointing column in the other table is the foreign key, and its values may repeat, since many rows can refer to one parent. A composite key is a primary key made of two or more columns together, so no single column of it need be unique. 'Unit key' is not a standard database term.
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