Hướng dẫn dùng postgres row_number trong PHP

Summary: in this tutorial, you will learn how to use the PostgreSQL ROW_NUMBER() function to assign a unique integer value to each row in a result set.

Introduction to the PostgreSQL ROW_NUMBER() function

The ROW_NUMBER() function is a window function that assigns a sequential integer to each row in a result set. The following illustrates the syntax of the ROW_NUMBER() function:

ROW_NUMBER() OVER(
    [PARTITION BY column_1, column_2,…]
    [ORDER BY column_3,column_4,…]
)
Code language: SQL (Structured Query Language) (sql)

The set of rows on which the ROW_NUMBER() function operates is called a window.

The PARTITION BY clause divides the window into smaller sets or partitions. If you specify the PARTITION BY clause, the row number for each partition starts with one and increments by one.

Because the PARTITION BY clause is optional to the ROW_NUMBER() function, therefore you can omit it, and ROW_NUMBER() function will treat the whole window as a partition.

The ORDER BY clause inside the OVER clause determines the order in which the numbers are assigned.

We will use the products table created in the PostgreSQL window function tutorial to demonstrate the functionality of the ROW_NUMBER() function.

The following is the data in the products table:

See the following query.

SELECT
	product_id,
	product_name,
	group_id,
	ROW_NUMBER () OVER (ORDER BY product_id)
FROM
	products;
Code language: SQL (Structured Query Language) (sql)

Because we did not use the PARTITION BY clause, the ROW_NUMBER() function considers the whole result set as a partition.

The ORDER BY clause sorts the result set by product_id, therefore, the ROW_NUMBER() function assigns integer values to the rows based on the  product_id order.

In the following query, we change the column in the ORDER BY clause to product_name, the ROW_NUMBER() function assigns the integer values to each row based on the product name order.

SELECT
	product_id,
	product_name,
	group_id,
	ROW_NUMBER () OVER (
           ORDER BY product_name
        )
FROM
	products;
Code language: SQL (Structured Query Language) (sql)

In the following query, we use the PARTITION BY clause to divide the window into subsets based on the values in the  group_id column. In this case, the ROW_NUMBER() function assigns one to the starting row of each partition and increases by one for the next row within the same partition.

The ORDER BY clause sorts the rows in each partition by the values in the product_name column.

SELECT
	product_id,
	product_name,
	group_id,
	ROW_NUMBER () OVER (
		PARTITION BY group_id
		ORDER BY
			product_name
	)
FROM
	products;



Code language: SQL (Structured Query Language) (sql)

PostgreSQL ROW_NUMBER() function and DISTINCT operator

The following query uses the ROW_NUMBER() function to assign integers to the distinct prices from the products table.

SELECT DISTINCT
	price,
	ROW_NUMBER () OVER (ORDER BY price)
FROM
	products
ORDER BY
	price;
Code language: SQL (Structured Query Language) (sql)

However, the result is not expected because it includes duplicate prices. The reason is that the ROW_NUMBER() operates on the result set before the DISTINCT is applied.

To solve this problem, we can get a list of distinct prices in a CTE, the apply the ROW_NUMBER() function in the outer query as follows:

WITH prices AS (
	SELECT DISTINCT
		price
	FROM
		products
) SELECT
	price,
	ROW_NUMBER () OVER (ORDER BY price)
FROM
	prices;
Code language: SQL (Structured Query Language) (sql)

Or we can use a subquery in the FROM clause to get a list of unique price, and then apply the ROW_NUMBER() function in the outer query.

SELECT
	price,
	ROW_NUMBER () OVER (ORDER BY price)
FROM
	(
		SELECT DISTINCT
			price
		FROM
			products
	) prices;
Code language: SQL (Structured Query Language) (sql)

Using ROW_NUMBER() function for pagination

In application development, you use the pagination technique for displaying a subset of rows instead of all rows in a table.

Besides using the LIMIT clause, you can also use the ROW_NUMBER() function for the pagination.

For example, the following query selects the five rows starting at row number 6:

SELECT
	*
FROM
	(
		SELECT
			product_id,
			product_name,
			price,
			ROW_NUMBER () OVER (ORDER BY product_name)
		FROM
			products
	) x
WHERE
	ROW_NUMBER BETWEEN 6 AND 10;
Code language: SQL (Structured Query Language) (sql)

Using ROW_NUMBER() function for getting the nth highest / lowest row

For example, to get the third most expensive products, first, we get the distinct prices from the products table and select the price whose row number is 3. Then, in the outer query, we get the products with the price that equals the 3rd highest price.

SELECT
	*
FROM
	products
WHERE
	price = (
		SELECT
			price
		FROM
			(
				SELECT
					price,
					ROW_NUMBER () OVER (
						ORDER BY price DESC
					) nth
				FROM
					(
						SELECT DISTINCT
							(price)
						FROM
							products
					) prices
			) sorted_prices
		WHERE
			nth = 3
	);
Code language: SQL (Structured Query Language) (sql)

In this tutorial, we have shown you how to use the PostgreSQL ROW_NUMBER() function to assign integer values to rows in a result set.

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