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Sql join?

  1. Oct 23, 2009 #1

    DaveC426913

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    Gold Member

    I've got 3 tables:
    Pics - table of my pictures
    Categories - table of categories that pictures will fall into
    PicsCategories - a many-to-many table relating pictures to categories. So the primary key of this table consists of a pic field and a category field

    So now I want to provide a couple of categories as params and have it return all rows from the Pics table that are in those categories.

    Code (Text):

    Table Pics
    id Name
    1 Bob
    2 Mary
    3 Jill
    4 Jane

    Table Categories
    id Name
    1 Male
    2 Female
    3 Parent

    Table PicsCategories
    Pic Cat
    1   1
    1   3
    2   2
    3   2
    3   3
    4   2
    4   3
     
    So if I supply Category1=2 OR Category2=3 (females and/or parents)
    It should return Pic rows:
    2
    3
    4
    I'm looking at JOIN but JOIN is meant to extract data from two tables. I do not need ANY data from the PicsCategories table.

    This is what I want:
    SELECT UNIQUE(*) FROM Pics WHERE id=(SELECT Pic FROM PicsCategories WHERE Categories=2 OR Categories=3)
     
    Last edited: Oct 23, 2009
  2. jcsd
  3. Oct 23, 2009 #2

    DaveC426913

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    Gold Member

    Ah. It's a subquery.

    Code (Text):

    SELECT DISTINCT *
    FROM Pics
    WHERE id
    IN (
    SELECT Pic
    FROM tblPicsCategories
    WHERE Category=2
    OR Category=3
    )
     
    Thanks for all your help! Couldn't have done it without you!
     
  4. Oct 23, 2009 #3

    DavidSnider

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    Gold Member

    I think this works as well:

    select distinct p.* from pics p, tblPicsCategories pc where p.ID = pc.pic and (pc.Category = 2 or pc.Category = 3)
     
  5. Oct 23, 2009 #4

    DaveC426913

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    Gold Member

    What are the p and pc?

    I mean, I get that p means pic and pc means piccategories but...

    select distinct tblPic.* from pics tblPic, tblPicsCategories tblPicsCategories where tblPic.ID = tblPicsCategories.pic and (tblPicsCategories.Category = 2 or tblPicsCategories.Category = 3)
     
  6. Oct 24, 2009 #5
    p and pc are called aliases in this case. Sometimes written with an 'as' i.e.

    ...from pics as p, ....

    I prefer the 'as' because I think it's clearer. There are two reasons for using them

    1. It's quicker
    2. You need it if the same table appears twice as a join

    i.e.

    select a.name, b.name
    from mytrees as a, mytrees as b
    where a.parent_id=b.id

    Where parent_id contains the id of the row that contains its parent. You need an alias just to refer to the individual tables.
     
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