Add content for constraints (#3059)

* Update 103-constraints.md

* Update content/roadmaps/116-aspnet-core/content/102-database-fundamentals/103-constraints.md

Co-authored-by: Kamran Ahmed <kamranahmed.se@gmail.com>

content/roadmaps/116-aspnet-core/content/102-database-fundamentals/103-constraints.md

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 # Constraints
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+Database constraints are rules that are used to limit the data that can be stored in a database table. These constraints can be used to ensure the integrity and accuracy of the data in the table, and they can be used to enforce business rules or other requirements. For example, a constraint might be used to ensure that a column only contains positive numbers, or to ensure that a column always has a unique value. Constraints can be specified at the time a table is created, or they can be added to an existing table. Some common types of constraints include primary keys, foreign keys, and NOT NULL constraints.
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+<BadgeLink colorScheme='blue' badgeText='Read' href='https://www.w3schools.com/sql/sql_constraints.asp'>SQL Constraints</BadgeLink>