9/26/2023 0 Comments Elixir ecto subquery![]() …and a User schema that looks like this: defmodule Registrar.User do use Ecto.Schema schema "users" do field ( :uuid, Ecto.UUID ) field ( :full_name, :string ) end end ![]() To start with, we have an AdmissionEvent schema that looks like this: defmodule do use Ecto.Schema schema "admission_events" do field ( :action, :string ) field ( :admission_id, :integer ) field ( :admitter_uuid, Ecto.UUID ) field ( :occurred_at, :naive_datetime ) end end Say you work at a school with an admissions department, and you’ve been tasked with displaying an event log showing all the events related to a given admission, organized into three columns: 1) date, 2) action taken, and 3) who the action was taken by. Let’s run through an example to see it in action. You can accomplish that same end result in a single query expression with Ecto.Query#select_merge/3. Happily, today I learned there’s a better way. In the past, whenever this happened, I’d usually spin up something I wasn’t totally satisfied with – maybe updating the schema(s), breaking it up into multiple queries, or building a multi-select statement if I was feeling fancy. ![]() SELECT w.Job AS Joboffer, (SELECT e.LastNameįROM Employee e WHERE e.Id = w.Working with Elixir and Ecto, I’ve run into scenarios where I needed to retrieve data from a table plus maybe a field or two from an unassociated table. As here it was used in SELECT clause and I’m right that it’s an example of correlated subquery and not a derived table?.Every derived table must have its own alias.īut I’ve bumped into these examples and I’m a but lost Used mostly in FROM clause and never in WHERE clause. Derived table – always returns a table.Also could be used in SELECT and FROM clause ( never returns a table) Subquery - simply nested query most often used in WHERE clause.I'm newbie and I'm trying to sum up my knowledge of subquery and derived tables so please correct me if I wrong or I missed something: Thanks to -sniff- for the awesome header image! You should post these to /r/learnsql instead. Note /r/SQL does not allow links to basic tutorials to be posted here. Please view the Wiki for online resources. Learning SQLĪ common question is how to learn SQL. SELECT count(a.field1), a.field2, SUM(b.field4) FROM a INNER JOIN b ON a.key1 = b.key1 WHERE a.field8 = 'test' GROUP by a.field1, a.field2 HAVING SUM(b.field4) > 5 ORDER by a.field.3įor those with SQL questions we recommend using SQLFiddle to provide a useful development and testing environment for those who wish to fully understand your problem and help devise a solution. Something as simple as line breaks and using reddit's built in code formatting (4 spaces at the start of each line) can turn this: This will greatly increase your chances of receiving the help you desire. If you are including actual code in a post or comment, please attempt to format it in a way that is readable for other users. We will gladly help where we can as long as you post the work you have already done or show that you have attempted to figure it out on your own. ![]() If you are a student or just looking for help on your code please do not just post your questions and expect the community to do all the work for you. While naturally we should endeavor to work as platform neutrally as possible many questions and answers require tailoring to the feature set of a specific platform. When requesting help or asking questions please prefix your title with the SQL variant/platform you are using within square brackets like so: The goal of /r/SQL is to provide a place for interesting and informative SQL content and discussions.
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