SQL aggregations

docs/sql/aggregations.md

@@ -0,0 +1,148 @@
+---
+layout: default
+title: Aggregation Functions
+parent: SQL
+nav_order: 11
+---
+
+# Aggregation functions
+
+Aggregate functions use the `GROUP BY` clause to group sets of values into subsets.
+
+## Group By
+
+Use the `GROUP BY` clause as an identifier, ordinal, or expression.
+
+### Identifier
+
+```sql
+SELECT gender, sum(age) FROM accounts GROUP BY gender;
+```
+
+| gender | sum (age)
+:--- | :---
+F | 28 |
+M | 101 |
+
+### Ordinal
+
+```sql
+SELECT gender, sum(age) FROM accounts GROUP BY 1;
+```
+
+| gender | sum (age)
+:--- | :---
+F | 28 |
+M | 101 |
+
+### Expression
+
+```sql
+SELECT abs(account_number), sum(age) FROM accounts GROUP BY abs(account_number);
+```
+
+| abs(account_number) | sum (age)
+:--- | :---
+| 1  | 32  |
+| 13 | 28  |
+| 18 | 33  |
+| 6  | 36  |
+
+## Aggregation
+
+Use aggregations as a select, expression, or an argument of an expression.
+
+### Select
+
+```sql
+SELECT gender, sum(age) FROM accounts GROUP BY gender;
+```
+
+| gender | sum (age)
+:--- | :---
+F | 28 |
+M | 101 |
+
+### Argument
+
+```sql
+SELECT gender, sum(age) * 2 as sum2 FROM accounts GROUP BY gender;
+```
+
+| gender | sum2
+:--- | :---
+F | 56 |
+M | 202 |
+
+### Expression
+
+```sql
+SELECT gender, sum(age * 2) as sum2 FROM accounts GROUP BY gender;
+```
+
+| gender | sum2
+:--- | :---
+F | 56 |
+M | 202 |
+
+### COUNT
+
+Use the `COUNT` function to accept arguments such as a `*` or a literal like `1`.
+The meaning of these different forms are as follows:
+
+- `COUNT(field)` - Only counts if given a field (or expression) is not null or missing in the input rows.
+- `COUNT(*)` - Counts the number of all its input rows.
+- `COUNT(1)` (same as `COUNT(*)`) - Counts any non-null literal.
+
+## Having
+
+Use the `HAVING` clause to filter out aggregated values.
+
+### HAVING with GROUP BY
+
+You can use aggregate expressions or its aliases defined in a `SELECT` clause in a `HAVING` condition.
+
+We recommend using a non-aggregate expression in the `WHERE` clause although you can do this in a `HAVING` clause.
+
+The aggregations in a `HAVING` clause are not necessarily the same as that in a select list. As an extension to the SQL standard, you're not restricted to using identifiers only in the `GROUP BY` list.
+For example:
+
+```sql
+SELECT gender, sum(age)
+FROM accounts
+GROUP BY gender
+HAVING sum(age) > 100;
+```
+
+| gender | sum (age)
+:--- | :---
+M | 101 |
+
+Here's another example for using an alias in a `HAVING` condition.
+
+```sql
+SELECT gender, sum(age) AS s
+FROM accounts
+GROUP BY gender
+HAVING s > 100;
+```
+
+| gender | s
+:--- | :---
+M | 101 |
+
+If an identifier is ambiguous, for example, present both as a select alias and as an index field (preference is alias). In this case, the identifier is replaced with an expression aliased in the `SELECT` clause:
+
+### HAVING without GROUP BY
+
+You can use a `HAVING` clause without the `GROUP BY` clause. This is useful because aggregations are not supported in a `WHERE` clause:
+
+```sql
+SELECT 'Total of age > 100'
+FROM accounts
+HAVING sum(age) > 100;
+```
+
+| Total of age > 100 |
+:--- |
+Total of age > 100 |