What is a constraint? Tell me about its various levels.
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The constraint is used to specify the rule and regulations that allows or restricts what values/data will be stored in the table. It ensures data accuracy and integrity inside the table. It enforces us to store valid data and prevents us from storing irrelevant data. If any interruption occurs between the constraint and data action, the action is failed. Some of the most commonly used constraints are NOT NULL, PRIMARY KEY, FOREIGN KEY, AUTO_INCREMENT, UNIQUE KEY, etc.