Which of the following relational database constructs is used to ensure valid values are entered for a column?
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A. B. C. D.C.
The correct answer is C. Constraint.
A constraint is a type of relational database construct used to ensure valid values are entered for a column. Constraints define rules that the data in a database must follow. They can be used to prevent invalid data from being entered into a column or to enforce relationships between tables.
There are several types of constraints that can be applied to a column:
NOT NULL constraint: This ensures that a column cannot contain NULL values.
UNIQUE constraint: This ensures that a column contains unique values, and no two rows in the table have the same value in that column.
PRIMARY KEY constraint: This is a combination of NOT NULL and UNIQUE constraints, and is used to uniquely identify each row in a table.
FOREIGN KEY constraint: This enforces referential integrity between two tables. It ensures that a value entered in a column in one table exists in a column of another related table.
CHECK constraint: This enforces a specific condition on the data in a column.
Constraints are an essential part of a relational database design, as they ensure data integrity and consistency. They help to prevent data inconsistencies and errors that can lead to data corruption and loss. By defining constraints, the database administrator can enforce rules that ensure the data is accurate and reliable, and that the data relationships are maintained.