Question

explain constraints ,​

Answer 1

Answer:

A constraint is a rule that is used for optimization purposes. There are five types of constraints: A NOT NULL constraint is a rule that prevents null values from being entered into one or more columns within a table. ... You can use a primary key and foreign key constraints to define relationships between tables.

Explanation:

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Answer 2

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The definition of a constraint is something that imposes a limit or restriction or that prevents something from occurring. An example of a constraint is the fact that there are only so many hours in a day to accomplish things. The threat or use of force to prevent, restrict, or dictate the action or thought of others.