We can verify this by asking Ecto to create the joins_repo database we've just defined in config.exs: $ mix ecto.create ![]() Now Ecto is configured to connect to the database server. The last things that is left is to make sure that the repo is included in our app's supervision tree by updating lib/joins/application.ex to say: # lib/joins/application.ex We need to update the file to tell the app which repo to use and what username and password are needed to connect to the instance of PostgreSQL we are running locally. This file is where we tell Ecto how to connect to the database so let's go ahead and update it now. Not only does it tell us that the repo module has indeed been created for us, it also says that config/config.exs has been updated as well. The output includes the following lines: * creating lib/joins Let's go ahead and run the following: $ mix -r Joins.Repo Fortunately, Ecto comes with a mix task which will generate the repo for us. In Ecto, all communication between our application and the database happens through a module called Repo. We have added Ecto and Postgrex, a PostgreSQL driver that Ecto uses to speak to PostgreSQL databases. Now, open up mix.exs and add Ecto as a dependency: # mix.exs Create a new Elixir project by running: $ mix new joins -sup We'll use Elixir 1.7.3 and PostgreSQL which is a popular open-source relational database. In order to let us play with Ecto in the IEx console we'll set up a sample project and add Ecto as a dependency. If you don't want to run code examples on your machine have a brief look at our sample data below and feel free to jump ahead to joins. This and the following section include steps required to set up a project using Ecto and populate the database with some sample data. On top of queries, Ecto provides numerous other features, such as Schemas, which we are deliberately not going to use here to keep things as simple as possible. It provides a feature-rich and readable query DSL which covers the vast majority of queries one might typically need. It allows us to access relational databases from Elixir applications without having to write plain SQL. What is Ecto?Įcto is a database wrapper library for the Elixir programming language. If we wanted to display a list of orders with corresponding customer names we'd need to join the two tables together. A somewhat classic example might be a customers table containing customer data and an orders table containing order details. Joins allow us to combine rows from two or more tables in order to answer questions that require data from multiple sources. In relational databases data is stored in tables. This is why in this article I am going to do both and we'll review the basics of SQL joins but we'll query the database using Ecto, a database wrapper for Elixir.īecause we are going to need some test data to play with, we'll also set up a sample Elixir project, add Ecto to it and configure it to connect to a local instance of Postgres. Whilst I think that learning new things is an important part of being a developer, I find it is also quite useful to go back to basics once in a while. ![]() I tried changing it to where: is_nil(ot.user_id), but get the same error.SQL joins with Ecto A review of SQL joins and how to do them in Ecto Going back to basicsĪs back-end developers, SQL and relational databases are often a significant part of our daily workflows. Left_join: ot in subquery(older_merchant_transactions_query(merchant_id, date)), Where: t.merchant_id = ^merchant_id and fragment("?::date", t.inserted_at) <= ^dateįrom t in subquery(new_merchant_transactions_query(merchant_id, date)), Where merchant_id = 1 and inserted_at = ^dateĭef older_merchant_transactions_query(merchant_id, date) do Where merchant_id = 1 and inserted_at > date '' From those, we will select from new_transactions what is NOT in older_transactions. The SQL Query does this by using WITH to select all the transactions for a certain date range in new_transactions, then select all transactions for another date range in older_transactions. In this instance, I want to know what users have transactions within a certain time period AND do not have transactions in another time period. The goal is to return what is in unique between the two queries. I have an SQL query that using the PostgreSQL WITH AS to act as an XOR or "Not" Left Join.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |