A SQL Server Database could have multiple tables and each table that has to be stored based on some rules. These rules are the possible checks & conditions which allows you to ensure the data validations before it is being inserted into a specific table.

SQL Server Constraints Types

SQL server constraints can be divided primarily into :

1. SQL Server Table level constraints
2. SQL Server Column level constraints

S. No	SQL Server Constraint	Table / Column Level	Description
1	NOT NULL	Column Level	Column value cannot be specified as NULL
2	UNIQUE	Column Level	all values in the column need to be UNIQUE
3	CHECK	Column Level	all values in a column satisfies a specific condition
4	DEFAULT	Column Level	specifies a DEFAULT value for the column
5	INDEX	Table Level	apply indexes make the data retrieval fast
6	PRIMARY KEY	Column Level	Unique identifier for the column. Combination of NOT NULL and UNIQUE values
7	FOREIGN KEY	Column Level	Identifies the Column with Unique values in another table referenced in another table.

SQL Server Constraints can be applied :

1. Create Table
2. Alter Table

Syntax: SQL Server ADD Constraints in Table

CREATE TABLE table_name (
table_col1 datatype constraint,
table_col2 datatype constraint,
table_col3 datatype constraint,n
….

table_col3 datatype constraint
);

CREATE TABLE oracleappshelpStudents
( StudentId INT IDENTITY PRIMARY KEY,
UserId INT REFERENCES oracleappshelpUsers (UserId),
Email VARCHAR(256) ,
CreatedBy VARCHAR(128) NOT NULL,
UpdatedBy VARCHAR(128) NOT NULL,
CreatedDate DATETIME NOT NULL,
UpdatedDate DATETIME NOT NULL )

We will be discussing in separate blog for each Constraint . Please refer the below given links

SQL Server Not Null Constraint in the table

SQL Server Primary Key / Foreign Key Constraint in the table

SQL Server Default Constraint Check