I'm fairly new to SQL and have started running into sub-queries as in this query below:
SELECT C.CustomerID
, C.Name
, ( Select PhoneNumber
FROM PhoneNumberTable P
WHERE P.CustomerID = C.CustomerID ) AS "PhoneNumber"
FROM CustomerTable C
Comparing to this query with a join below:
SELECT C.CustomerID
, C.Name
, P.PhoneNumber
FROM CustomerTable C
JOIN PhoneNumberTable P
ON P.customerID = C.customerID
Is there a difference in terms of efficiency/speed? The SQL I am working with has several sub-queries as I have shown above (no JOINs) and it is difficult to read.
joins in my experience tend to be faster, but sometimes you need a subquery.
you should also look into CTE they are very usefull and much easier (in my opinion) to manage
in your specific case i would use a join... because you are trying to join the 2 tables together.
Related
i have i would like to create a join over several tables.
table login : I would like to retrieve all the data from login
table logging : calculating the Nb_of_sessions for each db & for each a specific event type by user
table meeting : calculating the Nb_of_meetings for each db & for each user
table live : calculating the Nb_of_live for each db & for each user
I have those queries with the right results :
SELECT db.id,_id as userid,firstname,lastname
FROM "logins"."login",
UNNEST(dbs) AS a1 (db)
SELECT dbid,userid,count(distinct(sessionid)) as no_of_visits,
array_join(array_agg(value.from_url),',') as from_url
FROM "loggings"."logging"
where event='url_event'
group by db.id,userid;
SELECT dbid,userid AS userid,count(*) as nb_interviews,
array_join(array_agg(interviewer),',') as interviewer
FROM "meetings"."meeting"
group by dbid,userid;
SELECT dbid,r1.user._id AS userid,count(_id) as nb_chat
FROM "lives"."live",
UNNEST(users) AS r1 (user)
group by dbid,r1.user._id;
But when i begin to try put it all together, it seems i retrieve bad data (i have only on db retrieved) and it seems not efficient.
select a1.db.id,a._id as userid,a.firstname,a.lastname,count(rl._id) as nb_chat
FROM
"logins"."login" a,
"loggings"."logging" b,
"meetings"."meeting" c,
"lives"."live" d,
UNNEST(dbs) AS a1 (db),
UNNEST(users) AS r1 (user)
where a._id = b.userid AND a._id = c.userid AND a._id = r1.user._id
group by 1,2,3,4
Do you have an idea ?
Regards.
The easiest way is to work with with to structure the subquery and then reference them.
with parameter reference:
You can use WITH to flatten nested queries, or to simplify subqueries.
The WITH clause precedes the SELECT list in a query and defines one or
more subqueries for use within the SELECT query.
Each subquery defines a temporary table, similar to a view definition,
which you can reference in the FROM clause. The tables are used only
when the query runs.
Since you already have working sub queries, the following should work:
with logins as
(
SELECT db.id,_id as userid,firstname,lastname
FROM "logins"."login",
UNNEST(dbs) AS a1 (db)
)
,visits as
(
SELECT dbid,userid,count(distinct(sessionid)) as no_of_visits,
array_join(array_agg(value.from_url),',') as from_url
FROM "loggings"."logging"
where event='url_event'
group by db.id,userid
)
,meetings as
(
SELECT dbid,userid AS userid,count(*) as nb_interviews,
array_join(array_agg(interviewer),',') as interviewer
FROM "meetings"."meeting"
group by dbid,userid
)
,chats as
(
SELECT dbid,r1.user._id AS userid,count(_id) as nb_chat
FROM "lives"."live",
UNNEST(users) AS r1 (user)
group by dbid,r1.user._id
)
select *
from logins l
left join visits v
on l.dbid = v.dbid
and l.userid = v.userid
left join meetings m
on l.dbid = m.dbid
and l.userid = m.userid
left join chats c
on l.dbid = c.dbid
and l.userid = c.userid;
All of the documentation for Cosmos DB and it looks like it only supports the JOINkeyword, which seems to be a sort of INNER JOIN.
I have the following query:
SELECT * FROM
(
SELECT
DISTINCT(c.id),
c.OtherCollection,
FROM c
JOIN s IN c.OtherCollection
)
AS c order by c.id
This works fine and returns the data of documents that have OtherCollection populated. But It obviously does not return any documents that do not have it populated.
The reason for the join is that sometimes I execute the following query (queries are built up from user input)
SELECT * FROM
(
SELECT
DISTINCT(c.id),
c.OtherCollection,
FROM c
JOIN s IN c.OtherCollection
WHERE s.PropertyName = 'SomeValue'
)
AS c order by c.id
The question is how can I have a sort of LEFT JOIN operator in this scenario?
CosmosDB JOIN operation is limited to the scope of a single document. What possible is you can join parent object with child objects under same document.
It is totally different from SQL Join query which supports across two/many tables.
You can simulate LEFT JOIN with the EXISTS sentence.
