my_table has 4 columns: id integer, value integer, value2 integer, name character varying
I want all the records that:
have the same value2 as a record which name is 'a_name'
have a field value inferior to the one of a record which name is 'a_name'
And I have satisfying results with the following query:
select t.id
from my_table as t
where t.value < ( select value from my_table where name = 'a_name')
and s.value2 = (select value2 from my_table where name = 'a_name');
But is it possible to simplify this query with sql joins ?
Joining on the same table is still too much intricate in my mind. And I try to understand with this example.
What I happened so far trying, is a result full of dupplicates:
select t2.id
from my_table as t
inner join my_table as t2 on t2.value2 = t.value2
where t2.value < ( select value from my_table where name = 'a_name');
I think this will solve your problem.
select t1.id
from my_table as t1
join my_table as t2
on t1.value2 = t2.value2
and t2.name = 'a_name'
and t1.value < t2.value
You should use self join instead of inner join see this
http://msdn.microsoft.com/en-us/library/ms177490%28v=sql.105%29.aspx
You can always get distinct results by calling "SELECT distinct t2.id ..."
However, that will not enhance your understanding of inner joins. If you are willing, keep reading on. Let's start by getting all records with name = 'a_name'.
SELECT a.*
FROM my_table as a
WHERE a.name = 'a.name';
A simpler way to perform your inner joins is to understand that the result for the above query is yet another table, formally known as a relation. You can think of it as joining on the same table, but an easier way to think of it is as "joining on the result of this query". Lets put this to the test.
SELECT other.id
FROM my_table as a,
INNER JOIN my_table as other ON other.value2 = a.value2
WHERE a.name = 'a_name'
AND other.value < a.value;
If the first query (all rows with name = 'a_name') has many results, you stand a good chance of the second query having duplicates, because the inner join between aliases 'a' and 'other' is a subset of their cross product.
Edits: Grammar, Clarity
please try this
select t.id
from my_table as t
inner join
(select value from my_table where name = 'a_name')t1 on t.value<t1.value
inner join
(select value2 from my_table where name = 'a_name')t2 on t.value2=t2.value2
Related
Is it possible to remove the subquery from this SQL?
Table has 2 attributes "id" and "field"
Many field could have the same Id.
These table has many registers with the same Id and different Value
In need get all same Id values using one of them like filter.
select *
from Table
where id = (select id from Table where value = 'someValue')
I think it could be really easy but I don't know how to do.
Self Join can be done
select T.Id,T.Field
from Table T
INNER JOIN Table TT
ON T.ID = TT.ID
AND TT.Value = 'someValue'
Not sure if you over simplified your example too much but you could make this a little simpler.
select *
from Table
where value = 'someValue'
This should work
select T1.* from Table T1 JOIN Table T2 ON T1.id = T2.id AND T2.value = 'someValue'
Edited (Correct Answer):
What I assume your problem is:
You have a value. Let´s pretend it´s "testValue". Now you want to get the id of this value and find all other datasets with the same id.
What has to be cleared is that, "ID" is not the Primary Key and is not Unique.
You should be able to solve this by a simple self join:
select t.* from Table t right join Table tt on tt.id = t.id where tt.value = 'someValue';
So because of the join you will get a result that returns simply the table. With the where clause you shrink the result to your value. You should get the set of ids.
Old Answer:
This should do the trick:
select * from Table a inner join Table2 b on a.id = b.id where b.value = 'someValue';
You mentioned only one table in your question. I think this must be a mistake. If not, you have to change only the Table2 in my query. But that would have no sense as you could do a simple query, too:
select * from Table where value = 'someValue';
this would be the result of the first query with a self join.
this is a simplified version of a problem I'm having,
I have two tables:
Table1 has two columns (Stuff, YesNo) and
Table2 has one column (Stuff)
The records in the YesNo Column will either be 1 or 0
How could I select records in Table2 where the records in Table1.YesNo = 1
Many Thanks
SELECT Table2.*
FROM Table2
INNER JOIN Table1 ON Table1.Stuff = Table2.Stuff
WHERE Table1.YesNo = 1
If I understand you correctly, this would be your solution:
Select Stuff From Table2
Where Exists (
Select 'Y'
From Table1
Where Table1.Stuff = Table2.Stuff
And YesNo = 1
)
As I believe you'll need data from both tables and you may want to render fields unique to each table This seems like a likely response. However, as I don't believe STUFF accurately represents the relationship... you'll need to quantify/adjust the on a.stuff = b.stuff so that the join includes all necessary fields.
