what is a primary key ? define its rule for data entry ?
Answers
In the relational model of databases, a primary key is a specific choice of a minimal set of attributes (columns) that uniquely specify a tuple (row) in a relation (table). Informally, a primary key is "which attributes identify a record", and in simple cases are simply a single attribute: a unique id.
There are several rules that a primary key must follow: There is only one primary key for each table. If the primary key is a column, the value of this column must be unique and it must not NULL . If the primary key consists of multiple columns, each combination of values in these columns must be unique.
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Answer:
A primary key is a special relational database table column (or combination of columns) designated to uniquely identify each table record.
A primary key is used as a unique identifier to quickly parse data within the table. A table cannot have more than one primary key.
A primary key’s main features are:
It must contain a unique value for each row of data.
It cannot contain null values.
Every row must have a primary key value.
A primary key might use one or more fields already present in the underlying data model, or a specific extra field can be created to be the primary key
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