SQL: How to Dynamically Calculate a Value from a Separate Table?

When building complex applications, it’s often necessary to perform joins between multiple tables in a database. In this article, we’ll explore how to use SQL to dynamically calculate a value based on data from another table.

Understanding the Problem

The problem at hand is to retrieve a list of posts from the posts table and determine whether or not the current user has voted on each post. The votes table contains information about votes cast by users on posts, with columns for the user ID, post ID, and vote value.

Analyzing the Given Tables

We have two tables: posts and votes. The posts table contains information about individual posts, including their ID, publish status, and author ID. The votes table stores votes cast by users on posts, with columns for the user ID, post ID, and vote value.

+----+---------------+--------+
| id | publishStatus |authorId|
+----+---------------+--------+
|  1 | PUBLIC        |      1 |
|  2 | PUBLIC        |      2 |
|  3 | PUBLIC        |      3 |
|  4 | PUBLIC        |      4 |
|  5 | PUBLIC        |      2 |
|  6 | PUBLIC        |      1 |
|  7 | PUBLIC        |      1 |
|  8 | PUBLIC        |      1 |
|  9 | PUBLIC        |      1 |
+----+---------------+--------+

+----+--------+--------+-------+
| id | userId | postId | value |
+----+--------+--------+-------+
|  1 |      1 |      1 |     1 |
|  2 |      2 |      1 |     1 |
|  3 |      4 |      3 |     1 |
|  4 |      1 |      5 |     0 |
|  5 |      3 |      2 |     1 |
|  6 |      4 |      8 |     1 |
|  7 |      1 |      8 |     1 |
|  8 |      4 |      9 |     1 |
|  9 |      1 |      9 |     1 |
+----+--------+--------+-------+

Solved Solution

To solve this problem, we need to join the posts table with the votes table on the post ID column. We’ll use a case statement to determine whether or not the user has voted on each post.

SELECT DISTINCT p.*, 
       CASE 
           WHEN v.value = 0 
               THEN 'No'
           ELSE 'Yes'
           END AS userHasVoted
FROM posts p
INNER JOIN votes v ON p.id = v.postid;

This query will return a list of posts with an additional column indicating whether or not the current user has voted on each post.

How It Works

The SELECT DISTINCT clause ensures that we only retrieve unique rows from the results.
We join the posts table with the votes table on the postid column using an inner join. This allows us to access data from both tables in a single query.
The CASE statement is used to determine whether or not the user has voted on each post. If the vote value is 0, we return ‘No’, otherwise we return ‘Yes’.

Example Use Case

Suppose we want to retrieve all posts along with information about whether or not the current user (with ID 1) has voted on each post.

+----+---------------+--------+-----------+
| id | publishStatus |authorId|userHasVoted|
+----+---------------+--------+-----------+
|  1 | PUBLIC        |      1 |    Yes     |
|  2 | PUBLIC        |      2 |    No      |
|  3 | PUBLIC        |      3 |    Yes     |
|  4 | PUBLIC        |      4 |    Yes     |
|  5 | PUBLIC        |      2 |    Yes     |
|  6 | PUBLIC        |      1 |    Yes     |
|  7 | PUBLIC        |      1 |    Yes     |
|  8 | PUBLIC        |      1 |    Yes     |
|  9 | PUBLIC        |      1 |    Yes     |
+----+---------------+--------+-----------+

In this example, the current user (with ID 1) has voted on posts 1, 3, 4, 6, 7, 8, and 9.

Conclusion

Joining tables in a database allows you to retrieve complex data from multiple sources. By using case statements, you can dynamically calculate values based on data from another table. This technique is useful when building applications that require retrieving information from multiple tables, such as user profiles or voting systems.

Last modified on 2023-12-16