one user prefrom different roles is called alias 


eg
two dept  are there  sales , HR
user of sales working in HR  with different row is called 
alias

alias is logical popint to alternate to the table name.this
can be used to resolve the loops.

Aliases are references to existing tables in a schema. An 
Alias is a table that is an exact duplicate of the original 
table with a different name. The data in the table is 
exactly the same as the original table. An alias breaks a 
loop by using the same table twice in the same query for a 
different purpose.

An alias is a logical pointer to an alternate table name.
Exact copy of the original table.Establish a one-one join 
between the original table and the table which is aliased.
The aggregatre results should be canned on to the aliased 
table and not the original table.

alias is a logical table of existing look up table.using 
alias we can resolve looops