# Aggregate functions

Source: https://www.hotdata.dev/docs/sql-functions-aggregate
Site index: https://www.hotdata.dev/llms.txt

Reference for aggregate functions in [HotSQL](/docs/sql). Names, signatures, and examples match the engine exactly.

### approx_distinct

Returns the approximate number of distinct input values calculated using the HyperLogLog algorithm.

```
approx_distinct(expression)
```

```sql
> SELECT approx_distinct(column_name) FROM table_name;
+-----------------------------------+
| approx_distinct(column_name)      |
+-----------------------------------+
| 42                                |
+-----------------------------------+
```

### approx_median

Returns the approximate median (50th percentile) of input values. It is an alias of `approx_percentile_cont(0.5) WITHIN GROUP (ORDER BY x)`.

```
approx_median(expression)
```

```sql
> SELECT approx_median(column_name) FROM table_name;
+-----------------------------------+
| approx_median(column_name)        |
+-----------------------------------+
| 23.5                              |
+-----------------------------------+
```

### approx_percentile_cont

Returns the approximate percentile of input values using the t-digest algorithm.

```
approx_percentile_cont(percentile [, centroids]) WITHIN GROUP (ORDER BY expression)
```

**Arguments**

- `percentile`: Percentile to compute. Must be a float value between 0 and 1 (inclusive).
- `centroids`: Number of centroids to use in the t-digest algorithm. _Default is 100_. A higher number results in more accurate approximation but requires more memory.

```sql
> SELECT approx_percentile_cont(0.75) WITHIN GROUP (ORDER BY column_name) FROM table_name;
+------------------------------------------------------------------+
| approx_percentile_cont(0.75) WITHIN GROUP (ORDER BY column_name) |
+------------------------------------------------------------------+
| 65.0                                                             |
+------------------------------------------------------------------+
> SELECT approx_percentile_cont(0.75, 100) WITHIN GROUP (ORDER BY column_name) FROM table_name;
+-----------------------------------------------------------------------+
| approx_percentile_cont(0.75, 100) WITHIN GROUP (ORDER BY column_name) |
+-----------------------------------------------------------------------+
| 65.0                                                                  |
+-----------------------------------------------------------------------+
```
An alternate syntax is also supported:
```sql
> SELECT approx_percentile_cont(column_name, 0.75) FROM table_name;
+-----------------------------------------------+
| approx_percentile_cont(column_name, 0.75)     |
+-----------------------------------------------+
| 65.0                                          |
+-----------------------------------------------+

> SELECT approx_percentile_cont(column_name, 0.75, 100) FROM table_name;
+----------------------------------------------------------+
| approx_percentile_cont(column_name, 0.75, 100)           |
+----------------------------------------------------------+
| 65.0                                                     |
+----------------------------------------------------------+
```

### approx_percentile_cont_with_weight

Returns the weighted approximate percentile of input values using the t-digest algorithm.

```
approx_percentile_cont_with_weight(weight, percentile [, centroids]) WITHIN GROUP (ORDER BY expression)
```

**Arguments**

- `expression`: The expression.
- `weight`: Expression to use as weight. Can be a constant, column, or function, and any combination of arithmetic operators.
- `percentile`: Percentile to compute. Must be a float value between 0 and 1 (inclusive).
- `centroids`: Number of centroids to use in the t-digest algorithm. _Default is 100_. A higher number results in more accurate approximation but requires more memory.

```sql
> SELECT approx_percentile_cont_with_weight(weight_column, 0.90) WITHIN GROUP (ORDER BY column_name) FROM table_name;
+---------------------------------------------------------------------------------------------+
| approx_percentile_cont_with_weight(weight_column, 0.90) WITHIN GROUP (ORDER BY column_name) |
+---------------------------------------------------------------------------------------------+
| 78.5                                                                                        |
+---------------------------------------------------------------------------------------------+
> SELECT approx_percentile_cont_with_weight(weight_column, 0.90, 100) WITHIN GROUP (ORDER BY column_name) FROM table_name;
+--------------------------------------------------------------------------------------------------+
| approx_percentile_cont_with_weight(weight_column, 0.90, 100) WITHIN GROUP (ORDER BY column_name) |
+--------------------------------------------------------------------------------------------------+
| 78.5                                                                                             |
+--------------------------------------------------------------------------------------------------+
```
An alternative syntax is also supported:

```sql
> SELECT approx_percentile_cont_with_weight(column_name, weight_column, 0.90) FROM table_name;
+--------------------------------------------------+
| approx_percentile_cont_with_weight(column_name, weight_column, 0.90) |
+--------------------------------------------------+
| 78.5                                             |
+--------------------------------------------------+
```

