Question

I have two table for gallery system :

gallery_cat(
  gallery_cat_id    PK,
  gallery_cat_name
)

gallery(
  gallery_id        PK,
  gallery_cat_id    FK,
  gallery_name,
  gallery_file_name,
  gallery_date
)

I need to write a SQL query that return one picture from gallery table for each album, the purpose of this that I need to list the albums with one picture for each.

gallery_name | gallery_cat_name| gallery_file_name
-------------+-----------------+------------------
pic1         | Album1          | pic1.jpg 

Answer 1

This should do the trick:

SELECT g2.gallery_name, gc2.gallery_cat_name, g2.gallery_file_name
FROM gallery g2
INNER JOIN gallery_cat gc2 ON (g2.gallery_cat_id = gc2.gallery_cat_id)
WHERE g2.gallery_id IN (
  SELECT g.gallery_id     
  FROM gallery g
  GROUP BY g.gallery_cat_id)

Explanation:

At the end is a sub-select

IN (
  SELECT g.gallery_id     
  FROM gallery g
  GROUP BY g.gallery_cat_id) <<-- select 1 random g.id per gallery_cat.  

Here I select all g.id, but because of the group by clause it will reduce the results to 1 row per grouped by item. I.e. 1 row (chosen more or less at random) per g.gallery_cat_id.

Next I do a normal select with a join:

SELECT g2.gallery_name, gc2.gallery_cat_name, g2.gallery_file_name
FROM gallery g2
INNER JOIN gallery_cat gc2 ON (g2.gallery_cat_id = gc2.gallery_cat_id)
WHERE g2.gallery_id IN (

Because I refer to the same table twice in the same query you have to use an alias(*).
I select all names and all catnames and all filenames.
However in the where clause I filter these so that only rows from the sub-select are shown.

I have to do it this way, because the group by mixes rows into one messed up ow, if I select from that directly I will get values from different rows mixed together, not a good thing.
By first selecting the id’s I want and then matching full rows to those id I prevent this from happening.

*(in this case with this kind of subselect that’s not really 100% true, but trust me on the point that it’s always a good idea to alias your tables)