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More than two tables in an OUTER JOIN

I need one example to join more than two tables using outer join.
 

There are several ways, depending on specific needs (OUTER with OUTER, OUTER with INNER, and so on), but in each case you will need parentheses.

Consider the following tables --

CATEGORY
  id  descr
5  animal
  10  vegetable
  15  mineral

ITEM id descr 101 puppy 102 granite 104 quartz 110 iron 115 happiness 117 goldfish
CATITEM catid itemid 5 101 5 117 15 102 15 104 15 110

Notice that there is no item in the vegetable category.

There are two ways to write the outer join that lists all categories and the items in them --

select category.descr as catname
     , item.descr as itemname
  from category 
       left outer join 
       ( catitem 
         left outer join item 
         on catitem.itemid = item.id )
       on category.id = catitem.catid 

select category.descr as catname , item.descr as itemname from ( category left outer join catitem on category.id = catitem.catid ) left outer join item on catitem.itemid = item.id

Both of these produce the same result --

catname   itemname
animal    puppy
animal    goldfish
vegetable -- 
mineral   granite
mineral   quartz
mineral   iron

Notice there was no category for happiness. A double outer join in the "opposite" direction would be

select item.descr as itemname
     , category.descr as catname
  from item
       left outer join 
       ( catitem 
         left outer join category 
         on catitem.catid = category.id )
       on item.id = catitem.itemid

This produces --

itemname  catname
puppy     animal
granite   mineral
quartz    mineral
iron      mineral
happiness -- 
goldfish  animal

 

For More Information

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This was last published in May 2002

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