1 The Union Operator
?The union operator returns results form both queries after eliminating duplications.\
?select employee_id,job_id
?from employees
?uniion
?select employ_id ,job_id
?from job_history;
2 the all operator
?The union all opertor reutrn result from both queries,including all duplications
3 interset
?select ster_id,qty from sales where qty>20;
?intersect
?select ster_id,qty from sales where ster_id like '7%';
4 minus
?select ster_id,qty from sales where qty>20
?minus
?select ster_id from sales where ster_id like '7%'
5 set operator guidelines
?. teh expressions in the select list must match in number and data type;
?. Parentheses can be used to alter the sequence of the execution
?.The order by clause:
??? can appear only at the very end of the statement
??? will accept the column name,aliases from thee firest select statement ,or thee positional notation
?.Duplicate row are atuomatically eliminated except in union all.
?.Column names from the first query appear in the result
?.The output is sorted in ascending order by default except in union all
6 matching the select statement
?select department_id,to_number(null),location,hire_date
?from employees
?union
?select department_id,location_id,to_date(null)
?from departments;
?select employee_id,job_id,salary
?from employees
?union
?select employee_id,job_id,0
?from job_history;
7 Controlling the order of the rows
?select 'sing' as "my dream" ,3,a_dummy
?from dual
?union
?select 'like''d like to teach',1
?from dual
?union
?select 'the world to',2
?from dual
?order by 2;