### array_agg

Returns an array created from the expression elements. If ordering is required, elements are inserted in the specified order.
This aggregation function can only mix DISTINCT and ORDER BY if the ordering expression is exactly the same as the argument expression.

```
array_agg(expression [ORDER BY expression])
```

```sql
> SELECT array_agg(column_name ORDER BY other_column) FROM table_name;
+-----------------------------------------------+
| array_agg(column_name ORDER BY other_column)  |
+-----------------------------------------------+
| [element1, element2, element3]                |
+-----------------------------------------------+
> SELECT array_agg(DISTINCT column_name ORDER BY column_name) FROM table_name;
+--------------------------------------------------------+
| array_agg(DISTINCT column_name ORDER BY column_name)  |
+--------------------------------------------------------+
| [element1, element2, element3]                         |
+--------------------------------------------------------+
```

### avg

Returns the average of numeric values in the specified column.

```
avg(expression)
```

```sql
> SELECT avg(column_name) FROM table_name;
+---------------------------+
| avg(column_name)           |
+---------------------------+
| 42.75                      |
+---------------------------+
```

### bool_and

Returns true if all non-null input values are true, otherwise false.

```
bool_and(expression)
```

**Arguments**

- `expression`: The expression.

```sql
> SELECT bool_and(column_name) FROM table_name;
+----------------------------+
| bool_and(column_name)       |
+----------------------------+
| true                        |
+----------------------------+
```

### corr

Returns the coefficient of correlation between two numeric values.

```
corr(expression1, expression2)
```

**Arguments**

- `expression1`: First expression.
- `expression2`: Second expression.

```sql
> SELECT corr(column1, column2) FROM table_name;
+--------------------------------+
| corr(column1, column2)         |
+--------------------------------+
| 0.85                           |
+--------------------------------+
```

### count

Returns the number of non-null values in the specified column. To include null values in the total count, use `count(*)`.

```
count(expression)
```

```sql
> SELECT count(column_name) FROM table_name;
+-----------------------+
| count(column_name)     |
+-----------------------+
| 100                   |
+-----------------------+

> SELECT count(*) FROM table_name;
+------------------+
| count(*)         |
+------------------+
| 120              |
+------------------+
```

### covar_samp

Returns the sample covariance of a set of number pairs.

```
covar_samp(expression1, expression2)
```

**Arguments**

- `expression1`: First expression.
- `expression2`: Second expression.

```sql
> SELECT covar_samp(column1, column2) FROM table_name;
+-----------------------------------+
| covar_samp(column1, column2)      |
+-----------------------------------+
| 8.25                              |
+-----------------------------------+
```

### first_value

Returns the first element in an aggregation group according to the requested ordering. If no ordering is given, returns an arbitrary element from the group.

```
first_value(expression [ORDER BY expression])
```

```sql
> SELECT first_value(column_name ORDER BY other_column) FROM table_name;
+-----------------------------------------------+
| first_value(column_name ORDER BY other_column)|
+-----------------------------------------------+
| first_element                                 |
+-----------------------------------------------+
```

### grouping

Returns 1 if the data is aggregated across the specified column, or 0 if it is not aggregated in the result set.

```
grouping(expression)
```

**Arguments**

- `expression`: Expression to evaluate whether data is aggregated across the specified column. Can be a constant, column, or function.

```sql
> SELECT column_name, GROUPING(column_name) AS group_column
  FROM table_name
  GROUP BY GROUPING SETS ((column_name), ());
+-------------+-------------+
| column_name | group_column |
+-------------+-------------+
| value1      | 0           |
| value2      | 0           |
| NULL        | 1           |
+-------------+-------------+
```

### last_value

Returns the last element in an aggregation group according to the requested ordering. If no ordering is given, returns an arbitrary element from the group.

```
last_value(expression [ORDER BY expression])
```

```sql
> SELECT last_value(column_name ORDER BY other_column) FROM table_name;
+-----------------------------------------------+
| last_value(column_name ORDER BY other_column) |
+-----------------------------------------------+
| last_element                                  |
+-----------------------------------------------+
```

### max

Returns the maximum value in the specified column.

```
max(expression)
```

```sql
> SELECT max(column_name) FROM table_name;
+----------------------+
| max(column_name)      |
+----------------------+
| 150                  |
+----------------------+
```

