Set Operator

It allow to combine the results of multile separate queries into a single result set. 
    postgres=# select * from dept;   

    SET OPERATOR
    ------------
    UNION
    UNION ALL
    INTERSECT
    EXCEPT

    postgres=# select * from dept;   
    deptno |   dname    |   loc 
    --------+------------+----------      
    10 | ACCOUNTING | NEW YORK      
    20 | RESEARCH   | DALLAS      
    30 | SALES      | CHICAGO      
    40 | OPERATIONS | BOSTON  

    postgres=# select * from dept_tab;   
    deptno |   dname    |   loc 
    --------+------------+----------      
    10 | ACCOUNTING | NEW YORK      
    20 | RESEARCH   | DALLAS      
    50 | DEVELOPER  | LONDON      
    60 | MARKETING  | MEXICO
UNION :
It returns distinct rows selected by either query.
pg-set-op-1 
 
    postgres=# select * from dept 	
    union 	
    select * from dept_tab;   
    
    deptno |   dname    |   loc 
    --------+------------+----------      
    60 | MARKETING  | MEXICO      
    20 | RESEARCH   | DALLAS      
    10 | ACCOUNTING | NEW YORK      
    40 | OPERATIONS | BOSTON      
    30 | SALES      | CHICAGO      
    50 | DEVELOPER  | LONDON
UNION ALL :
It returns all rows selected by either query.
pg-set-op-2 
    postgres=# select * from dept 	
    union all 	
    select * from dept_tab;   
    
    deptno |   dname    |   loc 
    --------+------------+----------      
    10 | ACCOUNTING | NEW YORK      
    20 | RESEARCH   | DALLAS      
    30 | SALES      | CHICAGO      
    40 | OPERATIONS | BOSTON      
    10 | ACCOUNTING | NEW YORK      
    20 | RESEARCH   | DALLAS      
    50 | DEVELOPER  | LONDON      
    60 | MARKETING  | MEXICO
INTERSECT :
It returns all distinct rows selected by both query. 
    postgres=# select * from dept 	
    intersect 	
    select * from dept_tab;   
    
    deptno |   dname    |   loc 
    --------+------------+----------      
    10 | ACCOUNTING | NEW YORK      
    20 | RESEARCH   | DALLAS
EXCEPT :
It returns all distinct rows selected by first query only. 
    postgres=# select * from dept 	
    except 	
    select * from dept_tab;   
    
    deptno |   dname    |   loc 
    --------+------------+---------      
    40 | OPERATIONS | BOSTON      
    30 | SALES      | CHICAGO


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