Eg:
SELECT VALUE c
FROM c
WHERE (
(c.OtherCollection = null) OR EXISTS (--Like a "Left Join"
SELECT null
FROM s IN c.OtherCollection
WHERE s.PropertyName = 'SomeValue'
)
)
--AND/OR Some other c Node conditions
order by c.id
I am running a query with Oracle:
SELECT
c.customer_number,
COUNT(DISTINCT o.ORDER_NUMBER),
COUNT(DISTINCT q.QUOTE_NUMBER)
FROM
Customer c
JOIN Orders o on c.customer_number = o.party_number
JOIN Quote q on c.customer_number = q.account_number
GROUP BY
c.customer_number
This works beautifully and I can get the customer and their order and quote counts.
However, not all customers have orders or quotes but I still want their data. When I use LEFT JOIN I get this error from Oracle:
ORA-24347: Warning of a NULL column in an aggregate function
Seemingly this error is caused by the eventual COUNT(NULL) for customers that are missing orders and/or quotes.
How can I get a COUNT of null values to come out to 0 in this query?
I can do COUNT(DISTINCT NVL(o.ORDER_NUMBER, 0)) but then the counts will come out to 1 if orders/quotes are missing which is no good. Using NVL(o.ORDER_NUMBER, NULL) has the same problem.
Try using inline views:
SELECT
c.customer_number,
o.order_count,
q.quote_count
FROM
customer c,
( SELECT
party_number,
COUNT(DISTINCT order_number) AS order_count
FROM
orders
GROUP BY
party_number
) o,
( SELECT
account_number,
COUNT(DISTINCT quote_number) AS quote_count
FROM
quote
GROUP BY
account_number
) q
WHERE 1=1
AND c.customer_number = o.party_number (+)
AND c.customer_number = q.account_number (+)
;
Sorry, but I'm not working with any databases right now to test this, or to test whatever the ANSI SQL version might be. Just going on memory.
How to change this complex sql statement into JPQL?
select a.name, a.surname, b.street, c.location, c.location_desc
from table1 join table2 on b.id = a.some_fk
left join table3 d on d.id = a.id
left join table4 c on c.some2_id = d.some2_fk where a.id = 400;
If this is possible to present in the JPQL form?
Impossible to give a definitive answer without knowing the entities and their mapping, but the query would look like this:
select a.name, a.surname, b.street, c.location, c.locationDesc
from Entity1 a
join a.entity2 b
left join a.entity3 d
left join d.entity4 c
where a.id = 400;
provided the necessary associations between entities are there.
JPQL is object oriented, it operates against JPA entity objects ,not database tables. Either you need to change the Question and add an UML diagramm or provide the Entity classes.
I want to perform a simple join on two tables (BusinessUnit and UserBusinessUnit), so I can get a list of all BusinessUnits allocated to a given user.
The first attempt works, but there's no override of Select which allows me to restrict the columns returned (I get all columns from both tables):
var db = new KensDB();
SqlQuery query = db.Select
.From<BusinessUnit>()
.InnerJoin<UserBusinessUnit>( BusinessUnitTable.IdColumn, UserBusinessUnitTable.BusinessUnitIdColumn )
.Where( BusinessUnitTable.RecordStatusColumn ).IsEqualTo( 1 )
.And( UserBusinessUnitTable.UserIdColumn ).IsEqualTo( userId );
The second attept allows the column name restriction, but the generated sql contains pluralised table names (?)
SqlQuery query = new Select( new string[] { BusinessUnitTable.IdColumn, BusinessUnitTable.NameColumn } )
.From<BusinessUnit>()
.InnerJoin<UserBusinessUnit>( BusinessUnitTable.IdColumn, UserBusinessUnitTable.BusinessUnitIdColumn )
.Where( BusinessUnitTable.RecordStatusColumn ).IsEqualTo( 1 )
.And( UserBusinessUnitTable.UserIdColumn ).IsEqualTo( userId );
Produces...
SELECT [BusinessUnits].[Id], [BusinessUnits].[Name]
FROM [BusinessUnits]
INNER JOIN [UserBusinessUnits]
ON [BusinessUnits].[Id] = [UserBusinessUnits].[BusinessUnitId]
WHERE [BusinessUnits].[RecordStatus] = #0
AND [UserBusinessUnits].[UserId] = #1
So, two questions:
- How do I restrict the columns returned in method 1?
- Why does method 2 pluralise the column names in the generated SQL (and can I get round this?)
I'm using 3.0.0.3...
So far my experience with 3.0.0.3 suggests that this is not possible yet with the query tool, although it is with version 2.
I think the preferred method (so far) with version 3 is to use a linq query with something like:
var busUnits = from b in BusinessUnit.All()
join u in UserBusinessUnit.All() on b.Id equals u.BusinessUnitId
select b;
I ran into the pluralized table names myself, but it was because I'd only re-run one template after making schema changes.
Once I re-ran all the templates, the plural table names went away.
Try re-running all 4 templates and see if that solves it for you.