What will happen if we execute below queries?

Select emp_id,employee_name from employee_master e
inner join employee_address ea
on e.emp_id = ea.emp_id;

Select emp_id,emp_id,employee_name from employee_master e
inner join employee_address ea
on e.emp_id = ea.emp_id;

Answer:

It will give us error as Ambiguous column name 'emp_id'.Because 'emp_id' is present on both tables i.e. employee_master and employee_address table that's why when we execute above queries,tables will confuse which emp_id is refering to which Tables,then it will say Ambiguous column name or duplicate column name.

To overcome this problem just provide alias name in columns as below:-

Select e.emp_id,employee_name from employee_master e

inner join employee_address ea

on e.emp_id = ea.emp_id;



Select e.emp_id,ea.emp_id,employee_name from employee_master e

inner join employee_address ea

on e.emp_id = ea.emp_id;