In which constraints the value of the column cannot be duplicate and null
Answers
Answer:
A column with the NOT NULL PRIMARY KEY constraint should not contain null or duplicate values. Defines the column as a unique key that cannot contain null or duplicate values. Columns with NOT NULL UNIQUE constraints defined for them are also called candidate keys.
Answer:
Constraints in SQL Server are predefined rules and restrictions that are enforced in a single column or multiple columns, regarding the values allowed in the columns, to maintain the integrity, accuracy, and reliability of that column’s data. In other words, if the inserted data meets the constraint rule, it will be inserted successfully. If the inserted data violates the defined constraint, the insert operation will be aborted.
Constraints in SQL Server can be defined at the column level, where it is specified as part of the column definition and will be applied to that column only, or declared independently at the table level. In this case, the constraint rules will be applied to more than one column in the specified table. The constraint can be created within the CREATE TABLE T-SQL command while creating the table or added using ALTER TABLE T-SQL command after creating the table. Adding the constraint after creating the table, the existing data will be checked for the constraint rule before creating that constraint.
There are six main constraints that are commonly used in SQL Server that we will describe deeply with examples within this article and the next one. These constraints are:
SQL NOT NULL
UNIQUE
PRIMARY KEY
FOREIGN KEY
CHECK
DEFAULT