SQL - UniqueKeyConstraint - Page7

-

                                                                                         


👉Unique Key Constraint

Difference between Unique and Primary Key:
Primary Key doesn't allow null values whereas unique column constraint permits multiple entries of null into the column. These null values are clubbed at the top of the column in the order in which they are entered into the table.

Features:
Unique key will not allow duplicate values.
Unique key can combine up to 16 columns in a composite unique key.
Unique index is created automatically.

Note:  Constraints can be bound to a table or a column using the Create Table command or Alter Table command.

✅Unique constraint at column level


✅Unique constraint at table level

Comments

Post a Comment

Popular posts from this blog

SQL-ForeignKeyConstraint-Page6

-
Image
                                                                              👉 Foreign Key Constraint Foreign Key represent relationships between tables. A foreign key is a column or group of columns whose values are derived from the primary key or unique key of some other table. ✅ The table in which the foreign key is defined is called Foreign table or Detail table. ✅ The table that defines the primary or unique key and is referenced by the foreign key is called the Primary table or Master table. 👉 Features 1)Master table cannot be updated if child records exists. 2)Records cannot be inserted or deleted into a detail table if corresponding records in the master table do not exist. 3)Records of the master table cannot be deleted if corresponding records in the detail table already exist. 👉 O...
Read more

Javascript-OOPs(Part3)-Inheritance - (9)

-
Image
   👉 Javascript with OOPs (Inheritance) In Javascript, Inheritance is done using prototype object. A prototype object is an object pointing to another object. We don't have a direct inheritance in Javascript, we can mimic inheritance using prototype. Let's say, we have one more class that inherits from Customer, and it submits to Server2. function CustomerChild(){         this.submit = function() {         alert ("this submits the data to Server2" + " "      +  this.CustomerName + " "                                       + this.CustomerCode");   }}             CustomerChild.prototype = new Customer(); var cust = new Customer(); cust.CustomerName ("John"); cust.Submit();  --this is a call to the submit method of the Customer class.  cust = new CustomerChild(); cust...
Read more

Javascript-Unobtrusive - (6)

-
Image
  👉 Unobtrusive Javascript In web pages, we have presentation and behavior. Presentation is controlled by HTML and behavior is controlled by javascript. <input type = "button" onclick = "Fun1();" id = btn1/> <script language = javascript>              Function Fun1()                       {alert ("Hi");} </script> Here, button is tightly linked with the behavior. Unobtrusive javascript seperates presentation from behavior, making HTML more sleek. <input type = "button"  id = btn1/> <script src = "jscript1.js" type = "text/ javascript"> </script> code of jscript1.js file: <script language = "javascript">            Function Fun1() {                      alert("Hi");             } var e1 = document.getElementByID("btn1"); e1.onclick = ...
Read more