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On conflict Do Something Clause in Postgres

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Advanced SQL for Professional Developers

Instead of first checking to see if a record already exists within your table, we can do a on conflict do update. In this command, we can ether insert a row into our table, if it does exist, then check to see if all of the columns match up. This saves us a database call and is pretty straightforward to understand. 

Instructor: [0:00] Let's say that you have an eCommerce site and you allow the creation of an account to happen during the checkout process. Whenever a user signs up, we store some basic info about that person into our users table.

[0:12] When a user enters in an email to create a new account, we want to check to see if that email is already being saved for a different user because two different users should not have the same email for security purposes and account recovery.

[0:25] Before we create a new account, we want to select out of our users table to see if that email already exists. To insert a new user, we'll have to run two separate queries, a SELECT to see if it exists and then the INSERT if it does not.

[0:41] However, there is an easier way to do this. We can write INSERT INTO users listing out the column headers, then our values, a UUID, Lucy Jones with an email lucyjones@Gmail. Then we'll say ON CONFLICT DO NOTHING.

[1:01] When we run this, we'll see that we are inserting nothing. We'll see insert , . If we didn't add this ON CONFLICT DO NOTHING clause, we would see an error printed to the page, rather than this insert of , .

[1:13] Another cool thing about swallowing these errors with this ON CONFLICT clause is we can instead of doing an INSERT or not doing anything, we can UPDATE a row. Let's say that our friend Lucy got married and she has a different last name, but she kept her original email. We see that we have Lucy Hawkins going into our users table, but the same email.

[1:33] We're going to say ON CONFLICT email DO UPDATE, SET first_name = excluded.first_name and last_name = excluded.last_name. We'll see that here we do get an INSERT. When we select that Lucy again by looking up her email, we'll see that her last name has been updated to Hawkins and that she still has the same email.

[1:58] Let's go back and look at our query again. When we want to do an update whenever there's a conflict with our INSERT statement, we have to specify the conflict. This is called the conflict target. In our case, the conflict is a duplicate value on the email column that I have previously set to be a unique constrain.

[2:16] Next up is the conflict action. In our case, we're doing an update. We want to specifically update first_name and last_name to whatever values are coming in with the INSERT statement. In our case, that is these values here.

[2:30] We reference them by this excluded terminology. By saying first_name = excluded.first_name, we're saying update the row that has the conflict with the excluded value or the value that's coming in on the INSERT statement.

[2:43] We're basically guarantying that we are doing either an INSERT or an UPDATE, which is also commonly known as an UPSERT. Taking this even one step further because it's cool and interesting to note that we can use a WHERE clause within this UPDATE as well.

[2:59] Backing up, we can alias the users table with an U. Then we can say WHERE U.first_name is not Lucy. This has a couple conditions now, right? If there's a conflict with email, do update setting these columns to the incoming data. Unless, the original row's first name is Lucy, then don't update the row.

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Advanced SQL for Professional Developers

On conflict Do Something Clause in Postgres

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