MySQL - UPDATE query based on SELECT Query

MySQL - Updating with a SELECT Query

👋 Hey there tech enthusiasts! In this blog post, we'll tackle a common question about MySQL - how to use an UPDATE query based on a SELECT query. 🤔

The specific problem we'll address is checking if there is an association between two events based on their date-time in the same table. We'll explore easy solutions and provide a step-by-step guide to make your SQL journey a breeze! 💪

📋 The Scenario

Let's break down the context around this question. We have two sets of data in the same table:

• Set A: Contains the ending date-time of certain events.

• Set B: Contains the starting date-time for other events.

The goal is to link up events if the first one completes before the second one. 🗓️

🔍 Understanding the SELECT Queries

The first step is to extract the necessary information using SELECT queries. Here's what we have so far:

SELECT name as name_A, date-time as end_DTS, id as id_A 
FROM tableA 
WHERE criteria = 1

SELECT name as name_B, date-time as start_DTS, id as id_B 
FROM tableA 
WHERE criteria = 2

These queries fetch the required fields for both sets of data. The "name_A" and "name_B" columns represent the event names, while "end_DTS" and "start_DTS" denote the ending and starting date-times, respectively.

🔗 Joining the Results

Next, we need to join the results of the two SELECT queries to link the events together. Here's the query to accomplish this:

SELECT name_A, name_B, id_A, id_B, 
if(start_DTS > end_DTS, 'VALID', '') as validation_check
FROM tableA
LEFT JOIN tableB ON name_A = name_B

This query joins the "tableA" and "tableB" based on the common event name. The "validation_check" column is calculated using the logical condition "start_DTS > end_DTS". If the condition holds true, it assigns the value 'VALID'; otherwise, it remains an empty string.

⚡ Updating the Rows

Now that we have the validation result in the "validation_check" column, we want to update the rows accordingly. Here comes the exciting part!

Yes, you can perform an UPDATE query based on a SELECT nested inside it. Here's the query structure:

UPDATE tableA
SET column_to_update = (SELECT result_column FROM other_table WHERE condition)
WHERE condition;

Now, let's modify it to match our scenario:

UPDATE tableA
SET column_to_update = (SELECT validation_check FROM (previous SELECT query) AS subquery)
WHERE condition;

In the "column_to_update", specify the column you want to update based on the result of the SELECT nested query. The "condition" determines which rows should be updated. Make sure to adapt these parts to your specific database schema.

🚀 Ready to Update the Associations

In your case, you can update the association between the events using the following query structure:

UPDATE tableA
SET association_column = (
    SELECT validation_check 
    FROM (
        SELECT name_A, name_B, id_A, id_B, 
        if(start_DTS > end_DTS, 'VALID', '') as validation_check
        FROM tableA
        LEFT JOIN tableB ON name_A = name_B
    ) AS subquery
    WHERE tableA.id = subquery.id_A
)
WHERE association_column IS NOT NULL;

Replace "association_column" with the actual column name where you want to store the association information. The WHERE clause ensures that only valid associations are updated.

💡 Wrapping Up

You've made it to the end of this guide! Now you should be fully equipped to update rows in MySQL based on a SELECT query. You've learned how to join results, calculate values with logical conditions, and update your table like a pro! 💪

Got any additional questions or suggestions? Share your thoughts in the comments below and let's discuss this fascinating MySQL topic together. Don't forget to hit that share button to spread the knowledge to your friends in the tech community! 🌟