updating table rows in postgres using subquery

📝 Updating Table Rows in Postgres Using Subquery: A Complete Guide 🔄

Hey there! 👋 Are you struggling to update table rows in Postgres using a subquery? Don't worry, we've got your back! In this blog post, we'll walk you through common issues and provide easy solutions to help you successfully update existing rows using values returned from a select statement. Let's dive in! 💪

The Scenario

To set the stage, let's take a look at the context of this question. We have a table called public.dummy in a Postgres 8.4 database. It has various columns, including customer, supplier, and partner. The goal is to update these columns based on specific conditions using values returned from the SELECT statement.

Here's the CREATE TABLE statement for our public.dummy table:

CREATE TABLE public.dummy 
(
  address_id SERIAL,
  addr1 character(40),
  addr2 character(40),
  city character(25),
  state character(2),
  zip character(5),
  customer boolean,
  supplier boolean,
  partner boolean
)
WITH (OIDS=FALSE);

Initially, the questioner tested their query using the INSERT statement. However, they are now seeking an UPDATE statement to modify existing rows based on the values returned from a SELECT query.

Converting to an UPDATE Statement

To accomplish the desired update using a subquery, we can leverage the power of the UPDATE statement in combination with a subquery. Here's an example of how you can do it:

UPDATE public.dummy d
SET 
  customer = (SELECT CASE WHEN cust.addr1 IS NOT NULL THEN TRUE ELSE FALSE END 
              FROM cust_original cust 
              WHERE d.addr1 = cust.addr1 AND d.addr2 = cust.addr2 AND d.city = cust.city 
              AND d.state = cust.state AND substring(cust.zip, 1, 5) = d.zip),
  supplier = (SELECT CASE WHEN suppl.addr1 IS NOT NULL THEN TRUE ELSE FALSE END 
              FROM supp_original suppl 
              WHERE d.addr1 = suppl.addr1 AND d.addr2 = suppl.addr2 AND d.city = suppl.city 
              AND d.state = suppl.state AND d.zip = substring(suppl.zip, 1, 5)),
  partner = (SELECT CASE WHEN partn.addr1 IS NOT NULL THEN TRUE ELSE FALSE END 
              FROM partner_original partn
              WHERE d.addr1 = partn.addr1 AND d.addr2 = partn.addr2 AND d.city = partn.city 
              AND d.state = partn.state AND d.zip = substring(partn.zip, 1, 5))
WHERE d.address_id = address_id;

In the UPDATE statement, we perform the following steps:

1. We update the customer column using values returned from the cust_original table.

2. We update the supplier column using values returned from the supp_original table.

3. We update the partner column using values returned from the partner_original table.

Giving It a Try!

Now that you have the update statement, you can try it out on your own database! Feel free to replace the table and column names with your specific ones, and adjust the conditions in the subqueries accordingly. Always remember to backup your data before making any updates to ensure nothing goes wrong. Safety first! 🔒

Wrapping Up

Updating table rows in Postgres using a subquery can seem a bit challenging at first, but with the right approach and syntax, it becomes a breeze. We hope this guide clarified any confusion you had and provided easy solutions to your problem.

If you found this guide helpful, please consider sharing it with your fellow techies facing a similar issue. Also, drop a comment below and let us know your thoughts or any other tech topics you'd like us to cover. Your engagement and feedback mean the world to us! 😊

Until next time! Happy updating! 🚀