COMPUTER SCIENCE AND ENGINEERING
SQL
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Question
[CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
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Unique
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Not null
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Both Unique and Not nul
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Either Unique or Not null
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Detailed explanation-1: -What is Primary Key? The primary key is a unique or non-null key that uniquely identifies every record in that table or relation. The primary key column cannot store duplicate values that mean primary key column values are always unique.
Detailed explanation-2: -Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).
Detailed explanation-3: -There can be one or many unique keys, while there can only be one primary key.
Detailed explanation-4: -Primary keys A primary key contains unique values and identifies each row in a table. For some databases, the primary key cannot contain NULL values. A table can have only one primary key and this primary key can consist of single or multiple columns.