I'm working on an SQL project (involving a library database) and I'm having a hard time figuring out how to make a conditional count.
So, I have 4 tables: Imprumuturi, Cititori, Autori, Carti. I need to list the 'Cititori' that have more than one borrowed 'Carti' at the current time.
I tried to use
SELECT cititori.nume_cititor,COUNT(imprumuturi.pk_cititor)
AS numar_imprumuturi FROM cititori, imprumuturi
WHERE imprumuturi.data_return IS NULL GROUP BY cititori.nume_cititor
HAVING COUNT(imprumuturi.pk_cititor)>1
ORDER BY cititori.nume_cititor;
And while it lists all the 'Cititori", it doesn't count the number of active borrowed 'Carti' as it should.
Can I get a hint or some help on how to make it work?
These are the fields of my database
Seems you missed the relation between the tables:
SELECT cititori.nume_cititor,COUNT(imprumuturi.pk_cititor)
AS numar_imprumuturi
FROM cititori
INNER JOIN imprumuturi ON imprumuturi.pk_cititori = cititori.pk_cititori
WHERE imprumuturi.data_return IS NULL
GROUP BY cititori.nume_cititor
HAVING COUNT(imprumuturi.pk_cititor)>1
ORDER BY cititori.nume_cititor;
As suggested, you should not use the old implicit join syntax based on comma-separated table names and where condition, but use explicit join syntax.
I just registered and want to ask.
I learn sql queries not so long time and I got a trouble when I decided to move a table to another database. A few articles were read about building long subqueries , but they didn't help me.
Everything works perfect before that my action.
I just moved the table and tried to rewrite the query while whole day.
update [dbo].Full
set [salary] = 1000
where [dbo].Full.id in (
select distinct k1.id
from (
select id, Topic, User
from Full
where User not in (select distinct topic_name from [DB_1].dbo.S_School)
) k1
where k1.id not in (
select distinct k2.id
from (
select id, Topic, User
from Full
where User not in (select distinct topic_name from [DB_1].dbo.Shool)
) k2,
List_School t3
where charindex (t3.NameApp, k2.Topic)>5
)
)
I moved table List_School to database [DB_1] and I can't to bend with it.
I can't write [DB_1].dbo.List_School. Should I use one more subquery?
I even thought about create a few temporary tables but it can influence on speed of execution.
Sql gurus , please invest some your time on me. Thank you in advance.
I will be happy for each hint, which you give me.
There appear to be a number of issues. You are comparing the user column to the topic_name column. An expected meaning of those column names would suggest you are not comparing the correct columns. But that is a guess.
In the final subquery you have an ansi join on table List_School but no join columns which means the join witk k2 is a cartesian product (aka cross join) which is not what you would want in most situations. Again a guess as no details of actual problem data or error messages was provided.
I am having some trouble converting my SQL queries into Hive queries.
Relational schema:
Suppliers(sid, sname, address)
Parts(pid, pname, color)
Catalog(sid, pid, cost)
Query 1: Find the pnames of parts for which there is some supplier.
I have attempted one of the query conversions for query 1 and I think it is correct If someone can let me know if it is correct or incorrect I would really appreciate it. They seem to be the same to me based on the info I have looked up for Hive.
Query 1: SQL
SELECT pname
FROM Parts, Catalog
WHERE Parts.pid = Catalog.pid
Query 1: Converted to Hive
SELECT pname
FROM Parts, Catalog
WHERE Parts.pid = Catalog.pid;
Query 2: Find the sids of suppliers who supply only red parts.
For the second query I am having trouble. Mainly I am having trouble with the "not exists" part and the defining what color we want part. Can someone help me figure this out? I need to put the SQL into a Hive query.
Query 2: SQL
SELECT DISTINCT C.sid
FROM Catalog C
WHERE NOT EXISTS ( SELECT *
FROM Parts P
WHERE P.pid = C.pid AND P.color <> ‘Red’)
If someone can help me get these into the correct Hive format I would really appreciate it.
Thank you.
Although I have never used HiveQL in looking up some of its documentation it appears to support outer joins written in plain sql. In that case this should work: (an outer join where there is no match)
select distinct c.id
from catalog c
left outer join parts p
on (c.pid = p.pid
and p.color <> 'Red')
where p.pid is null
Edit -- enclosed on clause in () , this is not normally required of any major databases but seems to be needed in hiveql -- (https://cwiki.apache.org/confluence/display/Hive/LanguageManual+Joins).
