SQL query help on joining 2 tables - sql

Question

I have a table of items and another table with item groups (not all items are in a group).
I want to get all from the items table and if it exists in the groups table, I only want to get one item per group please show me SQL query for this.

Answer 1

I am defining a sample items and itemGroups table.
items
itemID, itemName, ItemDescription
itemGroups
itemGroupId, itemGroupname, ItemGroupDescription, itemID
Now to get all items from the item table if it exists in the itemsgroups table you have to compare the itemID which should your primary key in items table and compare it to the foreigh key for items which should be stored in the item groups table. So the SQL query could look like this
select * from items
where itemID in (select itemID from itemGroups)
Your coloumns names might be different but this is the general concept

Answer 2

If you have a groupid stored with each item row, try this:
select *
from items i
where groupid is null or itemid = (
select min(itemid) from items i2
where i2.groupid = i.groupid
)
If you didn't establish the linkage that way you might have to take this approach:
select *
from items i
where itemid = (
select coalesce(min(ig.itemid), i.itemid)
from itemgroups ig
where ig.groupid = (select groupid from itemgroups ig2 where ig2.itemid = i.itemid)
)

Answer 3

Assuming you have a table structure similar to this:
create table items (itemid int, itemname varchar2(50));
insert all
into items (itemid, itemname) values (1,'Apples')
into items (itemid, itemname) values (2,'Bananas')
into items (itemid, itemname) values (3,'Ford')
into items (itemid, itemname) values (4,'Honda')
into items (itemid, itemname) values (5,'Football')
select * from dual;
create table itemgroups (itemgroupid int, itemid int, itemgroupname varchar2(50));
insert all
into itemgroups (itemgroupid, itemid, itemgroupname) values (1,1,'Fruits')
into itemgroups (itemgroupid, itemid, itemgroupname) values (1,2,'Fruits')
into itemgroups (itemgroupid, itemid, itemgroupname) values (2,3,'Cars')
into itemgroups (itemgroupid, itemid, itemgroupname) values (2,4,'Cars')
select * from dual;
You could do a left join to get all the records from your items table (but only get one item per group). Then, do some kind of aggregation like MAX or MIN to only get one value per group like this:
select min(i.itemid) as itemid,
min(i.itemname) as itemname,
ig.itemgroupid,
ig.itemgroupname
from items i
left join itemgroups ig on i.itemid = ig.itemid
group by ig.itemgroupid,
ig.itemgroupname
order by min(i.itemid);
Result:
+--------+----------+-------------+---------------+
| itemid | itemname | itemgroupid | itemgroupname |
+--------+----------+-------------+---------------+
| 1 | Apples | 1 | Fruits |
| 3 | Ford | 2 | Cars |
| 5 | Football | (null) | (null) |
+--------+----------+-------------+---------------+
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