What are window functions and different types of window functions in MYSQL?

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Window functions in SQL, including MySQL, allow you to perform calculations across a set of table rows related to the current row. These functions can be very useful for tasks like calculating running totals, ranking items based on certain criteria, calculating differences between rows, and more. Window functions operate on a "window" of rows related to the current row.

In MySQL 8.0 and later versions, you can use window functions through the OVER() clause. Here's the basic syntax of a window function in MySQL:

sql code
SELECT column1, column2, ..., window_function(column) OVER (PARTITIONBY partition_expression ORDERBY sort_expression rows_frame) FROM table_name;

Explanation of the components:

  • window_function: The specific window function you want to use (e.g., SUM(), ROW_NUMBER(), RANK(), etc.).
  • column: The column on which the window function will be applied.
  • PARTITION BY: Divides the result set into partitions to which the window_function is applied separately.
  • ORDER BY: Specifies the order of rows within each partition.
  • rows_frame: Defines the range of rows used for each calculation within the partition (optional).

Here are some common window functions in MySQL:

  1. ROW_NUMBER(): Assigns a unique sequential integer to each row within a partition of a result set.

    sql code
    SELECTROW_NUMBER() OVER (PARTITIONBY partition_column ORDERBY sort_column) AS row_num, column1, column2, ... FROM table_name;

  2. RANK(): Assigns a unique rank to each distinct row within a partition of a result set, with tied rows receiving the same rank value, leaving gaps in the ranking sequence.

    sql code
    SELECTRANK() OVER (PARTITIONBY partition_column ORDERBY sort_column) AS rank, column1, column2, ... FROM table_name;

  3. DENSE_RANK(): Similar to RANK(), but it does not leave gaps in the ranking sequence for tied rows.

    sql code
    SELECTDENSE_RANK() OVER (PARTITIONBY partition_column ORDERBY sort_column) AS dense_rank, column1, column2, ... FROM table_name;

  4. SUM(): Calculates the sum of a numeric column's values within a window.

    sql code
    SELECTSUM(column) OVER (PARTITIONBY partition_column ORDERBY sort_column) AS sum_column, column1, column2, ... FROM table_name;

  5. LEAD(): Accesses the value of a subsequent row within the partition.

    sql code
    SELECTcolumn, LEAD(column) OVER (PARTITIONBY partition_column ORDERBY sort_column) AS next_value, ... FROM table_name;

  6. LAG(): Accesses the value of a previous row within the partition.

    sql
    SELECTcolumn, LAG(column) OVER (PARTITIONBY partition_column ORDERBY sort_column) AS previous_value, ... FROM table_name;

These examples demonstrate some of the window functions you can use in MySQL to perform complex calculations on specific windows of rows within your result set. Remember, the availability of window functions depends on the version of MySQL you're using, so ensure you're using a compatible version to leverage these features.

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