#Multiple inner join postgresql code#

I'm in industry too, though I did go through formal studies at university and databases was one of the topics that interested me the most back then (late 90s). What if we just returned the relational structures directly? A big ol' N+1 situation with 1 query to fetch recipes and then N additional queries to fetch the ingredients. One to fetch the recipes, and then one to fetch the ingredients once we know the id's of the recipies.ģ. Everything getting denormalized into a single, wasteful result table and then teased back out into Recipes and Ingredients by the ORMĢ. Print(f'')ĭepending on the ORM and the query options specified, this results in one of the following:ġ. This is an incredibly common pattern, particularly with ORMs.įor each recipe in Recipe.joins(Ingredient).select_all() doįor each ingredient in recipe.ingredients do What if my 'desired format' is a structure that maintains the relationships specified in the database? Might there be a better way for some use cases?ĭenormalize to present information in the desired format Well, that's how they've always worked, yes. I guess my overall bafflement is that the things they describe as 'not ideal' seem to me like features rather than problems, and these features have been highly beneficial in my practical experience of writing a lot of database-driven apps.

#Multiple inner join postgresql code#

More than that - you want your query to define and adhere to its output format regardless of the underlying database structure, precisely so you can refactor things in the database and rewrite the query but not need to rewrite the code that uses the query results.

Discarding irrelevant information is a feature, not a bug. You write your query to retrieve and differentiate precisely what you need and no more. You're not supposed to retrieve all possible relational information in query results. > which 'discard all schema and relational information on the way.'Īgain, this is a feature. And if round-trip latency is some kind of issue with running queries sequentially, you can always issue queries in parallel instead.) (And if you're dealing with what would be an overly-large amount of repeated values, you just run multiple queries yourself instead of one. Data should be stored as normalized as possible, but queries are supposed to denormalize to present information in the desired format. But it's genuinely hard to see this as a drawback - that's a feature. Yes, they're using the example of repeating information (denormalization), such as multiple courses taught by the same professor. > Either the result set will be larger than it needs to be You do a good job of summarizing their two main arguments, so allow me to rebut: Reading it closely, this is what I already disagree with. INNER JOIN sales ON customers.customer_id = sales.> First the authors show that for certain use cases, a single query is not ideal. The following statement uses the INNER JOIN clause to select data from both tables: SELECT The relationship between the two tables is established by the customer id column. Each purchase, however, belongs to a single customer. We have two tables: one for customers and one for sales.Įvery time a customer purchases something, a new row is added to the sales table.Įach customer may have made zero or numerous purchases. In case these values are not equal, the inner join just ignores them and moves to the next row.If these values are equal, the inner join creates a new row that contains all columns of both tables and adds it to the result set.Third, specify the second table (table sales) in the INNER JOIN clause and provide a join condition after the ON keyword.įor each row in the table customers, inner join compares the value in the customers.customer_id column with the value in the sales.customer_id column of every row in the table sales:.Second, specify the main table i.e., table customers in the FROM clause.First, specify columns from both tables that you want to select data in the SELECT clause.To join table customers with the table sales, you follow these steps: Kindly note the table names when the columns are named the same to help avoid the ambiguous column name error. INNER JOIN sales ON customers.customer_id = sales.customer_id To select data from both tables, you use the INNER JOIN clause in the SELECT statement as follows: