Okay, I'm relatively new to the more advanced uses of SQL Server.
I have several tables that I need to gather informations from, and several of these tables links to other tables where I need a specific information. As a result, I just want one row with all the information, preferential named with aliases.
For example:
Tab_Transcoders:
ID, VideoCamID, InputStreamID, OutputStreamID.
where InputStream links to another table where I need the row of the matching ID, where in this row are other ID's (e.g. StreamType_ID that belongs to a third table containing ID_StreamType and Description etc.)
Same with OutputStreamID, same with VideoCamID.
In the end, I need a row containing for example:
ID, VideoCamID, InputStreamID, InputStreamType, InputStreamTypeDesc,
OutputStreamID, OutputStreamType, OutputStreamDesc, VideoCamID, etc. etc. etc.
It is important for me that I can set aliases, as for example InputStreamID & OutputStreamID links to the same table where all my Streams are listed (with IP's, Descs..)
I can accomplish this with doing like 100 SELECTS & SUBSELECTS, but I don't think that's an appropriate way.
I read some informations about things like CURSOR, UNION, FETCH, JOIN etc. etc.. but I don't know which one I have to use for my purpose.
eli
I think you want something like the following....
Select
t.ID,
t.VideoCamID,
i.InputStreamID,
is.StreamType as InputStreamType,
is.StreamDesc as InputStreamDesc,
o.OutputStreamID,
os.StreamType as OutputStreamType,
os.StreamDesc as OutputStreamDesc,
v.VideoCamID
from
Tab_Transcoders t
inner join InputStreams i on i.InputStreamID=t.InputStreamId
inner join Streams is on is.StreamId=i.StreamId
inner join OutputStreams o on o.OutputStreamId=t.OutputStreamId
inner join Streams os on os.StreamID=o.StreamId
inner join VideoCams v on v.VideoCamId=t.VideoCamID
If there is a defined relationship between your tables, then Use Join.
e.g Customer Order
Order will have customer Id
Select Order.ID,Order.Quantity, Order.CustomerId, Customer.FullName, Customer.Address
From Orders Order
Join
Customer
On
Order.CustomerId = Customer.CustomerId
First start by getting data from two tables using the join and then if it works as per your requirement, add another required table in the join.
Read about SQl JOINS.. It is fairly simple.
I will recommend reading you some of the articles around CTE aka Common Table Expression.
Refer http://msdn.microsoft.com/en-us/library/ms190766%28v=sql.105%29.aspx.
Apart from this never use subqueries. Try to use inner join / any other join if possible.
Related
I have a DB2 database and I have to join three tables. I would like to join first two tables and after joining firs two tables, I would like to join the joined table to another third table. I tried using left join but couldn't find a result as I expected. I tried the following:
select AFJKAR as "ELR_Elig_Redirect_SchdID",
AFEZAM as "Priority",
AFTSAS as "ELC_Status",
AFT7CE as "ELC_From_Date",
AFT8CE as "ELC_Thru_Date",
AFTTAS as "ELC_Redirect_Action",
AFJLAR as "GPI_List",
AIZAHA as "GPI_List_ID",
AILUIG as "GPI_Number",
AICXHG as "GPI_From_Date",
AICYHG as "GPI_Thru_Date",
SUEFC4 as "GPI_ID",
SUB4T3 as "Drug_Name"
from CLMPRDFIL.RCELCP as RCE
left join CLMPRDFIL.RCGP2P as RCG on RCE.AFJLAR = RCG.AIZAHA
left join CLMPRDFIL.RCGPIP as RCGP on RCG.AIZAHA = RCGP.SUEFC4;
Basically, I would like to join RCE and RCGP2P tables first. After joining this, I would like to join it by RCGPIP.
Use a corresponding optimization profile / guideline for this.
Optimization profiles and guidelines.
You may specify the desired join order there, if you believe, that you may achieve better performance with particular join order.
Note, that you should try to collect statistics on these tables first to make the Db2 optimizer use correct join order. For example, try to create a statistical view on first 2 tables using their join keys and collect statistics on it. Look at the access plan of your original query afterwards to check, if you get desired join order.
I am trying to use the existing Except clause in Bigquery. Please find my query below
select * EXCEPT (b.hosp_id, b.person_id,c.hosp_id) from
person a
inner join hospital b
on a.hosp_id= b.hosp_id
inner join reading c
on a.hosp_id= c.hosp_id
As you can see I am using 3 tables. All the 3 tables have the hosp_id column, so I would like to remove duplicate columns which are b.hosp_id and c.hosp_id. Simlarly, I would like to remove b.person_id column as well.
