There are two ways of doing aggregation, one on the result of a whole query and one
on the separate groups of a GROUP BY query.
Aggregation Function will always return one result value for a single group.
You can imagine the matching rows of a `SELECT`

statement without `GROUP BY`

as one group. One row will always be returned.

For a short summary of aggregation functions see Data Aggregation.

This aggregation function simply returns the number of rows that match the query.

count(columName) is also possible, but currently only works on a primary key column.
The semantics are the same.
The return value is always of type `long`

.

```
cr> select count(*) from locations;
+----------+
| count(*) |
+----------+
| 13 |
+----------+
SELECT 1 row in set (... sec)
```

`count(*)`

can also be used on group by queries:

```
cr> select count(*), kind from locations group by kind order by kind asc;
+----------+-------------+
| count(*) | kind |
+----------+-------------+
| 4 | Galaxy |
| 5 | Planet |
| 4 | Star System |
+----------+-------------+
SELECT 3 rows in set (... sec)
```

In contrast to the count(*) function
the `count`

function used with a column name as parameter will return the
number of rows with a non-`NULL`

value in that column.

Example:

```
cr> select count(name), count(*), date from locations group by date
... order by count(name) desc, count(*) desc;
+-------------+----------+---------------+
| count(name) | count(*) | date |
+-------------+----------+---------------+
| 7 | 8 | 1373932800000 |
| 4 | 4 | 308534400000 |
| 1 | 1 | 1367366400000 |
+-------------+----------+---------------+
SELECT 3 rows in set (... sec)
```

The count aggregation function also supports the distinct keyword. This
keyword changes the behaviour of the function so that it will only count the number
of distinct values in this column that are not `NULL`

:

```
cr> select count(distinct kind), count(*), date
... from locations group by date
... order by count(distinct kind) desc, count(*) desc;
+----------------------+----------+---------------+
| count(DISTINCT kind) | count(*) | date |
+----------------------+----------+---------------+
| 3 | 8 | 1373932800000 |
| 3 | 4 | 308534400000 |
| 1 | 1 | 1367366400000 |
+----------------------+----------+---------------+
SELECT 3 rows in set (... sec)
```

```
cr> select count(distinct kind) from locations;
+----------------------+
| count(DISTINCT kind) |
+----------------------+
| 3 |
+----------------------+
SELECT 1 row in set (... sec)
```

The `min`

aggregation function returns the smallest value in a column that is not `NULL`

.
Its single argument is a column name and its return value is always of the type of that column.
Example:

```
cr> select min(position), kind
... from locations
... where name not like 'North %'
... group by kind order by min(position) asc, kind asc;
+---------------+-------------+
| min(position) | kind |
+---------------+-------------+
| 1 | Planet |
| 1 | Star System |
| 2 | Galaxy |
+---------------+-------------+
SELECT 3 rows in set (... sec)
```

```
cr> select min(date) from locations;
+--------------+
| min(date) |
+--------------+
| 308534400000 |
+--------------+
SELECT 1 row in set (... sec)
```

`min`

returns `NULL`

if the column does not contain any value but `NULL`

.
It is allowed on numeric columns (`byte`

, `short`

, `integer`

, `long`

, `float`

, `double`

),
on `timestamp`

and `string`

columns. On `string`

columns it will return the lexicographically
smallest.:

```
cr> select min(name), kind from locations
... group by kind order by kind asc;
+------------------------------------+-------------+
| min(name) | kind |
+------------------------------------+-------------+
| Galactic Sector QQ7 Active J Gamma | Galaxy |
| | Planet |
| Aldebaran | Star System |
+------------------------------------+-------------+
SELECT 3 rows in set (... sec)
```

It behaves exactly like `min`

but returns the biggest value in a column that is not `NULL`

.
Some Examples:

```
cr> select max(position), kind from locations
... group by kind order by kind desc;
+---------------+-------------+
| max(position) | kind |
+---------------+-------------+
| 4 | Star System |
| 5 | Planet |
| 6 | Galaxy |
+---------------+-------------+
SELECT 3 rows in set (... sec)
```

```
cr> select max(position) from locations;
+---------------+
| max(position) |
+---------------+
| 6 |
+---------------+
SELECT 1 row in set (... sec)
```

```
cr> select max(name), kind from locations
... group by kind order by max(name) desc;
+-------------------+-------------+
| max(name) | kind |
+-------------------+-------------+
| Outer Eastern Rim | Galaxy |
| Bartledan | Planet |
| Altair | Star System |
+-------------------+-------------+
SELECT 3 rows in set (... sec)
```

The `sum`

aggregation function returns the sum of all the values in a column that are not `NULL`

as a double value. Its single argument is the column name of a numeric column or timestamp column.
`sum`

can not be used on other column types.

```
cr> select sum(position), kind from locations
... group by kind order by sum(position) asc;
+---------------+-------------+
| sum(position) | kind |
+---------------+-------------+
| 10.0 | Star System |
| 13.0 | Galaxy |
| 15.0 | Planet |
+---------------+-------------+
SELECT 3 rows in set (... sec)
```

```
cr> select sum(position) as position_sum from locations;
+--------------+
| position_sum |
+--------------+
| 38.0 |
+--------------+
SELECT 1 row in set (... sec)
```

```
cr> select sum(name), kind from locations group by kind order by sum(name) desc;
SQLActionException[UnsupportedFeatureException: unknown function: sum(string)]
```

The `avg`

or `mean`

aggregation function returns the arithmetic mean, the *average*,
of all values in a column that are not `NULL`

as a double value. It accepts all numeric columns
and timestamp columns as single argument. Using `avg`

on other column types is not allowed.

