Constraint (SQL),”A rule associated with a column that the data entered into that column must follow. For example, a “”unique”” constraint specifies that no two records can have the same value in a particular column. They must all be unique. The other two most popular constraints are “”not null”” which specifies that a column can’t be left blank
and “”primary key””. A “”primary key”” constraint defines a unique identification of each record (or row) in a table.”