Regarding your first query I don't think it wouldn't work in hive based on example queries in those docs however as another commenter mentioned it is best practice to use explicit joins via the join clause rather than implicit joins in the where clause. Think of the where clause as where you filter based on various conditions (except for join conditions), and the join clause as where you put all join conditions. It helps organize your query's logic. In addition, you can only imply inner joins (in the where clause). The join clause is needed any time you want to work with outer joins (as in the case of your second query, above).
This is the same as your first query but with explicit join syntax:
select pname
from parts p
join catalog c
on (p.pid = c.pid)
I have a sql query that seems to work but I dont really understand why. Therefore I would very much appreciate if someone could help explain whats going on:
THE QUERY RETURNS: All organisations that dont have any comments that were not created by the consultant who created the organisation record.
SELECT \"organisations\".*
FROM \"organisations\"
WHERE \"organisations\".\"id\" NOT IN
(SELECT \"comments\".\"commentable_id\"
FROM \"comments\"
WHERE \"comments\".\"commentable_type\" = 'Organisation'
AND (comments.author_id != organisations.consultant_id)
ORDER BY \"comments\".\"created_at\" ASC
)
It seems to do so correctly.
The part I dont understand is why (comments.author_id != organisations.consultant_id) is working!? I dont understand how postgres even knows what "organisations" is inside that subselect? It is not defined in here.
If this was written as a join where I had joined comments to organisations then I would totally understand how you could do something like this but in this case its a subselect. How does it know how to map the comments and organisations table and exclude the ones where (comments.author_id != organisations.consultant_id)
That subselect happens in a row so it can see all columns of that row. You will probably get better performance with this
select organisations.*
from organisations
where not exists (
select 1
from comments
where
commentable_type = 'organisation' and
author_id != organisations.consultant_id
)
Notice that it is not necessary to qualify commentable_type since the one in comments has priority over any other outside the subselect. And if comments does not have a consultant_id column then it would be possible to take its qualifier out, although not recommended for better legibility.
The order by in your query buys you nothing, just added cost.
You are running a correlated subquery. http://technet.microsoft.com/en-us/library/ms187638(v=sql.105).aspx
This is commonly used in all databases. A subquery in the WHERE clause can refer to tables used in the parent query, and they often do.
That being said, your current query could likely be written better.
Here is one way, using an outer join with comments, where no matches are found based on your criteria -
select o.*
from organizations o
left join comments c
on c.commentable_type <> 'Organisation'
and c.author_id = o.consultant_id
where c.commentable_id is null
My boss found a bug in a query I created, and I don't understand the reasoning behind the bug, although the query results prove he's correct. Here's the query (simplified version) before the fix:
select PTNO,PTNM,CATCD
from PARTS
left join CATEGORIES on (CATEGORIES.CATCD=PARTS.CATCD);
and here it is after the fix:
select PTNO,PTNM,PARTS.CATCD
from PARTS
left join CATEGORIES on (CATEGORIES.CATCD=PARTS.CATCD);
The bug was, that null values were being shown for column CATCD, i.e. the query results included results from table CATEGORIES instead of PARTS.
Here's what I don't understand: if there was ambiguity in the original query, why didn't Oracle throw an error? As far as I understood, in the case of left joins, the "main" table in the query (PARTS) has precedence in ambiguity.
Am I wrong, or just not thinking about this problem correctly?
Update:
Here's a revised example, where the ambiguity error is not thrown:
CREATE TABLE PARTS (PTNO NUMBER, CATCD NUMBER, SECCD NUMBER);
CREATE TABLE CATEGORIES(CATCD NUMBER);
CREATE TABLE SECTIONS(SECCD NUMBER, CATCD NUMBER);
select PTNO,CATCD
from PARTS
left join CATEGORIES on (CATEGORIES.CATCD=PARTS.CATCD)
left join SECTIONS on (SECTIONS.SECCD=PARTS.SECCD) ;
Anybody have a clue?
