I have a bad result with using query in Quicksight.
I have one table with campaign, if I query just on this table, it is ok; I have all campaigns in my list. But when I use left join, i have just the results which match with join table.
Is this normal?
I have already tried all join possibilities and it is the same.
Related
I am trying to optimize the following query in postgresql
SELECT ci.country_id, ci.ci_id,ci.name
FROM customer c
INNER JOIN address a ON c.a_id = a.a_id
INNER JOIN city ci ON ci.ci_id = a.ci_id
The columns customer.a_id, address.a_id, city.ci_id and adress.ci_id all have an btree index.
I wanted to use a merge join instead of a hash join as I read that a hash join not really uses indexes so I turned of the hash joins with Set enable_hashjoin=off.
My query is now according to the query plan using a merge join but it is performing always a quick sort before the merge join. I know that for merge join the columns need to be sorted but they should already be sorted through the index. Is there a way to force Postgres to use the index and not to perform the sort?
You are joining three tables. It is using two merge joins to do that, with the output of one merge join being one input of the other. The intermediate table is joined using two different columns, but it can't be ordered on two different columns simultaneously, so if you are only going to use merge joins, you need at least one sort.
This whole thing seems pointless, as the query is already very fast, and why do you care if it uses a hash join or not?
I'm trying to join two tables together and one table has two identifiers that I want.
select stations.id, stations.name, count (rentals.startstation_id) as station_starts, count (rentals.endstation_id) as station_ends
from ny.station as stations
Left Join ny.rental as rentals on stations.id = rentals.startstation_id
Left Join ny.rental as rentals on station.id = rentals.endstation_id
group by 1,2
However the ny.rental table is very large and when I run this query my SQL Workbench/J crashes. The stations table is rather small.
What is the optimal way to construct this query?
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 have two very big tables for example tab1 and tab2 in Postgres. I need to perform join and group by operation on these tables, to make my query fast i need indexing. Is there any way how to use index over join and group by queries ?? (as I heard indexing is not possible over join and group by)
I am developing a t-sql query to return left join of two tables, but when I just select records from Table A, it gives me only 2 records. The problem though is when I left join it Table B, it gives me 4 records. How can I reduce this to just 2 records?
One problem though is that I am only aware of one PK/FK to link these two tables.
The field you are using for the join must exist more than once in table B - this is why multiple rows are being returned in the join. In order to reduce the row count you will have to either add further fields to the join, or add a where clause to filter out rows not required.
Alternatively you could use a GROUP BY statement to group the rows up, but this may not be what you need.
Remember that the left join brings you null fields from joined table.
Also you can use select(distinct), but i can't see well you issue. Can you give us more details?