When I execute the above query, I get the syntax error as shown below
Syntax error: Expected ")" or "," but got "." at [9:19]
Please note that all the columns that I am using in Except clause is present in the tables used. Additional info is all the tables used are temp tables created using with clause. When I do the same manually by selecting column of interest, it works fine. But I have several columns and can't do this manually.
Can you help? I am trying to learn Bigquery. Your inputs would help
I use the EXCEPT on a per-table basis:
select p.* EXCEPT (hosp_id, person_id),
h.*,
r.* EXCEPT (hosp_id)
from person p inner join
hospital h
on p.hosp_id = h.hosp_id inner join
reading r
on p.hosp_id = r.hosp_id;
Note that this also uses meaningful abbreviations for table aliases, which makes the query much simpler to understand.
In your case, I don't think you need EXCEPT at all if you use the USING clause.
Try this instead:
select * EXCEPT (person_id) from
person a
inner join hospital b
using (hosp_id)
inner join reading c
using (hosp_id)
You can only put column names (not paths) in the EXCEPT list, and you can simply avoid projecting the duplicate columns with USING instead of ON.
I have a SQL query that works in Access 2016:
SELECT
Count(*) AS total_tests,
Sum(IIf(score>=securing_threshold And score<mastering_threshold,1,0)) AS total_securing,
Sum(IIf(score>=mastering_threshold,1,0)) AS total_mastering,
total_securing/Count(*) AS percent_securing,
total_mastering/Count(*) AS percent_mastering,
(Count(*)-total_securing-total_mastering)/Count(*) AS percent_below,
subjects.subject,
students.year_entered,
IIf(Month(Date())<9,Year(Date())-students.year_entered+6,Year(Date())-students.year_entered+7) AS current_form,
groups.group
FROM
((subjects
INNER JOIN tests ON subjects.ID = tests.subject)
INNER JOIN (students
INNER JOIN test_results ON students.ID = test_results.student) ON tests.ID = test_results.test)
LEFT JOIN
(SELECT * FROM group_membership LEFT JOIN groups ON group_membership.group = groups.ID) As g
ON students.ID = g.student
GROUP BY subjects.subject, students.year_entered, groups.group;
However, I wish to filter out irrelevant groups before joining them to my table. The table groups has a column subject which is a foreign key.
When I try changing ON students.ID = g.student to ON students.ID = g.student And subjects.ID = g.subject I get the error 'JOIN expression not supported'.
Alternatively, when I try adding WHERE subjects.ID = groups.subject to the subquery, it asks me for the parameter value of subjects.ID, although it is a column in the parent query.
Googling reveals many similar errors but they were all resolved by changing the brackets. That didn't help.
Just in case the table relationships help:
Thank you.
EDIT: Sample database at https://www.dropbox.com/s/yh80oooem6gsni7/student%20tracker.ACCDB?dl=0
MS Access queries with many joins are difficult to update by SQL alone as parenetheses pairings are required unlike other RDBMS's and these pairings must follow an order. Moreover, some pairings can even be nested. Hence, for beginners it is advised to build queries with complex and many joins using the query design GUI in the MS Office application and let it build out the SQL.
For a simple filter on the g derived table, you could filter subject on the derived table, g, but likely you want want all subjects:
...
(SELECT * FROM group_membership
LEFT JOIN groups ON group_membership.group = groups.ID
WHERE groups.subject='Earth Science') As g
...
So for all subjects, consider re-building query from scratch in GUI that nearly mirrors your table relationships which actually auto-links joins in the GUI. Then, drop unneeded tables.
Usually you want to begin with the join table or set like groups and group_membership or tests and test_results. In fact, consider saving the g derived table as its own query.
Then add the distinct record primary source tables like students and subjects.
You may even need to play around with order in FROM and JOIN clauses to attain desired results, and maybe even add the same table in query. And be careful with adding join tables like group_membership (two one-to-many links), to GROUP BY queries as it leads to the duplicate record aggregation. So you may need to join aggregates queries by subject.
Unless you can post content of all tables, from our perspective it is difficult to help from here.
Your subquery g uses a LEFT JOIN, but there is a enforced 1:n relation between the two tables, so there will always be a matching group. Use a INNER JOIN instead.
With g.subject you are trying to join on a column that is on the right side of a left join, that cannot really work.
Also you shouldn't use SELECT * on a join of tables with identical column names. Include only the qualified column names that you need.
LEFT JOIN
(SELECT group_membership.student, groups.group, groups.subject
FROM group_membership INNER JOIN groups
ON group_membership.group = groups.ID) As g
ON (students.ID = g.student AND subjects.ID = g.subject)
I would call the columns in group_membership group_ID and student_ID to avoid confusion.
I don't have the database to test, but I would use subject table as subquery:
(SELECT * FROM subject WHERE filter out what you don't need) Subj
Then INNER JOIN this new Subj Table in your query which would exclude irrelevant groups.