Here's the query (simplified version)
I think by simplifying the query you removed the real cause of the bug :-)
What oracle version are you using? Oracle 10g ( 10.2.0.1.0 ) gives:
create table parts (ptno number , ptnm number , catcd number);
create table CATEGORIES (catcd number);
select PTNO,PTNM,CATCD from PARTS
left join CATEGORIES on (CATEGORIES.CATCD=PARTS.CATCD);
I get ORA-00918: column ambiguously defined
Interesting in SQL server that throws an error (as it should)
select id
from sysobjects s
left join syscolumns c on s.id = c.id
Server: Msg 209, Level 16, State 1, Line 1
Ambiguous column name 'id'.
select id
from sysobjects
left join syscolumns on sysobjects.id = syscolumns.id
Server: Msg 209, Level 16, State 1, Line 1
Ambiguous column name 'id'.
From my experience if you create a query like this the data result will pull CATCD from the right side of the join not the left when there is a field overlap like this.
So since this join will have all records from PARTS with only some pull through from CATEGORIES you will have NULL in the CATCD field any time there is no data on the right side.
By explicitly defining the column as from PARTS (ie left side) you will get a non null value assuming that the field has data in PARTS.
Remember that with LEFT JOIN you are only guarantied data in fields from the left table, there may well be empty columns to the right.
This may be a bug in the Oracle optimizer. I can reproduce the same behavior on the query with 3 tables. Intuitively it does seem that it should produce an error. If I rewrite it in either of the following ways, it does generate an error:
(1) Using old-style outer join
select ptno, catcd
from parts, categories, sections
where categories.catcd (+) = parts.catcd
and sections.seccd (+) = parts.seccd
(2) Explicitly isolating the two joins
select ptno, catcd
from (
select ptno, seccd, catcd
from parts
left join categories on (categories.CATCD=parts.CATCD)
)
left join sections on (sections.SECCD=parts.SECCD)
I used DBMS_XPLAN to get details on the execution of the query, which did show something interesting. The plan is basically to outer join PARTS and CATEGORIES, project that result set, then outer join it to SECTIONS. The interesting part is that in the projection of the first outer join, it is only including PTNO and SECCD -- it is NOT including the CATCD from either of the first two tables. Therefore the final result is getting CATCD from the third table.
But I don't know whether this is a cause or an effect.
I'm afraid I can't tell you why you're not getting an exception, but I can postulate as to why it chose CATEGORIES' version of the column over PARTS' version.
As far as I understood, in the case of left joins, the "main" table in the query (PARTS) has precedence in ambiguity
It's not clear whether by "main" you mean simply the left table in a left join, or the "driving" table, as you see the query conceptually... But in either case, what you see as the "main" table in the query as you've written it will not necessarily be the "main" table in the actual execution of that query.
My guess is that Oracle is simply using the column from the first table it hits in executing the query. And since most individual operations in SQL do not require one table to be hit before the other, the DBMS will decide at parse time which is the most efficient one to scan first. Try getting an execution plan for the query. I suspect it may reveal that it's hitting CATEGORIES first and then PARTS.
I am using Oracle 9.2.0.8.0. and it does give the error "ORA-00918: column ambiguously defined".
This is a known bug with some Oracle versions when using ANSI-style joins. The correct behavior would be to get an ORA-00918 error.
It's always best to specify your table names anyway; that way your queries don't break when you happen to add a new column with a name that is also used in another table.
It is generally advised to be specific and fully qualify all column names anyway, as it saves the optimizer a little work. Certainly in SQL Server.
From what I can gleen from the Oracle docs, it seems it will only throw if you select the column name twice in the select list, or once in the select list and then again elsewhere like an order by clause.
Perhaps you have uncovered an 'undocumented feature' :)
Like HollyStyles, I cannot find anything in the Oracle docs which can explain what you are seeing.
PostgreSQL, DB2, MySQL and MSSQL all refuse to run the first query, as it's ambiguous.
#Pat: I get the same error here for your query. My query is just a little bit more complicated than what I originally posted. I'm working on a reproducible simple example now.
A bigger question you should be asking yourself is - why do I have a category code in the parts table that doesn't exist in the categories table?
This is a bug in Oracle 9i. If you join more than 2 tables using ANSI notation, it will not detect ambiguities in column names, and can return the wrong column if an alias isn't used.
As has been mentioned already, it is fixed in 10g, so if an alias isn't used, an error will be returned.