primary key vs unique key in SQL
primary key and unique key are two important concept in relational database, and used to uniquely identify a row in a table. Both primary key and unique key can identify a row uniquely but there are some subtle difference between them which we will see in this article. In fact primary key vs unique is a popular SQL interview questions along with classics like truncate vs delete and  How to manage transaction in database, mostly asked to fresher and 2 to 3 years experience guys in any programming language. SQL is not just limited to any DBA or PLSQL developer but its an important skill even for Java programmer and you can expect SQL interview question even in many Java interviews. Some time programmer also confuse between foreign key and unique key, which is primary key of other table in relation.