PostgreSQL - How to Have a Condition on Join Model Result

When working with join operations in SQL, it can be challenging to achieve the desired result. In this article, we will explore how to create a conditional model result using PostgreSQL’s bool_or function and other advanced techniques.

Background and Context

The problem presented in the Stack Overflow post revolves around two tables: user and list. These tables have a many-to-many relationship through the userList table, where users can be connected to lists with specific roles (owner or collaborator). The request is to retrieve all lists of another user, considering both public lists and lists for which the requester has a role.

We will delve into the intricacies of PostgreSQL’s join operations, filter functions, and grouping clauses to solve this problem.

Understanding the Join Operation

The basic join operation in SQL joins two or more tables based on a common column. In this case, we have three tables: list, userList, and user. The join operation is as follows:

SELECT "list".*
FROM "list"
INNER JOIN "userList" ul ON ul.listId = l.id
INNER JOIN "user" u ON u.id = ul.userId;

This will return all columns from the list table, along with the corresponding values from the userList and user tables.

The Problem: Filtering Results

We want to filter the results to include only lists that meet certain conditions. Let’s break down these conditions:

1. Public Lists: We want to include public lists (i.e., isPublic = true) regardless of the user.
2. Lists with Collaborator or Owner Role for Requester: For non-public lists, we want to include those for which the requester has a collaborator or owner role.

Using bool_or Function

PostgreSQL provides the bool_or function, which returns true if any of its arguments are true. We can utilize this function in conjunction with conditional logic to filter our results.

Here’s an example using the bool_or function:

SELECT l.*
FROM list
INNER JOIN userlist ul ON ul.listId = l.id
GROUP BY l.id
HAVING 
    bool_or(ul.role = 'OWNER' AND u.id = 'john')  -- owned by John
    AND (l.ispublic OR bool_or(ul.userid = 'anna'));     -- public or allowed to Anna

This query does exactly what we want: it returns all lists for which the requester has an owner role (ul.role = 'OWNER') and either is public (l.ispublic) or allowed to access by Anna (bool_or(ul.userid = 'anna')).

However, there’s a catch. The first condition (ul.role = 'OWNER' AND u.id = 'john') will only return the list if both conditions are met (i.e., John is an owner of that list). But this might not be what we want.

Combining Conditions with Logical Operators

We can use logical operators (AND, OR) to combine multiple conditions. However, keep in mind that PostgreSQL performs these operations on a boolean scale.

Let’s try another approach using the same logic:

SELECT l.*
FROM list l
INNER JOIN userlist ul ON ul.listId = l.id
JOIN user u ON u.id = ul.userId
WHERE 
    (l.ispublic OR ul.userid = 'anna')  -- public or Anna allowed to see it
    AND (ul.role = 'OWNER' OR ul.userid = 'john');  -- owned by John or John allowed to see

Here, we’ve used the JOIN clause to combine conditions from multiple tables and then applied a logical condition to filter our results.

However, there’s still room for improvement. We can utilize subqueries or Common Table Expressions (CTEs) to make the code more readable and easier to maintain.

Advanced Techniques: Using Subqueries

One approach is to use a subquery:

SELECT l.*
FROM list l
INNER JOIN userlist ul ON ul.listId = l.id
WHERE 
    l.ispublic OR (
        ul.userid = 'anna'  -- Anna allowed to see this list
        AND EXISTS (SELECT 1 FROM userlist ul2 WHERE ul2.listid = ul.listid AND ul2.role IN ('OWNER', 'COLLABORATOR') AND ul2.userid = 'john')
    );

This approach is straightforward but may not be the most efficient.

Another option is to use a CTE:

WITH filtered_userlists AS (
    SELECT * FROM userlist WHERE userid IN ('anna', 'john')
)
SELECT l.*
FROM list l
INNER JOIN filtered_userlists ul ON ul.listId = l.id;

In this case, we’ve defined a temporary view (CTE) containing the users whose lists should be included in our final result. We can then join this CTE with our main query.

Conclusion

When working with complex queries and conditional logic, PostgreSQL provides several tools to help us achieve the desired results. By combining bool_or, logical operators, and advanced techniques like subqueries or CTEs, we can create efficient and readable SQL code that meets specific requirements.

Remember to consider indexing strategies for large datasets and database performance when implementing these queries in production environments.

Last modified on 2023-07-09