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