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How to explain a not very complex query to an SQL beginner

Posted on 2011-04-24
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Last Modified: 2012-05-11
Consider the following:

item_categories
(item_id int(11),
 category_id int(11)
)

category_relations
(category1_id int(11),
  category2_id int(11),
  relation varchar(20)
)

Then we have the query that uses item1_id and item2_id as inputs:

select cr.relation
from category_relations cr, item_categories ic1, item_categories ic2
where ic1.item_id=item1_id
and ic2.item_id=item2_id
and cr.category1_id=ic1.category_id
and cr.category2_id=ic2.category_id;

How would you best explain the need for calling item_categories twice to an SQL beginner?  Things like this has become pretty second-nature to me, but having to explain it in simple terms suddenly got me stumped.  Thanks.
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Question by:johanntagle
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Ryan McCauley earned 2000 total points
ID: 35457873
It's not really clear from your query why you're linking to that table twice, unless you're just trying to validate the integrity of both sides of the relationship in the category_relations table.

If that's the case, then I'd just explain it like that - you have a row in the table representing a relationship, but you don't know what two categories its between (since you only have IDs) unless you link back to the item_categories table to get more information. Since you have two different relationships, you need to link to the categories table two different times - one for each side of the relationship.

Is that clear, or am I missing the point of what you're trying to explain?
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Author Closing Comment

by:johanntagle
ID: 35457885
Clear enough!  Thanks!
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