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Learn how to effectively join two tables in PostgreSQL, transforming an array of IDs into an array of corresponding names using simple SQL queries.
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Join Two Tables in PostgreSQL with Array Aggregation

When working with databases, especially in a system like PostgreSQL, it's common to encounter scenarios where you need to join data from multiple tables. This becomes even more interesting when dealing with array fields. In this guide, we will explore how to join two tables in PostgreSQL, specifically focusing on a case where one table contains an array of IDs, while the other is a master table of those IDs and their corresponding names.

The Problem

Let's set the scene with a practical example. Suppose you have two tables:

Fruit Table: This contains the information about fruits, with each fruit having a unique FruitID and a FruitName.

Food Table: This table keeps track of meals, where each meal may include multiple fruits identified by their FruitID, stored in a string array.

Our goal is to retrieve data from the Food table and translate the array of FruitIDs into the corresponding fruit names.

Given the data structured as follows:

Fruit Table

FruitIDFruitNameA1MangoA2OrangeA3BananaFood Table

IDDayFruitA1Sunday{A1,A2}A2Monday{A2,A3}A3Tuesday{A1,A3}The desired output after performing the join operation should look like this:

Desired Output

IDDayFruit NameA1Sunday{Mango, Orange}A2Monday{Orange, Banana}A3Tuesday{Mango, Banana}The Solution

To achieve this, we will use SQL to join the tables and aggregate the fruit names. The key steps are as follows:

Step 1: Expand the Food Table

We will first expand the array in the Food table into multiple rows using the unnest function. This allows us to create a row for each FruitID.

Step 2: Join with the Fruit Table

Once we have the FruitIDs expanded, we will join these rows with the Fruit table to fetch the corresponding FruitName.

Step 3: Aggregate the Fruit Names

Finally, we will use the array_agg() function to group the fruit names back into an array format based on the original rows from the Food table.

Complete SQL Query

Putting this all together, here’s the complete SQL query to join the tables:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Query

Common Table Expression (CTE) - expand: This part takes the Food table and expands its Fruit field into individual rows for each FruitID.

UNNEST(fruit) effectively creates multiple rows per ID for each FruitID.

Joining Tables: In the lookup CTE, we join the expanded FruitIDs with the Fruit table to fetch the FruitName.

Aggregation: The final selection uses array_agg(fruitname) to collect fruit names into an array, aggregated by original id and day.

Conclusion

This technique of using PostgreSQL to join tables containing arrays is a powerful method for data manipulation. By expanding arrays and joining tables, you can easily transform IDs into meaningful names, greatly enhancing the readability and usability of your database queries.

Now that you understand how to achieve this, you can apply similar strategies to other arrays and tables in your PostgreSQL database for more insightful data retrieval!