SQL Interview Question and Answers | ROW NUMBER Function in SQL
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The `ROW_NUMBER()` function is a
The `ROW_NUMBER()` function is a window function in SQL that assigns a unique integer value to each row within the result set of a query. The primary purpose of the `ROW_NUMBER()` function is to generate a sequential integer for each row, typically based on a specified column's order. This function is commonly used for tasks like pagination, ranking, and identifying or numbering rows within a result set.
Here's the basic syntax of the `ROW_NUMBER()` function:
```sql
ROW_NUMBER() OVER (ORDER BY column1 [ASC|DESC], column2 [ASC|DESC], ...)
```
- `ROW_NUMBER()`: This is the function that generates the row number.
- `OVER`: This keyword is used to define the window or partition of the result set over which the row number is generated.
- `ORDER BY`: You specify the column or columns by which you want to order the rows. The `ROW_NUMBER()` function will assign row numbers based on this order.
- `[ASC|DESC]`: You can specify the sort order as ascending (ASC) or descending (DESC) for each column in the `ORDER BY` clause.
Here are a few common use cases for the `ROW_NUMBER()` function:
1. **Pagination**: You can use `ROW_NUMBER()` to implement pagination in your queries. By assigning row numbers to rows in a result set, you can easily retrieve a specific range of rows from the result, such as the first 10 rows, the next 10 rows, etc.
2. **Ranking**: You can use `ROW_NUMBER()` to assign rankings to rows based on specific criteria. For example, you can rank products by their price or customers by their purchase history.
3. **Removing Duplicates**: `ROW_NUMBER()` can be used to identify and remove duplicate rows from a result set. You can assign row numbers and then filter for rows with a row number of 1 to keep only one instance of each unique row.
Here's an example of how to use the `ROW_NUMBER()` function to implement pagination:
```sql
WITH OrderedData AS (
SELECT
column1,
column2,
ROW_NUMBER() OVER (ORDER BY column1) AS RowNum
FROM
YourTable
)
SELECT
column1,
column2
FROM
OrderedData
WHERE
RowNum BETWEEN 11 AND 20; -- Retrieve rows 11 to 20
```
In this example, the `ROW_NUMBER()` function assigns a unique row number to each row in the result set ordered by `column1`. The outer query then filters for rows with row numbers between 11 and 20 to implement pagination.
Keep in mind that the exact syntax and availability of window functions like `ROW_NUMBER()` may vary between different database management systems (e.g., SQL Server, PostgreSQL, MySQL), so you should consult the documentation for your specific DBMS for precise details on how to use this function.
Here's the basic syntax of the `ROW_NUMBER()` function:
```sql
ROW_NUMBER() OVER (ORDER BY column1 [ASC|DESC], column2 [ASC|DESC], ...)
```
- `ROW_NUMBER()`: This is the function that generates the row number.
- `OVER`: This keyword is used to define the window or partition of the result set over which the row number is generated.
- `ORDER BY`: You specify the column or columns by which you want to order the rows. The `ROW_NUMBER()` function will assign row numbers based on this order.
- `[ASC|DESC]`: You can specify the sort order as ascending (ASC) or descending (DESC) for each column in the `ORDER BY` clause.
Here are a few common use cases for the `ROW_NUMBER()` function:
1. **Pagination**: You can use `ROW_NUMBER()` to implement pagination in your queries. By assigning row numbers to rows in a result set, you can easily retrieve a specific range of rows from the result, such as the first 10 rows, the next 10 rows, etc.
2. **Ranking**: You can use `ROW_NUMBER()` to assign rankings to rows based on specific criteria. For example, you can rank products by their price or customers by their purchase history.
3. **Removing Duplicates**: `ROW_NUMBER()` can be used to identify and remove duplicate rows from a result set. You can assign row numbers and then filter for rows with a row number of 1 to keep only one instance of each unique row.
Here's an example of how to use the `ROW_NUMBER()` function to implement pagination:
```sql
WITH OrderedData AS (
SELECT
column1,
column2,
ROW_NUMBER() OVER (ORDER BY column1) AS RowNum
FROM
YourTable
)
SELECT
column1,
column2
FROM
OrderedData
WHERE
RowNum BETWEEN 11 AND 20; -- Retrieve rows 11 to 20
```
In this example, the `ROW_NUMBER()` function assigns a unique row number to each row in the result set ordered by `column1`. The outer query then filters for rows with row numbers between 11 and 20 to implement pagination.
Keep in mind that the exact syntax and availability of window functions like `ROW_NUMBER()` may vary between different database management systems (e.g., SQL Server, PostgreSQL, MySQL), so you should consult the documentation for your specific DBMS for precise details on how to use this function.
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