Table Alias:
   It is used to identify the similar column names uniqly across different tables. Especially when both the queries are running parallel & the query contains the column name which is similar in multiple tables used in the single query. To differenctiate the same column name present across different/same tables we may need alias name.

   
Rules for alias:
1. The alias name can be used/given to the table name directly without giving the alias name to the columns

Ex: select ename, job, sal from emp e;

2. If you give the alias name to the column name then the same alias name must be provided to the table name as well.
Ex: select e.ename, e.sal, e.deptno from emp e;

Note: Alias name is not required in the sub/nested query as the multile queries runs individually here.

But alias name is mandatory in corelated query as here both the queries run simultaneously/parallel based on the condition specified.

