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# PostgreSQL FIRST_VALUE Function

**Summary**: in this tutorial, you will learn how to use the PostgreSQL `FIRST_VALUE()` function to return the first value in a sorted partition of a result set.

## Introduction to PostgreSQL FIRST_VALUE() function

The `FIRST_VALUE()` function returns a value evaluated against the first row in a sorted partition of a result set.

The following is the syntax of the `FIRST_VALUE()` function:

```sql
FIRST_VALUE ( expression )
OVER (
    [PARTITION BY partition_expression, ... ]
    ORDER BY sort_expression [ASC | DESC], ...

```

```sql
FIRST_VALUE ( expression )
OVER (
    [PARTITION BY partition_expression, ... ]
    ORDER BY sort_expression [ASC | DESC], ...
)
```

In this syntax:

### expression

The `expression` can be an expression, column, or subquery evaluated against the value of the first row of a sorted partition of a result set. The `expression` must return a single value. And it cannot be a window function.

### PARTITION BY clause

The `PARTITION BY` clause divides rows in a result set into partitions to which the `FIRST_VALUE()` function is applied.

When you the `PARTITION BY` clause, the `FIRST_VALUE()` function treats the whole result set as a single partition.

### ORDER BY clause

The `ORDER BY` clause specifies the sort order of rows in each partition to which the `FIRST_VALUE()`function is applied.

### rows_range_clause

The `rows_range_clause` further limits the rows within the partition by defining the start and end in the partition

## PostgreSQL FIRST_VALUE() function examples

We will use the `products` table created in the window function tutorial for the demonstration:

![](https://neon.com/postgresqltutorial/products_product_groups_tables.png)The data of the `products` table is as follows:

![](https://neon.com/postgresqltutorial/products-table-sample-data.png)

### 1) Using PostgreSQL FIRST_VALUE() function over a result set example

The following statement uses the `FIRST_VALUE()` function to return all products and also the product which has the lowest price:

```sql
SELECT
    product_id,
    product_name,
    group_id,
    price,
    FIRST_VALUE(product_name)
    OVER(
        ORDER BY price
    ) lowest_price
FROM
    products;
```

Here is the result set:

![PostgreSQL FIRST\_VALUE Function over a result set](https://neon.com/postgresqltutorial/PostgreSQL-FIRST_VALUE-Function-over-a-result-set.png)
In this example:

- Since we skipped the `PARTITION BY` clause in the `FIRST_VALUE()` function, the function treated the whole result set as a single partition.
- The `ORDER BY` clause sorted products by prices from low to high.
- The `FIRST_VALUE()` function is applied to the whole result set and picked the value in the `product_name` column of the first row.

### 2) Using FIRST_VALUE() function over a partition example

This statement uses the `FIRST_VALUE()` function to return all products grouped by the product group. And for each product group, it returns the product with the lowest price:

```sql
SELECT
    product_id,
    product_name,
	group_id,
    price,
    FIRST_VALUE(product_name)
    OVER(
	PARTITION BY group_id
        ORDER BY price
        RANGE BETWEEN
            UNBOUNDED PRECEDING AND
            UNBOUNDED FOLLOWING
    ) lowest_price
FROM
    products;
```

![PostgreSQL FIRST\_VALUE Function over partition](https://neon.com/postgresqltutorial/PostgreSQL-FIRST_VALUE-Function-over-partition.png)
In this example:

- The `PARTITION BY` clause distributed products by product group.
- The `ORDER BY` clause sorted products in each product group (partition) by prices from low to high.
- The `RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING` clause defined the frame in each partition, starting from the first row and ending at the last row.
- The `FIRST_VALUE()` function is applied to each partition separately.

In this tutorial, you have learned how to use the PostgreSQL `FIRST_VALUE()` function to return the first value in a sorted partition of a result set.

---

## Related docs (Window Functions)

- [CUME_DIST](https://neon.com/postgresql/postgresql-window-function/postgresql-cume_dist-function)
- [DENSE_RANK](https://neon.com/postgresql/postgresql-window-function/postgresql-dense_rank-function)
- [LAG](https://neon.com/postgresql/postgresql-window-function/postgresql-lag-function)
- [LAST_VALUE](https://neon.com/postgresql/postgresql-window-function/postgresql-last_value-function)
- [LEAD](https://neon.com/postgresql/postgresql-window-function/postgresql-lead-function)
- [NTH_VALUE](https://neon.com/postgresql/postgresql-window-function/postgresql-nth_value-function)
- [NTILE](https://neon.com/postgresql/postgresql-window-function/postgresql-ntile-function)
- [PERCENT_RANK](https://neon.com/postgresql/postgresql-window-function/postgresql-percent_rank-function)
- [RANK](https://neon.com/postgresql/postgresql-window-function/postgresql-rank-function)
- [ROW_NUMBER](https://neon.com/postgresql/postgresql-window-function/postgresql-row_number)