How to check if similar values exist across multiple tables - php

Question

I've read and reviewed numerous posts on here (and elsewhere) about how to do this, but my results seem to be working backwards from what makes sense.
I have 4 tables that have different columns/data, but the columns I need to select from are the same and have the same structure.
Table 1: | id | item_id | user | other table 1 specific columns...<br>
Table 2: | id | item_id | user | other table 2 specific columns...<br>
Table 3: | id | item_id | user | other table 3 specific columns...<br>
Table 4: | id | item_id | user | other table 4 specific columns...<br>
I am trying to check to see if a specific user/item_id combination exists across all 4 tables in 1 query instead of 4.
If I'm reading all the posts correctly, I should do a UNION ALL between the SELECT statements, however, I'm actually getting the reverse of what is making sense.
To keep things simple, I am just testing this with Table 1 and Table 2.
Table 1 does NOT have the combination I'm looking for.
Table 2 does have the combination I'm looking for.
SELECT item_id AS table1ID
FROM table1
WHERE item_id=35 AND user='usernamehere'
UNION ALL
SELECT item_id AS table2ID
FROM table2
WHERE item_id=35 AND user='usernamehere'
So, what I was expecting to see was a null result from Table 1 and a result with the item_id from Table 2, but what's happening is I'm getting no result at all from Table 2 and a "hit" from Table 1.
Seems backwards and confusing since Table 1 doesn't actually have a record with item_id=35 and user='usernamehere'.
Any advice or insights are greatly appreciated.

Answer 1

Your query is just listing item_id's in the specific table. It does not specify where they come from. For two table, you can solve the problem using a join.
I think for more tables, you will want something like this:
select item_id, group_concat(which order by which) as tables
from ((select item_id, user, 'table1' as which from table1) union all
(select item_id, user, 'table2' from table1) union all
(select item_id, user, 'table3' from table1) union all
(select item_id, user, 'table4' from table1)
) t
where user = 'usernamehere'
group by item_id;
For each item_id, this will list the tables it is in. If you want items in all four tables. then add a having clause:
having count(distinct which) = 4;
Similarly, if you want examples where one or more are missing:
having count(distinct which) < 4;

Answer 2

Assuming $var has your search field:
select
inp.ID,
count(t1.itemID) as TableOne,
count(t2.itemID) as TableTwo,
count(t2.itemID) as TableThree,
count(t3.itemID) as TableFour
from
(
select
".$var." as ID
from
dual
) inp
left outer join
table1 t1
on inp.ID=t1.item_id
left outer join
table2 t2
on inp.ID=t1.item_id
left outer join
table3 t3
on inp.ID=t1.item_id
left outer join
table4 t4
on inp.ID=t1.item_id
group by
inp.ID
This will give you a count of whether the item is in your tables via a select you create and outer joins to your other tables which you can count nicely.

Answer 3

UNION will merge results (rows), I don't think it's what you need.
Consider using JOIN statement:
SELECT exTable.ID
FROM exTable as t1
JOIN t2 as t2 ON t1.col = t2.col // we have value of col in t2 same as in t1
JOIN t3 as t3 ON t1.col = t3.col
JOIN t4 as t4 ON t1.col = t4.col
WHERE t1.ID = 35 AND t1.user = 'usernamehere'
In some cases you may use LEFT OR RIGHT joins, you can read about them on W3C.