SELECT A.Stuff, B.Stuff, B.YesNo
FROM table1 B
INNER JOIN table2 A
on A.Stuff = B.Stuff
WHERE B.YesNo = 1
SELECT T2.*
FROM TABLE1 T1
JOIN TABLE2 T2
ON T1.Stuff = T2.Stuff
WHERE T1.YesNo = 1
I have this query in which i want to find who are all the other authors are for each title in which the author is Mr.X
The query I wrote for that is:
SELECT DISTINCT (author_name) as AUTHORS
from table1
where title = (Select title from table1 where (author_name) = 'X');
I got the ERROR: more than one row returned by a subquery used as an expression
I think to avoid this error i should use a self join but I'm not able to figure out how to do it.
your subquery returning more than 1 record and in such case you can't use = operator rather You should use IN operator to check against multiple values like below
where title in (Select title from table1 where (author_name) = 'X')
So, your query should look like
SELECT DISTINCT (author_name) as AUTHORS
from table1
where title in (Select title from table1 where (author_name) = 'X');
To change it to join instead
SELECT DISTINCT (t1.author_name) as AUTHORS
from table1 t1
join table1 t2
on t1.title = t2.title
An extra answer, in my opinion the performance of your query improves using: "count"
SELECT DISTINCT (author_name) as AUTHORS
from table1 t1
where ISNULL((Select COUNT(t2.title) from table1 t2 where (author_name) = 'X' AND t1.title = t2.title), 0) > 0
I have a query, that should return all records in T1 that not linked to records in T2:
SELECT DISTINCT fldID, fldValue FROM T1
WHERE NOT EXISTS
(
SELECT T1.fldID, T1.fldValue
FROM T2
JOIN T1 ON T2.fldID = T1.fldPtr
)
But it returns empty set -- should be one record.
If I use query like this (clause on one field):
SELECT DISTINCT fldID FROM T1
WHERE fldID NOT IN
(
SELECT T1.fldID
FROM T2
JOIN T1 ON T2.fldID = T1.fldPtr
)
It returns correct result.
But the SQL Server do not support syntax
WHERE ( fldID, flrValue ) NOT IN ....
Help me please to figure out how to compose query that will check several columns?
Thanks!
You can also use EXCEPT for this:
SELECT DISTINCT fldID, fldValue FROM T1
EXCEPT
SELECT T1.fldID, T1.fldValue
FROM T2
JOIN T1 ON T2.fldID = T1.fldPtr
A more efficient and elegant query that will work with every database is:
SELECT T1.*
FROM T1
LEFT JOIN T2
ON T2.fldID = T1.fldPtr
AND T2.flrValue = T1.flrValue
WHERE T2.fldID IS NULL
The LEFT JOIN attempts to match using both criteria, then the WHERE clause filters the joins, and only non-joins have NULL values for the LEFT JOINed table.
This approach is IMHO pretty much the industry standard for finding non-matches. It is usually more efficient than a NOT EXIstS(), although several databases optimize a NOT EXISTS() to this query anyway.
Use both those columns if sub-query join:
SELECT DISTINCT fldID, fldValue FROM T1
WHERE NOT EXISTS
(
SELECT *
FROM T2
JOIN T1 ON T2.fldID = T1.fldPtr
AND T1.fldValue = T2.flrValue
)
Something like (I think, as I'm not sure I 100% understand your question):
SELECT DISTINCT fldID FROM T1
WHERE fldID NOT IN
(
SELECT T1.fldID
FROM T2
JOIN T1 ON T2.fldID = T1.fldPtr
WHERE T2.flrValue = T1.flrValue
)
If you have the same structure in both tables you can use the EXCEPT operator http://technet.microsoft.com/en-us/library/ms188055.aspx
In a more general case, you have to to use left join and find null elements in second table.
try the below Query.
