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# PostgreSQL UPDATE Join

**Summary**: in this tutorial, you will learn how to use the PostgreSQL `UPDATE` join syntax to update data in a table based on values in another table.

## Introduction to the PostgreSQL UPDATE join syntax

Sometimes, you need to update data in a table based on values in another table. In this case, you can use the PostgreSQL [`UPDATE`](https://neon.com/postgresql/postgresql-tutorial/postgresql-update) join.

Here's the basic syntax of the `UPDATE` join statement:

```sql
UPDATE table1
SET table1.c1 = new_value
FROM table2
WHERE table1.c2 = table2.c2;
```

To join a table (table1) with another table (table2) in the `UPDATE` statement, you specify the joined table (table2) in the `FROM` clause and provide the join condition in the `WHERE` clause. The `FROM` clause must appear immediately after the `SET` clause.

For each row of the table `table1`, the `UPDATE` statement examines every row of the table `table2`.

If the values in the `c2` column of table `table1` equals the values in the `c2` column of table `table2`, the `UPDATE` statement updates the value in the `c1` column of the table `table1` with the new value (`new_value`).

## PostgreSQL UPDATE JOIN example

Let's take a look at an example to understand how the PostgreSQL `UPDATE` join works. We will use the following database tables for the demonstration:

![PostgreSQL UPDATE join sample database](https://neon.com/postgresqltutorial/PostgreSQL-UPDATE-JOIN-Sample-Database.png)First, [create a new table](https://neon.com/postgresql/postgresql-tutorial/postgresql-create-table) called `product_segment` that stores the product segments such as grand luxury, luxury, and mass.

The `product_segment` table has the `discount` column that stores the discount percentage based on a specific segment. For example, products with the grand luxury segment have `5%` discount while luxury and mass products have `6%` and `10%` discounts respectively.

```sql
CREATE TABLE product_segment (
    id SERIAL PRIMARY KEY,
    segment VARCHAR NOT NULL,
    discount NUMERIC (4, 2)
);


INSERT INTO
    product_segment (segment, discount)
VALUES
    ('Grand Luxury', 0.05),
    ('Luxury', 0.06),
    ('Mass', 0.1);

```

Second, create another table named `product` that stores the product data. The `product` table has the [foreign key column](https://neon.com/postgresql/postgresql-tutorial/postgresql-foreign-key) `segment_id` that links to the `id` of the `segment` table.

```sql
CREATE TABLE product(
    id SERIAL PRIMARY KEY,
    name VARCHAR NOT NULL,
    price NUMERIC(10,2),
    net_price NUMERIC(10,2),
    segment_id INT NOT NULL,
    FOREIGN KEY(segment_id) REFERENCES product_segment(id)
);


INSERT INTO
    product (name, price, segment_id)
VALUES
    ('diam', 804.89, 1),
    ('vestibulum aliquet', 228.55, 3),
    ('lacinia erat', 366.45, 2),
    ('scelerisque quam turpis', 145.33, 3),
    ('justo lacinia', 551.77, 2),
    ('ultrices mattis odio', 261.58, 3),
    ('hendrerit', 519.62, 2),
    ('in hac habitasse', 843.31, 1),
    ('orci eget orci', 254.18, 3),
    ('pellentesque', 427.78, 2),
    ('sit amet nunc', 936.29, 1),
    ('sed vestibulum', 910.34, 1),
    ('turpis eget', 208.33, 3),
    ('cursus vestibulum', 985.45, 1),
    ('orci nullam', 841.26, 1),
    ('est quam pharetra', 896.38, 1),
    ('posuere', 575.74, 2),
    ('ligula', 530.64, 2),
    ('convallis', 892.43, 1),
    ('nulla elit ac', 161.71, 3);
```

Third, suppose you have to calculate the net price of every product based on the discount of the product segment. To do this, you can apply the `UPDATE` join statement as follows:

```sql
UPDATE product
SET net_price = price - price * discount
FROM product_segment
WHERE product.segment_id = product_segment.id;
```

You can utilize the table aliases to make the query shorter like this:

```sql
UPDATE
    product p
SET
    net_price = price - price * discount
FROM
    product_segment s
WHERE
    p.segment_id = s.id;
```

This statement joins the `product` table to the `product_segment` table. If there is a match in both tables, it gets a discount from the `product_segment` table, calculates the net price based on the following formula, and updates the `net_price` column.

```
net_price = price - price * discount;
```

The following [`SELECT`](https://neon.com/postgresql/postgresql-tutorial/postgresql-select) statement retrieves the data of the `product` table to verify the update:

```sql
SELECT * FROM product;
```

![PostgreSQL UPDATE Join Example](https://neon.com/postgresqltutorial/PostgreSQL-UPDATE-JOIN-example.png)
The output indicates that the `net_price` column has been updated with the correct values.

## Summary

- Use the PostgreSQL `UPDATE` join statement to update data in a table based on values in another table.

---

## Related docs (Modifying Data)

- [INSERT](https://neon.com/postgresql/postgresql-tutorial/postgresql-insert)
- [INSERT Multiple Rows](https://neon.com/postgresql/postgresql-tutorial/postgresql-insert-multiple-rows)
- [UPDATE](https://neon.com/postgresql/postgresql-tutorial/postgresql-update)
- [DELETE](https://neon.com/postgresql/postgresql-tutorial/postgresql-delete)
- [DELETE Join](https://neon.com/postgresql/postgresql-tutorial/postgresql-delete-join)
- [DELETE CASCADE](https://neon.com/postgresql/postgresql-tutorial/postgresql-delete-cascade)
- [Delete Duplicate Rows](https://neon.com/postgresql/postgresql-tutorial/how-to-delete-duplicate-rows-in-postgresql)
- [Upsert](https://neon.com/postgresql/postgresql-tutorial/postgresql-upsert)
- [MERGE](https://neon.com/postgresql/postgresql-tutorial/postgresql-merge)