PERCENTILE_CONT

The PERCENTILE_CONT() window function was first introduced with in MariaDB 10.3.3.

Description

`PERCENTILE_CONT()` (standing for continuous percentile) is an ordered set aggregate function which can also be used as a window function. It returns a value which corresponds to the given fraction in the sort order. If required, it will interpolate between adjacent input items.

Essentially, the following process is followed to find the value to return:

• Get the number of rows in the partition, denoted by N
• RN = p*(N-1), where p denotes the argument to the PERCENTILE_CONT function
• calculate the FRN(floor row number) and CRN(column row number for the group( FRN= floor(RN) and CRN = ceil(RN))
• look up rows FRN and CRN
• If (CRN = FRN = RN) then the result is (value of expression from row at RN)
• Otherwise the result is
• (CRN - RN) * (value of expression for row at FRN) +
• (RN - FRN) * (value of expression for row at CRN)

The MEDIAN function is a specific case of `PERCENTILE_CONT`, equivalent to `PERCENTILE_CONT(0.5)`.

Examples

```CREATE TABLE book_rating (name CHAR(30), star_rating TINYINT);

INSERT INTO book_rating VALUES ('Lord of the Ladybirds', 5);
INSERT INTO book_rating VALUES ('Lord of the Ladybirds', 3);
INSERT INTO book_rating VALUES ('Lady of the Flies', 1);
INSERT INTO book_rating VALUES ('Lady of the Flies', 2);
INSERT INTO book_rating VALUES ('Lady of the Flies', 5);

SELECT name, PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY star_rating)
OVER (PARTITION BY name) AS pc
FROM book_rating;
+-----------------------+--------------+
| name                  | pc           |
+-----------------------+--------------+
| Lord of the Ladybirds | 4.0000000000 |
| Lord of the Ladybirds | 4.0000000000 |
| Lady of the Flies     | 2.0000000000 |
| Lady of the Flies     | 2.0000000000 |
| Lady of the Flies     | 2.0000000000 |
+-----------------------+--------------+

SELECT name, PERCENTILE_CONT(1) WITHIN GROUP (ORDER BY star_rating)
OVER (PARTITION BY name) AS pc
FROM book_rating;
+-----------------------+--------------+
| name                  | pc           |
+-----------------------+--------------+
| Lord of the Ladybirds | 5.0000000000 |
| Lord of the Ladybirds | 5.0000000000 |
| Lady of the Flies     | 5.0000000000 |
| Lady of the Flies     | 5.0000000000 |
| Lady of the Flies     | 5.0000000000 |
+-----------------------+--------------+

SELECT name, PERCENTILE_CONT(0) WITHIN GROUP (ORDER BY star_rating)
OVER (PARTITION BY name) AS pc
FROM book_rating;
+-----------------------+--------------+
| name                  | pc           |
+-----------------------+--------------+
| Lord of the Ladybirds | 3.0000000000 |
| Lord of the Ladybirds | 3.0000000000 |
| Lady of the Flies     | 1.0000000000 |
| Lady of the Flies     | 1.0000000000 |
| Lady of the Flies     | 1.0000000000 |
+-----------------------+--------------+

SELECT name, PERCENTILE_CONT(0.6) WITHIN GROUP (ORDER BY star_rating)
OVER (PARTITION BY name) AS pc
FROM book_rating;
+-----------------------+--------------+
| name                  | pc           |
+-----------------------+--------------+
| Lord of the Ladybirds | 4.2000000000 |
| Lord of the Ladybirds | 4.2000000000 |
| Lady of the Flies     | 2.6000000000 |
| Lady of the Flies     | 2.6000000000 |
| Lady of the Flies     | 2.6000000000 |
+-----------------------+--------------+
```

• MEDIAN() - a special case of `PERCENTILE_CONT` equivalent to `PERCENTILE_CONT(0.5)`