Example:

```
cr> select avg(position), kind from locations
... group by kind order by kind;
+---------------+-------------+
| avg(position) | kind |
+---------------+-------------+
| 3.25 | Galaxy |
| 3.0 | Planet |
| 2.5 | Star System |
+---------------+-------------+
SELECT 3 rows in set (... sec)
```

The `geometric_mean`

aggregation function computes the geometric mean,
a mean for positive numbers. For details see: Geometric Mean.

`geometric mean`

is defined on all numeric types and on timestamp. It always
returns double values. If a value is negative, all values were null or we got no
value at all `NULL`

is returned. If any of the aggregated values is `0`

the result will be `0.0`

as well.

Note

Due to java double precision arithmetic it is possible that any two executions of the aggregation function on the same data produce slightly differing results.

Example:

```
cr> select geometric_mean(position), kind from locations
... group by kind order by kind;
+--------------------------+-------------+
| geometric_mean(position) | kind |
+--------------------------+-------------+
| 2.6321480259049848 | Galaxy |
| 2.6051710846973517 | Planet |
| 2.213363839400643 | Star System |
+--------------------------+-------------+
SELECT 3 rows in set (... sec)
```

The `variance`

aggregation function computes the Variance of the set of non-null
values in a column. It is a measure about how far a set of numbers is spread.
A variance of `0.0`

indicates that all values are the same.

`variance`

is defined on all numeric types and on timestamp. It returns a
double value. If all values were null or we got no value at all `NULL`

is
returned.

Example:

```
cr> select variance(position), kind from locations
... group by kind order by kind desc;
+--------------------+-------------+
| variance(position) | kind |
+--------------------+-------------+
| 1.25 | Star System |
| 2.0 | Planet |
| 3.6875 | Galaxy |
+--------------------+-------------+
SELECT 3 rows in set (... sec)
```

Note

Due to java double precision arithmetic it is possible that any two executions of the aggregation function on the same data produce slightly differing results.

The `stddev`

aggregation function computes the Standard Deviation of the set
of non-null values in a column. It is a measure of the variation of data values.
A low standard deviation indicates that the values tend to be near the mean.

`stddev`

is defined on all numeric types and on timestamp. It always
returns double values. If all values were null or we got no
value at all `NULL`

is returned.

Example:

```
cr> select stddev(position), kind from locations
... group by kind order by kind;
+--------------------+-------------+
| stddev(position) | kind |
+--------------------+-------------+
| 1.920286436967152 | Galaxy |
| 1.4142135623730951 | Planet |
| 1.118033988749895 | Star System |
+--------------------+-------------+
SELECT 3 rows in set (... sec)
```

Note

Due to java double precision arithmetic it is possible that any two executions of the aggregation function on the same data produce slightly differing results.

The `percentile`

aggregation function computes a Percentile over numeric
non-null values in a column.

Percentiles show the point at which a certain percentage of observed values occur. For example, the 98th percentile is the value which is greater than 98% of the observed values. The result is defined and computed as an interpolated weighted average. According to that it allows the median of the input data to be defined conveniently as the 50th percentile.

The function expects a single fraction or an array of fractions and a column name. Independent
of the input column data type the result of `percentile`

always returns a double. If the
value at the specified column is `null`

the row is ignored. Fractions must be double
precision values between 0 and 1. When supplied a single fraction, the function will return
a single value corresponding to the percentile of the specified fraction:

```
cr> select percentile(position, 0.95), kind from locations
... group by kind order by kind;
+----------------------------+-------------+
| percentile(position, 0.95) | kind |
+----------------------------+-------------+
| 5.699999999999999 | Galaxy |
| 4.8 | Planet |
| 3.8499999999999996 | Star System |
+----------------------------+-------------+
SELECT 3 rows in set (... sec)
```

When supplied an array of fractions, the function will return an array of values corresponding to the percentile of each fraction specified:

```
cr> select percentile(position, [0.0013, 0.9987]) as perc from locations;
+--------------------------+
| perc |
+--------------------------+
| [1.0, 5.984400000000001] |
+--------------------------+
SELECT 1 row in set (... sec)
```

When a query with `percentile`

function won’t match any rows then a null
result is returned.

To be able to calculate percentiles over a huge amount of data and to scale out crate calculates
approximate instead of accurate percentiles. The algorithm used by the percentile metric is called
TDigest. The accuracy/size trade-off of the algorithm is defined by a single compression
parameter which has a constant value of `100`

. However, there are a few guidelines to keep in mind
in this implementation:

- Extreme percentiles (e.g. 99%) are more accurate
- For small sets percentiles are highly accurate
- It’s difficult to generalize the exact level of accuracy, as it depends on your data distribution and volume of data being aggregated

The `arbitrary`

aggregation function returns a single value of a column.
Which value it returns is not defined.
It accepts references to columns of all primitive types.
Using `arbitrary`

on `Object`

columns is not supported.
Its return type is the type of its parameter column and can be `NULL`

if the column contains `NULL`

values.

Example:

```
cr> select arbitrary(position) from locations;
+---------------------+
| arbitrary(position) |
+---------------------+
| ... |
+---------------------+
SELECT 1 row in set (... sec)
```

```
cr> select arbitrary(name), kind from locations
... where name != ''
... group by kind order by kind desc;
+-...-------------+-------------+
| arbitrary(name) | kind |
+-...-------------+-------------+
| ... | Star System |
| ... | Planet |
| ... | Galaxy |
+-...-------------+-------------+
SELECT 3 rows in set (... sec)
```

An example use case is to group a table with many rows per user by
`user_id`

and get the `username`

for every group, that means every
user. This works as rows with same `user_id`

have the same
`username`

. This method performs better than grouping on
`username`

as grouping on number types is generally faster than on
strings. The advantage is that the `arbitrary`

function does very little
to no computation as for example `max`

aggregation function would
do.