What are Constraints in SQL Tables?

In the SQL Server database, when you create tables, it is common for one or more table columns to have constraints associated with them. What is this constraint? A constraint is actually a rule, which is basically associated with a column. The data entered into that column must follow this rule.

Most Common Constraints in SQL Tables: For example, one is the ‘unique’ constraint that is a rule wherein all records should be unique. The “unique” constraint specifies that no two records shall have the same value in a specific column. They all must be totally unique. The other two most common and most popular constraints in SQL are “not null” and “primary key”. In ‘not null’, it is specified that a column cannot be left blank. In ‘primary key’ constraint, unique identification of every record (or row) in a table is specified.