I have these tables setup:
NOMINATIONS: A table of award nominations
NOMINATION_NOMINEES: A table of users with a FK on NOMINATIONS.ID
One Nomination can be referenced by many nominees via the ID field.
SELECT a.*, COUNT(SELECT all records from NOMINATION_NOMINEES with this ID) AS "b"
FROM NOMINATIONS a
LEFT JOIN NOMINATION_NOMINEES b on a.ID = b.ID
The results would look like:
ID | NOMINATION_DESCRIPTION | ... | NUMBER_NOMINEES
Where NUMBER_NOMINEES is the number of rows in the NOMINATION_NOMINEES table with the current row's ID.
This is a tricky one, we are feeding this into a larger system so I'm hoping to get this in one query with a bunch of subqueries. Implementing subqueries into this has twisted my mind. Anyone have an idea of where to head with this?
I'm sure the above way is not close to a decent approach to this one, but I can't quite wrap my mind around this one.
It can be done with a single correlated sub-query in SELECT clause.
SELECT a.*,
( SELECT COUNT(b.ID) FROM NOMINATION_NOMINEES b WHERE a.ID= b.ID )
FROM NOMINATIONS a
You should be able to use count as an analytic function:
select a.*,
count(b.id) over (partition by b.id)
from nominations a
left join nomination_nominees b on a.id = b.id
Related
I'd like to know if anyone would know an elegant and scalable method to full outer join multiple tables, given that I might want to regularly add new tables to the join?
For now my method consists in full joining table A with table B, store the result as a cte, then full joining the cte to table C, store the result as a cte2, full joining cte2 to table D... you got it.
Creating a new cte every time i want to add another table to the join is not very practical, but every other solutions i found so far have the same issue, there's always some kind of infinite looping either on ctes or in selects (like SELECT blabla FROM (SELECT blabla2 FROM..)).
Is there any way that i don't know that would help me perform this multiple full join without falling in an infinite recursive loop of ctes?
Thanks
EDIT: Sorry it seems it wasn't clear enough
When i perform a multiple full join in one query like:
SELECT
a.*, b.*, c.*
FROM
tableA a
FULL JOIN
tableB b
ON
a.id = b.id
FULL JOIN
tableC c
ON
a.id = c.id
If the id is present in tableB and tableC but not tableA, my result will create two lines where there should be one, because i joined b to a and c to a but not b to c. That's why i need to full join the result of the full join of a and b to c.
So if i have let's say five table instead of three, i need to full join the result of the full join of the result of the full join of the result of the full join... x)
This fiddle illustrates the problem.
If you want the rows from tables B and C to join, you need to accomodate the fact that maybe the data comes from table B and not A. The easiest is probably to use COALESCE.
Your join should therefore look like:
SELECT a.*, b.*, c.*
FROM tableA a
FULL JOIN tableB b ON a.id = b.id
FULL JOIN tableC c ON COALESCE(a.id, b.id) = c.id
-- FULL JOIN tableD d ON COALESCE(a.id, b.id, c.id) = d.id
-- FULL JOIN tableE e ON COALESCE(a.id, b.id, c.id, d.id) = e.id
Most databases that support FULL JOIN also support USING, which is the simplest way to do what you want:
SELECT *
FROM tableA a FULL JOIN
tableB b
USING (id) FULL JOIN
tableC c
USING (id);
The semantics of USING mean that only non-NULL values are used, if such a value is available.
I saw answers to a related question, but couldn't really apply what they are doing to my specific case.
I have a large table (300k rows) that I need to join with another even larger (1-2M rows) table efficiently. For my purposes, I only need to know whether a matching row exists in the second table. I came up with a nested query like so:
SELECT
id,
CASE cnt WHEN 0 then 'NO_MATCH' else 'YES_MATCH' end as match_exists
FROM
(
SELECT
A.id as id, count(*) as cnt
FROM
A, B
WHERE
A.id = B.foreing_id
GROUP BY A.id
) AS id_and_matches_count
Is there a better and/or more efficient way to do it?
Thanks!
