sql - Primary key or Unique index? -

At work we have a large database with a unique index instead of the primary key and all functions are fine.

I am preparing a new database for a new project and I have a dilemma:

In DB theory, the primary key is the fundamental element, it is fine, but the real projects What are the advantages and disadvantages of both?

What do you use in projects?

Edit: ... and what about the primary key and replication on MS SQL Server?

What is a unique index?

A unique index on the column The index that applies the barrier to the column, in that column, you can not have two identical values ​​in two similar lines Example:

 Create table table 1 (foo int, bar int); Create unique index ux_table1_foo on Table 1 (Foo); - Create unique index on foo Table 1 (FU, bar) Include in values ​​(1, 2); - Insert table 1 (AF, band) values ​​right (2, 2); - Insert the table 1 (FU, bar) values ​​correctly (3, 1); - INSERT (1, 4) right in Table 1 (AF, bar) values; - Fail! The duplicate entry '1' for the key 'ux_table1_foo' fails because the column violates the unique index on  foo , when it tries to insert value 1 into this column second time. 
 
 A unique obstacle in MySQL allows multiple plays. 
 
 It is possible to create a unique index on multiple columns. 
 
  Primary key vs. unique index  
 
 Things that are similar: 
 
 
 The meaning of the primary key is a unique index . 
 
 
 Things are different: 
 
 
 The meaning of a primary key is also not zero, but a unique index can be null. 
 
 Only one primary key can be, but there may be more than one unique index. 
 
 If no package index is defined then the primary key will be the package index.