Skip to main content

How to use Primary Key Constraint on Column in SQL

In SQL, Primary Key constraint is defined on a column or a set of columns whose values uniquely identify all the rows in a table. These columns are referred to as the primary key columns. A primary key column cannot contain NULL values since it is used to uniquely identify rows in a table. The primary key constraint ensures entity integrity.

You can define a primary key constraint while creating the table or you can add it later by altering the table. However, if you define the primary key constraint after inserting rows, the SQL Server will give an error if the rows contain duplicate values in the column. While defining a primary key constraint, you need to specify a name for the constraint. If a name is not specified, the SQL Server automatically assigns a name to the constraint.

If a primary key constraint is defined on a column that already contains data, then the existing data in the column is screened. If any duplicate values are found, then the primary key constraint is rejected. The syntax of applying the primary key constraint when creating table is:

CREATE TABLE table_name
(
Col_name [CONSTRAINT constraint_name PRIMARY KEY
 [CLUSTERED|NONCLUSTERED]
Col_name [, col_name [, col_name [, … ] ] ]
)

Where,

  • Constraint_name specifies the name of the constraint to be created.
  • CLUSTERED|NONCLUSTERED are keywords that specify if a clustered or a nonclustered index is to be created for the primary key constraint.
  • Col_name specifies the name of the column(s) on which the primary key constraint is to be defined.

In the preceding example of the EmployeeLeave table, you can add a primary key constraint, while creating the table. You can set the EmployeeID and the LeaveStartDate columns of the EmployeeLeave table as a composite primary key. You can use the following statement to apply the primary key constraint:

CREATE TABLE HumanResources.EmployeeLeave
(
EmployeeID int,
LeaveStartDate datetime CONSTRAINT cpkLeaveStartDate PRIMARY KEY (EmployeeID, LeaveStartDate),



)

The preceding statement creates the EmployeeLeave table with a composite primary key constraint on EmployeeID and LeaveStartDate. The name of the constraint is cpkLeaveStartDate.

Comments

Popular posts from this blog

difference between structure and union in C Language

In c language article we will see the difference between union and structure. Both are the user define datatype in c language. See the table which is mentioned below: ASP.NET Video Tutorial Series Structure Union1.The keywordstruct is used to define a structure 1. The keyword union is used to define a union. 2. When a variable is associated with a structure, the compiler allocates the memory for each member. The size of structure is greater than or equal to the sum ofsizes of its members. The smaller members may end with unused slack bytes. 2. When a variable is associated with a union, thecompiler allocates thememory by considering the size of the largest memory. So, size of union is equal to the size of largest member. 3. Each member within a structure is assigned unique storage area of location. 3. Memory allocated is shared by individual members of union. 4. The address of each member will be in ascending order This indicates that memory for each member will start at different offset v…

Difference between Linear search and Binary Search in c language

SQL Video Channel : Download all SQL Video



Binary Search Linear Search Works only on sorted items. such as  1,2,3,4,5,6  etc
Works on sorted as well as unsorted items. 12,4,5,3,2,1 etc Very efficient if the items are sorted Very efficient if the items are less and present in the beginning of the list. such as Suppose your list items are : 12,3,4,5,1 and you want to search 12 number then you get beginning in the list. Works well with arrays and not on linked lists. Works with arrays and linked lists.
Number of comparisons are less More number of comparisons are required if the items are present in the later part of the array or its elements are more.

Memory representation of Linked List Data Structures in C Language

Memory representation of Linked List

             In memory the linked list is stored in scattered cells (locations).The memory for each node is allocated dynamically means as and when required. So the Linked List can increase as per the user wish and the size is not fixed, it can vary.

               Suppose first node of linked list is allocated with an address 1008. Its graphical representation looks like the figure shown below:


      Suppose next node is allocated at an address 506, so the list becomes,



  Suppose next node is allocated with an address with an address 10,s the list become,


The other way to represent the linked list is as shown below:




 In the above representation the data stored in the linked list is “INDIA”, the information part of each node contains one character. The external pointer root points to first node’s address 1005. The link part of the node containing information I contains 1007, the address of next node. The last node …