You just want a left outer join:
SELECT
A.id as id, count(B.foreing_id) as cnt
FROM A
LEFT OUTER JOIN B ON
A.id = B.foreing_id
GROUP BY A.id
What should I do if I want to:
For now, there are table A and table B,
A:
id, name, address //the id is unique
B
id, contact, email
Since one person may have more than one contact and email, or have no contact and email(which means no record in table B)
Now I want to count how many records for each id, even 0:
And the result will look like:
id, name, contact_email_total_count
How can I do that(for now the only place I can not figure out is how to count 0 record since there is no record in table B)?
For that case you will want to use a LEFT JOIN, then add an aggregate and a GROUP BY:
select a.id,
a.name,
count(b.id) as contact_email_total_count
from tablea a
left join tableb b
on a.id = b.id
group by a.id, a.name
See SQL Fiddle with Demo
If you need help learning join syntax here is a great visual explanation of joins.
Based on your comment the typical order of execution is as follows:
FROM
ON
JOIN
WHERE
GROUP BY
HAVING
SELECT
ORDER BY
Need to do a left join to maintain the records in table A regardless of B:
PostgreSQL: left outer join syntax
Need to aggregate the count of records in B:
PostgreSQL GROUP BY different from MySQL?
I was wondering, is there a way to make a kind of one to one left outer join:
I need a join that matches say table A with table B, for each record on table A it must search for its pair on table B, but there exists only 1 record that matches that condition, so when it has found its pair on B, it must stop and continue with the next row at table A.
What I have is a simple LEFT OUTER JOIN.
select * from A left outer join B on A.ID = B.ID order by (NAME) asc
Thanks in advance!
SQL doesn't work this way. In the first place it does not look at things row-by-row. In the second place what defines the record you want to match on?
Assuming you don't really care which row is selcted, something like this might work:
SELECT *
From tableA
left outer join
(select b.* from tableb b1
join (Select min(Id) from tableb group by id) b2 on b1.id - b2.id) b
on a.id = b.id
BUt it still is pretty iffy that you wil get the records you want when there are multiple records with the id in table b.
The syntax you present in your question is correct. There is no difference in the query for joining on a one-to-one relationship than on a one-to-many.
Assume I have two data tables and a linking table as such:
A B A_B_Link
----- ----- -----
ID ID A_ID
Name Name B_ID
2 Questions:
I would like to write a query so that I have all of A's columns and a count of how many B's are linked to A, what is the best way to do this?
Is there a way to have a query return a row with all of the columns from A and a column containing all of linked names from B (maybe separated by some delimiter?)
Note that the query must return distinct rows from A, so a simple left outer join is not going to work here...I'm guessing I'll need nested select statements?
For your first question:
SELECT A.ID, A.Name, COUNT(ab.B_ID) AS bcount
FROM A LEFT JOIN A_B_Link ab ON (ab.A_ID = A.ID)
GROUP BY A.ID, A.Name;
This outputs one row per row of A, with the count of matching B's. Note that you must list all columns of A in the GROUP BY statement; there's no way to use a wildcard here.
An alternate solution is to use a correlated subquery, as #Ray Booysen shows:
SELECT A.*,
(SELECT COUNT(*) FROM A_B_Link
WHERE A_B_Link.A_ID = A.A_ID) AS bcount
FROM A;
This works, but correlated subqueries aren't very good for performance.
For your second question, you need something like MySQL's GROUP_CONCAT() aggregate function. In MySQL, you can get a comma-separated list of B.Name per row of A like this:
SELECT A.*, GROUP_CONCAT(B.Name) AS bname_list
FROM A
LEFT OUTER JOIN A_B_Link ab ON (A.ID = ab.A_ID)
LEFT OUTER JOIN B ON (ab.B_ID = B.ID)
GROUP BY A.ID;
There's no easy equivalent in Microsoft SQL Server. Check here for another question on SO about this:
"Simulating group_concat MySQL function in MS SQL Server 2005?"
Or Google for 'microsoft SQL server "group_concat"' for a variety of other solutions.
For #1
SELECT A.*,
(SELECT COUNT(*) FROM A_B_Link WHERE A_B_Link.A_ID = AOuter.A_ID)
FROM A as AOuter
SELECT A.*, COUNT(B_ID)
FROM A
LEFT JOIN A_B_Link ab ON ab.A_ID=A.ID