Is there a way to tell Talend not to remove the prefix of column names especially when they are specified in the query to retrieve data from data source and keep the names mentioned in the query itself?
Thanks!
Assuming you are using the 'guess schema' feature with a query that joins some tables. Further assuming your tables have columns with the same names you run into trouble with the guessed schema. There is no way to have talend use or even know the names of the tables the colums come from, because they are part of a 'projection' and could result from transformation and/or aggregation. Thus, you'll need to help talend guessing the correct schema, which means a) you cant use the * to select all columns and b) you should assign each column an alias that hints at the table the column comes from.
So instead of select * from employee join department on employee.department_id = department.id you'd have something like select e.id as emp_id, e.name as emp_name, d.id as department_id, d.name as department_name from employee e join department d on e.department_id = d.id. The id from employee will be emp_id in the guessed schema.
Related
Okay. So the question that I got asked by the teacher was this:
(5 marks) Construct a SQL query on the dvdrental database that uses a natural join of two or more tables and an additional where condition. (E.g. find the titles of films rented by a particular customer.) Note the hints on the course news page if your query returns nothing.
Here is the layout of the database im working with:
http://www.postgresqltutorial.com/wp-content/uploads/2013/05/PostgreSQL-Sample-Database.png
The hint to us was this:
PostgreSQL hint:
If a natural join doesn't produce any results in the dvdrental DB, it is because many tables have the last update: timestamp field, and thus the natural join tries to join on that field as well as the intended field.
e.g.
select *
from film natural join inventory;
does not work because of this - it produces an empty table (no results).
Instead, use
select *
from film, inventory
where film.film_id = inventory.film_id;
This is what I did:
select *
from film, customer
where film.film_id = customer.customer_id;
The problem is I cannot get a particular customer.
I tried doing customer_id = 2; but it returns a error.
Really need help!
Well, it seems that you would like to join two tables that have no direct relation with each other, there's your issue:
where film.film_id = customer.customer_id
To find which films are rented by which customer you would have to join customer table with rental, then with inventory and finally with film.
The task description states
Construct a SQL query on the dvdrental database that uses a natural join of two or more tables and an additional where condition.quote
I'm trying to build a request to get the data from a table, but some of those columns have foreign keys I would like to replace by the associated keyword in one request.
Basically there's
table A with column 1:PKA-ID and column 2:name.
table B with column 1:PKB-ID, column 2:FKA-ID, column 3:amount.
I want to get all the lines in table B but with all foreign keys replaced by the associated names in table A.
I started building a request with a subrequest + alias to get that, but ofc I have more than one result per subrequest, yet I can't find a way to link that subrequest to the ID of table B [might be exhausted, dumb or both] from the main request. I did something like that:
SELECT (SELECT "NAME" FROM A JOIN B ON ID = FKA-ID) AS name, amount FROM TABLEB;
it feels so simple of a request yet...
You don't need a join in the subselect.
SELECT pkb_id,
(SELECT name FROM a WHERE a.pka_id = b.fka_id),
amount
FROM b;
(See it live in SQL Fiddle).
The subselect query runs for each and every row of its parent select and has the parent row available from the context.
You can also use a simple join.
SELECT b.pkb_id, a.name, b.amount
FROM b, a
WHERE a.pka_id = b.fka_id;
Note that the join version puts less restrictions on the PostgreSQL query optimizer so in some cases the join version might work faster. (For example, in PostgreSQL 9.6 the join might utilize multiple CPU units, cf. Parallel Query).
I have the following schema dataset which i want to transform into a table that can be exported to SQL. I am using HIVE. Input as follows
call_id,stat1,stat2,stat3
1,a,b,c,
2,x,y,z,
3,d,e,f,
1,j,k,l,
The output table needs to have call_id as its primary key so it needs to be unique. The output schema should be
call_id,stat2,stat3,
1,b,c, or (1,k,l)
2,y,z,
3,e,f,
The problem is that when i use the keyword DISTINCT in the HIVE query, the DISTINCT applies to the all the colums combined. I want to apply the DISTINCT operation only to the call_id. Something on the lines of
SELECT DISTINCT(call_id), stat2,stat3 from intable;
However this is not valid in HIVE(I am not well-versed in SQL either).
The only legal query seems to be
SELECT DISTINCT call_id, stat2,stat3 from intable;
But this returns multiple rows with same call_id as the other columns are different and the row on the whole is distinct.
NOTE: There is no arithmetic relation between a,b,c,x,y,z, etc. So any trick of averaging or summing is not viable.
Any ideas how i can do this?
One quick idea,not the best one, but will do the work-
hive>create table temp1(a int,b string);
hive>insert overwrite table temp1
select call_id,max(concat(stat1,'|',stat2,'|',stat3)) from intable group by call_id;
hive>insert overwrite table intable
select a,split(b,'|')[0],split(b,'|')[1],split(b,'|')[2] from temp1;
,,I want to apply the DISTINCT operation only to the call_id"
But how will then Hive know which row to eliminate?
Without knowing the amount of data / size of the stat fields you have, the following query can the job:
select distinct i1.call_id, i1.stat2, i1.stat3 from (
select call_id, MIN(concat(stat1, stat2, stat3)) as smin
from intable group by call_id
) i2 join intable i1 on i1.call_id = i2.call_id
AND concat(i1.stat1, i1.stat2, i1.stat3) = i2.smin;
I am using a PostgreSQL database. I have one select query:
select userid, name, age from tbluser;
Now there is another table, tblcalculatedtax, which is generated on the fly and their column names are not predefined, the only mapping between that table and this table is userid. I want to get records after joining two tables. How can I get that?
Simpler:
SELECT *
FROM tbluser
JOIN tblcalculatedtax USING (userid)
Details in the fine manual about SELECT.
Your need SQL Joins. Here's a W3Schools tutorial: http://www.w3schools.com/sql/sql_join.asp
To quickly answer your question, though:
SELECT * FROM tbluser
INNER JOIN tblcalculatedtax
ON tbluser.userid=tblcalculatedtext.userid
The * selects all the columns, so you don't need to know their names. Of course, I'm not sure what use a column is to you if you don't know it's name: do you know what data it contains?
(I don't think I have titled this question correctly - but I don't know how to describe it)
Here is what I am trying to do:
Let's say I have a Person table that has a PersonID field. And let's say that a Person can belong to many Groups. So there is a Group table with a GroupID field and a GroupMembership table that is a many-to-many join between the two tables and the GroupMembership table has a PersonID field and a GroupID field. So far, it is a simple many to many join.
Given a list of GroupIDs I would like to be able to write a query that returns all of the people that are in ALL of those groups (not any one of those groups). And the query should be able to handle any number of GroupIDs. I would like to avoid dynamic SQL.
Is there some simple way of doing this that I am missing?
Thanks,
Corey
select person_id, count(*) from groupmembership
where group_id in ([your list of group ids])
group by person_id
having count(*) = [size of your list of group ids]
Edited: thank you dotjoe!
Basically you are looking for Persons for whom there is no group he is not a member of, so
select *
from Person p
where not exists (
select 1
from Group g
where not exists (
select 1
from GroupMembership gm
where gm.PersonID = p.ID
and gm.GroupID = g.ID
)
)
You're basically not going to avoid "dynamic" SQL in the sense of dynamically generating the query at query time. There's no way to hand a list around in SQL (well, there is, table variables, but getting them into the system from C# is either impossible (2005 & below) or else annoying (2008)).
One way that you could do it with multiple queries is to insert your list into a work table (probably a process-keyed table) and join against that table. The only other option would be to use a dynamic query such as the ones specified by Jonathan and hongliang.