Is it possible to write this query in ROR?
SELECT column_1,
(SELECT name FROM table_2 WHERE table_2.column_1 = table_1.column_1) as name
FROM table_1;
Yes, it is possible:
Table_1.select("column_1, (SELECT name FROM table_2 WHERE table_2.column_1 = table_1.column_1) as name")
If you will user Arel it will seems yet more complicated then this.
But exists other ways to simplify this query:
split it to two query and merge it together in Rails
using joins method for join table_1 and table_2 and select field table_2.name.
Related
I'm currently using StandardSQL in BigQuery, I tried to join two sets of table one of which is a pseudo-column table partitioned by day.
I tried to use this query below:
SELECT
DISTINCT DATE(create_time) AS date,
user_id,
city_name,
transaction_id,
price
FROM
table_1 a
LEFT JOIN (SELECT user_id, city_name FROM table_2) b
ON (a.user_id = b.user_id AND DATE(create_time) = _PARTITIONDATE)
I've tried this kind of JOIN (using _PARTITIONDATE) and worked out, but for this particular query I got an error message:
Unrecognized name: _PARTITIONDATE
Can anyone tell me why this happened, and how could I solve this? Thanks in advance.
The issue is that you are not selecting the _PARTITIONDATE field from table_2 when joining it so it can't recognize it:
SELECT user_id, city_name FROM table_2
In order to solve it you can add it as follows:
SELECT
DISTINCT DATE(create_time) AS date,
user_id,
city_name,
transaction_id,
price
FROM
table_1 a
LEFT JOIN (SELECT _PARTITIONDATE AS pd, user_id, city_name FROM table_2) b
ON (a.user_id = b.user_id AND DATE(create_time) = pd)
Note that you'll need an alias such as pd as it's a pseudocolumn
Probably it was working in the past if you were joining two tables directly such as in (you don't get selectivity benefits in that case):
FROM
table_1 a
LEFT JOIN table_2 b
ON (a.user_id = b.user_id AND DATE(create_time) = _PARTITIONDATE)
I would like to join tables . Could you please help?
Select Number, OwnerId from DNIS.numbers
select ID,Name from DNIS.owners
Thank you.
Normally, SQL servers allow you to join tables from different databases as long as the former all belong to them. Here is an example showing you how to do this (all you have to do is to explicitly write the database names associated to each table in the query):
SELECT N.Number, N.OwnerId, O.ID, O.Name
FROM DB1.[dbo].DNIS numbers N
JOIN DB2.[dbo].DNIS owners O ON O.ID = N.OwnerId
You can also use the following syntax:
SELECT N.Number, N.OwnerId, O.ID, O.Name
FROM DB1..DNIS numbers N
JOIN DB2..DNIS owners O ON O.ID = N.OwnerId
In order to accomplish that you will have to specify the table and column names in your join statement, like so:
SELECT db1.tablename.column, db2.tablename.column
FROM db1.tablename INNER JOIN db2.tablename
ON db1.tablename.id = db2.tablename.id;
I am writing a hive query to join two tables; table1 and table2. In the result I just need all columns from table1 and no columns from table2.
I know the solution where I can select all the columns manually by specifying table1.column1, table1.column2.. and so on in the select statement. But I have about 22 columns in table 1. Also, I have to do the same for multiple other tables ans its painful process.
I tried using "SELECT table1.*", but I get a parse exception.
Is there a better way to do it?
Hive 0.13 onwards the following query syntax works:
SELECT a.* FROM a JOIN b ON (a.id = b.id)
This query will select all columns from a. So instead of typing all the column names (making the query cumbersome), it is a better idea to use tablealias.*
I want to expand this question.
order by foreign key in activerecord
I'm trying to order a set of records based on a value in a really large table.
When I use join, it brings all the "other" records data into the objects.. As join should..
#table users 30+ columns
#table bids 5 columns
record = Bid.find(:all,:joins=>:users, :order=>'users.ranking DESC' ).first
Now record holds 35 fields..
Is there a way to do this without the join?
Here's my thinking..
With the join I get this query
SELECT * FROM "bids"
left join users on runner_id = users.id
ORDER BY ranking LIMIT 1
Now I can add a select to the code so I don't get the full user table, but putting a select in a scope is dangerous IMHO.
When I write sql by hand.
SELECT * FROM bids
order by (select users.ranking from users where users.id = runner_id) DESC
limit 1
I believe this is a faster query, based on the "explain" it seems simpler.
More important than speed though is that the second method doesn't have the 30 extra fields.
If I build in a custom select inside the scope, it could explode other searches on the object if they too have custom selects (there can be only one)
What you would like to achieve in active record writing is something along
SELECT b.* from bids b inner join users u on u.id=b.user_id order by u.ranking desc
In active record i would write such as:
Bids.joins("inner join users u on bids.user_id=u.id").order("u.ranking desc")
I think it's the only to make a join without fetching all attributes from the user models.
So I'm getting a bunch of Volunteers records, with some filtering and sorting, which is fine. But I'd like to also get a count of the number of Children each volunteer is helping (using volunteer_id on children table), as a sub-query in the select clause to avoid having to perform a separate query for each record. As a bonus it would be good to be able to sort by this count too!
I'd like to end up with a generated query like this and be able to access the 'kids' column:
SELECT id, name, (SELECT COUNT(*) FROM children WHERE volunteer_id = volunteers.id) AS kids FROM volunteers
Is there any way of doing this with Arel? I've had a bit of a scout around and haven't found anything yet.
Alternatively, is it possible to join to the children table and get: count(children.id) ?
Thanks for any help :)
The proper way of doing this with SQL is with a GROUP BY clause:
SELECT v.id, v.name, COUNT(*) AS kids
FROM volunteers v
LEFT OUTER JOIN children c ON v.id = c.volunteer_id
GROUP BY v.id, v.name
There is a method .group() in AR for using GROUP BY queries.