### median

Returns the median value in the specified column.

```
median(expression)
```

**Arguments**

- `expression`: The expression.

```sql
> SELECT median(column_name) FROM table_name;
+----------------------+
| median(column_name)   |
+----------------------+
| 45.5                 |
+----------------------+
```

### min

Returns the minimum value in the specified column.

```
min(expression)
```

```sql
> SELECT min(column_name) FROM table_name;
+----------------------+
| min(column_name)      |
+----------------------+
| 12                   |
+----------------------+
```

### nth_value

Returns the nth value in a group of values.

```
nth_value(expression, n ORDER BY expression)
```

**Arguments**

- `expression`: The column or expression to retrieve the nth value from.
- `n`: The position (nth) of the value to retrieve, based on the ordering.

```sql
> SELECT dept_id, salary, NTH_VALUE(salary, 2) OVER (PARTITION BY dept_id ORDER BY salary ASC) AS second_salary_by_dept
  FROM employee;
+---------+--------+-------------------------+
| dept_id | salary | second_salary_by_dept   |
+---------+--------+-------------------------+
| 1       | 30000  | NULL                    |
| 1       | 40000  | 40000                   |
| 1       | 50000  | 40000                   |
| 2       | 35000  | NULL                    |
| 2       | 45000  | 45000                   |
+---------+--------+-------------------------+
```

### percentile_cont

Returns the exact percentile of input values, interpolating between values if needed.

```
percentile_cont(percentile) WITHIN GROUP (ORDER BY expression)
```

**Arguments**

- `expression`: The expression.
- `percentile`: Percentile to compute. Must be a float value between 0 and 1 (inclusive).

```sql
> SELECT percentile_cont(0.75) WITHIN GROUP (ORDER BY column_name) FROM table_name;
+----------------------------------------------------------+
| percentile_cont(0.75) WITHIN GROUP (ORDER BY column_name) |
+----------------------------------------------------------+
| 45.5                                                     |
+----------------------------------------------------------+
```

An alternate syntax is also supported:
```sql
> SELECT percentile_cont(column_name, 0.75) FROM table_name;
+---------------------------------------+
| percentile_cont(column_name, 0.75)    |
+---------------------------------------+
| 45.5                                  |
+---------------------------------------+
```

### stddev

Returns the standard deviation of a set of numbers.

```
stddev(expression)
```

```sql
> SELECT stddev(column_name) FROM table_name;
+----------------------+
| stddev(column_name)   |
+----------------------+
| 12.34                |
+----------------------+
```

### stddev_pop

Returns the population standard deviation of a set of numbers.

```
stddev_pop(expression)
```

```sql
> SELECT stddev_pop(column_name) FROM table_name;
+--------------------------+
| stddev_pop(column_name)   |
+--------------------------+
| 10.56                    |
+--------------------------+
```

### string_agg

Concatenates the values of string expressions and places separator values between them. 
If ordering is required, strings are concatenated in the specified order. 
This aggregation function can only mix DISTINCT and ORDER BY if the ordering expression is exactly the same as the first argument expression.

```
string_agg([DISTINCT] expression, delimiter [ORDER BY expression])
```

**Arguments**

- `expression`: The string expression to concatenate. Can be a column or any valid string expression.
- `delimiter`: A literal string used as a separator between the concatenated values.

```sql
> SELECT string_agg(name, ', ') AS names_list
  FROM employee;
+--------------------------+
| names_list               |
+--------------------------+
| Alice, Bob, Bob, Charlie |
+--------------------------+
> SELECT string_agg(name, ', ' ORDER BY name DESC) AS names_list
  FROM employee;
+--------------------------+
| names_list               |
+--------------------------+
| Charlie, Bob, Bob, Alice |
+--------------------------+
> SELECT string_agg(DISTINCT name, ', ' ORDER BY name DESC) AS names_list
  FROM employee;
+--------------------------+
| names_list               |
+--------------------------+
| Charlie, Bob, Alice |
+--------------------------+
```

### sum

Returns the sum of all values in the specified column.

```
sum(expression)
```

```sql
> SELECT sum(column_name) FROM table_name;
+-----------------------+
| sum(column_name)       |
+-----------------------+
| 12345                 |
+-----------------------+
```

### var

Returns the statistical sample variance of a set of numbers.

```
var(expression)
```

**Arguments**

- `expression`: Numeric expression.

### var_pop

Returns the statistical population variance of a set of numbers.

```
var_pop(expression)
```

**Arguments**

- `expression`: Numeric expression.