ROW_NUMBER() is a window function in SQL that assigns a unique sequential integer to each row in the result set returned by a query. It is commonly used to generate a unique identifier for each row, or to rank or sort rows based on a specified ordering.
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The syntax for using the ROW_NUMBER() function is as follows:
ROW_NUMBER() OVER (ORDER BY column_name [ASC|DESC])
n this syntax, the ORDER BY clause specifies the column or columns to use as the sorting criteria for the row numbering. The optional ASC or DESC keyword can be used to specify the sort order.
The ROW_NUMBER() function always starts counting from 1 for the first row in the result set, and increments by 1 for each subsequent row. If there are ties in the ordering, the function will assign the same row number to all tied rows.
Here is an example of using ROW_NUMBER() to assign a unique identifier to each row in a table:
SELECT ROW_NUMBER() OVER (ORDER BY id) AS rn, *
FROM my_table;
This query will generate a result set that includes a new column called row_number, which contains a unique sequential integer for each row in the my_table table, sorted by the id column.
ROW_NUMBER() with PARTITION BY in SQL
The ROW_NUMBER() function in SQL can be used in conjunction with the PARTITION BY clause to assign unique sequential integers to rows based on groups within a result set.
The PARTITION BY clause divides the result set into partitions or groups based on one or more column values. The ROW_NUMBER() function then assigns a unique sequential integer to each row within each partition.
The syntax for using the ROW_NUMBER() function with the PARTITION BY clause is as follows:
ROW_NUMBER() OVER (PARTITION BY column ORDER BY column [ASC|DESC])
In this syntax, the PARTITION BY clause specifies the column or columns to use as the grouping criteria for the row numbering. The ORDER BY clause specifies the column or columns to use as the sorting criteria within each partition. The optional ASC or DESC keyword can be used to specify the sort order.
Here is an example of using ROW_NUMBER() with PARTITION BY to assign a unique identifier to each row in a table based on a grouping column:
SELECT
ROW_NUMBER() OVER(PARTITION BY categoryid ORDER BY productid) AS rn,
*
FROM products
This query will generate a result set that includes a new column called rn, which contains a unique sequential integer for each row in the products table, grouped by the category_id column and sorted within each group by the product_id column.
The use of ROW_NUMBER() with PARTITION BY is useful when you need to identify and rank items within groups or partitions of a large dataset.
From the N0rthwind database in SQL Server, try:
select QuantityPerUnit,
ROW_NUMBER() over(partition by QuantityPerUnit order by QuantityPerUnit) as rn
from products
This query will generate a result set that includes a new column called rn, which contains a unique sequential integer for each row in the products table, grouped by the QuantityPerUnit column and sorted within each group by the QuantityPerUnit column.
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