select DISTINCT fldID
from Table1
WHERE cast(fldID as varchar(100))+'~'+cast(flrValue as varchar)
NOT IN (select cast(fldID as varchar(100))+'~'+cast(flrValue as varchar) from table2)
This is more easy query. It returns all T1.fldID that not linked to records in T2
SELECT DISTINCT T1.fldID
FROM T1
LEFT JOIN T2 ON T2.fldID = T1.fldPtr
WHERE T2.fldID IS NULL
Using IN to exclude a large number of values is terrible for performance. Try the following:
SELECT T1.*
FROM T1
LEFT JOIN T2 ON T2.fldID = T1.fldPtr AND T1.fldValue = T2.fldvalue
WHERE T2.fldID IS NULL
(from my comment:) you do not have to reference t1 again in the subquery. Doing so would cause a logic of the form select all the records from t1 that don't exist in t1 ..., which is always empty, just like select all blue balls that are not blue, or select all odd numbers that are even ...
The first query should be:
SELECT DISTINCT fldID, fldValue
FROM T1
WHERE NOT EXISTS (
SELECT * FROM T2
WHERE T2.fldID = T1.fldPtr
);
And: in your original query, the subquery is uncorrelated: The t1 in the subquery shadows the t1 in the main query, making the subquery not referring any table or alias from the main query: it returns either True (some row exists) or False, the result being totally uncorrelated to the rows in the main query. (yet another good reason to use aliases instead of real table names in your queries)
Hope the title makes some kind of sense - I'd basically like to do a nested select, based on a value in the original select, like so:
SELECT MAX(iteration) AS maxiteration,
(SELECT column
FROM table
WHERE id = 223652
AND iteration = maxiteration)
FROM table
WHERE id = 223652;
I get an ORA-00904 invalid identifier error.
Would really appreciate any advice on how to return this value, thanks!
It looks like this should be rewritten with a where clause:
select iteration,
col
from tbl
where id = 223652
and iteration = (select max(iteration) from tbl where id = 223652);
You can circumvent the problem alltogether by placing the subselect in an INNER JOIN of its own.
SELECT t.iteration
, t.column
FROM table t
INNER JOIN (
SELECT id, MAX(iteration) AS iteration
FROM table
WHERE id = 223652
) tm ON tm.id = t.id AND tm.iteration = t.iteration
Since you're using Oracle, I'd suggest using analytic functions for this:
SELECT * FROM (
SELECT col,
iteration,
row_number() over (partition by id order by iteration desc) rn
FROM tab
WHERE id = 223652
) WHERE rn = 1
do it like this:
with maxiteration as
(
SELECT MAX(iteration) AS maxiteration
FROM table
WHERE id = 223652
)
select
column,
iteration
from
table
where
id = 223652
AND iteration = maxiteration
;
Not 100% sure on Oracle syntax, but isn't it something like:
select iteration, column from table where id = 223652 order by iteration desc limit 1
I would approach this problem in a slightly different way. You're basically looking for the row that has no other iterations greater than it. There are at least 3 ways I can think of to do this:
SELECT
T1.iteration AS maxiteration,
T1.column
FROM
Table T1
WHERE
T1.id = 223652 AND
NOT EXISTS
(
SELECT *
FROM Table T2
WHERE
T2.id = 223652 AND
T2.iteration > T1.iteration
)
Or...
SELECT
T1.iteration AS maxiteration,
T1.column
FROM
Table T1
LEFT OUTER JOIN Table T2 ON
T2.id = T1.id AND
T2.iteration > T1.iteration
WHERE
T1.id = 223652 AND
T2.id IS NULL
Or...
SELECT
T1.iteration AS maxiteration,
T1.column
FROM
Table T1
INNER JOIN (SELECT id, MAX(iteration) AS maxiteration FROM Table T2 GROUP BY id) SQ ON
SQ.id = T1.id AND
SQ.maxiteration = T1.iteration
WHERE
T1.id = 223652
EDIT: I didn't see the ORA error the first time reading the question and it wasn't tagged as Oracle specific. I think that there may be some differences in the syntax and use of aliases in Oracle, so you may need to tweak some of the above queries.
The Oracle error is telling you that it doesn't know what maxiteration is, because the column alias isn't available yet inside the subquery. You need to refer to it by the table alias and column name instead of the column alias I believe.
You do something like
select maxiteration,column from table a join (select max(iteration) as maxiteration from table where id=1) b using (id) where b.maxiteration=a.iteration;
This could of course return multiple rows for one maxiteration unless your table has a constraint against it.