Also I would never create join in WHERE clause (WHERE subjects.ID = groups.subject), what this does it creates cartesian product (table with all the possible combinations of subjects.ID and groups.subject) then it filters out records to satisfy your join. When dealing with huge data it might take forever or crash.
Error related to "Join expression may not be supported"; do datatypes match in those fields?
I solved it by (a lot of trial and error and) taking the advice here to make queries in the GUI and joining them. The end result is 4 queries deep! If the database was bigger, performance would be awful... but now its working I can tweak it eventually.
Thank you everybody!
The following sql query below produces the specified result.
select product.product_no,product_type,salesteam.rep_name,salesteam.SUPERVISOR_NAME
from product
inner join salesteam
on product.product_rep=salesteam.rep_id
ORDER BY product.Product_No;
However my intensions are to further produce a more detailed result which will include all the attributes in the PRODUCT table. my approach is to list all the attributes in the first line of the query.
select product.product_no,product.product_date,product.product_colour,product.product_style,
product.product_age product_type,salesteam.rep_name,salesteam.SUPERVISOR_NAME
from product
inner join salesteam
on product.product_rep=salesteam.rep_id
ORDER BY product.Product_No;
Is there another way it can be done instead of listing all the attributes of PRoduct table one by one?
You can use * to select all columns from all tables, or you can use [table/alias].* to select all columns from the specified table. In your case, you can use product.*:
select product.*,salesteam.rep_name,salesteam.SUPERVISOR_NAME
from product
inner join salesteam
on product.product_rep=salesteam.rep_id
ORDER BY product.Product_No;
It is important to note that you should only do this if you are 100% sure you need every single column, and always will. There are performance implications associated with this; if you're selecting 100 columns from a table when you really only need 4 or 5 of them, you're adding a lot of overhead to the query. The DBMS has to work harder, and you're also sending more data across the wire (if your database is not on the same machine as your executing code).
If any columns are later added to the product table, those columns will also be returned by this query in the future.
select
product.*,
salesteam.rep_name,
salesteam.SUPERVISOR_NAME
from product inner join salesteam on
product.product_rep=salesteam.rep_id
ORDER BY
product.Product_No;
This should do.
You can write like this
select P.* --- all Product columns
,S.* --- all salesteam columns
from product P
inner join salesteam S
on P.product_rep=S.rep_id
ORDER BY P.Product_No;
I'm studying SQL for a database exam and the way I've seen SQL is they way it looks on this page:
http://en.wikipedia.org/wiki/Star_schema
IE join written the way Join <table name> On <table attribute> and then the join condition for the selection. My course book and my exercises given to me from the academic institution however, use only natural join in their examples. So when is it right to use natural join? Should natural join be used if the query can also be written using JOIN .. ON ?
Thanks for any answer or comment
A natural join will find columns with the same name in both tables and add one column in the result for each pair found. The inner join lets you specify the comparison you want to make using any column.
IMO, the JOIN ON syntax is much more readable and maintainable than the natural join syntax. Natural joins is a leftover of some old standards, and I try to avoid it like the plague.
A natural join will find columns with the same name in both tables and add one column in the result for each pair found. The inner join lets you specify the comparison you want to make using any column.
The JOIN keyword is used in an SQL statement to query data from two or more tables, based on a relationship between certain columns in these tables.
Different Joins
* JOIN: Return rows when there is at least one match in both tables
* LEFT JOIN: Return all rows from the left table, even if there are no matches in the right table
* RIGHT JOIN: Return all rows from the right table, even if there are no matches in the left table
* FULL JOIN: Return rows when there is a match in one of the tables
INNER JOIN
http://www.w3schools.com/sql/sql_join_inner.asp
FULL JOIN
http://www.w3schools.com/sql/sql_join_full.asp
A natural join is said to be an abomination because it does not allow qualifying key columns, which makes it confusing. Because you never know which "common" columns are being used to join two tables simply by looking at the sql statement.
A NATURAL JOIN matches on any shared column names between the tables, whereas an INNER JOIN only matches on the given ON condition.
The joins often interchangeable and usually produce the same results. However, there are some important considerations to make:
If a NATURAL JOIN finds no matching columns, it returns the cross
product. This could produce disastrous results if the schema is
modified. On the other hand, an INNER JOIN will return a 'column does
not exist' error. This is much more fault tolerant.
An INNER JOIN self-documents with its ON clause, resulting in a
clearer query that describes the table schema to the reader.
An INNER JOIN results in a maintainable and reusable query in
which the column names can be swapped in and out with changes in the
use case or table schema.
The programmer can notice column name mis-matches (e.g. item_ID vs itemID) sooner if they are forced to define the ON predicate.
Otherwise, a NATURAL JOIN is still a good choice for a quick, ad-hoc query.