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Jack Ye
fe651ae003
The function catalogue is now real rather than a pointer. Scalar functions are listed by taxonomy -- math, string, unicode, regex, date/time, array, map, struct, crypto, type -- and the names come from what datafusion-functions 54 actually registers, not from what DataFusion documents on main. Aggregate and window pages gain the same treatment. Two things the audit corrected rather than copied. SHOW TABLES does not work. DataFusion implements it over information_schema, which this session does not enable, so it fails at plan time. It was documented as working here, on the SHOW page and in three Manage tables; all four are fixed, and the SQL overview now lists the absence alongside SHOW COLUMNS. Prepared statements are a Flight SQL protocol feature, not a PREPARE statement. A script carrying PREPARE ... AS SELECT will not run, while the same intent expressed through a client will. The new page says so rather than copying DataFusion's syntax. Every section of the Documentation tab that has a SQL counterpart now links to it -- 27 of 76 non-SQL pages, each pointing at the specific statement rather than at the reference index.
119 lines
3.3 KiB
Plaintext
119 lines
3.3 KiB
Plaintext
---
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title: "Aggregation"
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sidebarTitle: "Aggregation"
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description: "GROUP BY, HAVING, and the aggregate function set."
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icon: layer-group
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keywords: ["group by", "having", "count", "sum", "avg", "aggregate", "approximate"]
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---
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<Badge color="red">Enterprise</Badge> [SQL runs on a deployment.](/sql)
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Aggregation is one of the two main reasons to reach for SQL rather than a query
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builder -- the SDKs project, filter and search, but they do not group. The other
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is [joins](/sql/join).
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## Grouping {#grouping}
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```sql
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SELECT category, count(*) AS n, avg(score) AS mean_score
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FROM documents
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GROUP BY category
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HAVING count(*) > 10
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ORDER BY n DESC
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```
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`WHERE` filters rows before grouping; `HAVING` filters groups after. Putting a
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row-level predicate in `HAVING` still gives the right answer but does more work,
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because rows that could have been discarded early are grouped first.
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Grouping by several expressions produces one row per distinct combination:
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```sql
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SELECT category, source, count(*) FROM documents GROUP BY category, source
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```
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## Aggregate without grouping {#whole-table}
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Omitting `GROUP BY` aggregates the whole table into one row -- the usual way to
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ask how big something is:
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```sql
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SELECT count(*), min(created_at), max(created_at) FROM documents
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```
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`count(*)` counts rows; `count(col)` counts non-null values, which is how you
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measure how populated a [computed column](/feature-engineering/add-computed-column)
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is before deciding to refresh it again.
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## The function set {#functions}
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Registered in DataFusion 54. Names below are what is actually available.
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**General**
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```text
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count sum min max avg median
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bit_and bit_or bit_xor bool_and bool_or
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first_value last_value nth_value grouping
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```
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**Statistical**
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```text
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stddev stddev_pop stddev_samp
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var var_pop var_samp
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corr covar_pop covar_samp
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regr_slope regr_intercept regr_count regr_r2
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regr_avgx regr_avgy regr_sxx regr_syy regr_sxy
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```
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**Approximate**
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```text
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approx_distinct approx_median
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approx_percentile_cont approx_percentile_cont_with_weight
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percentile_cont
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```
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**Collecting**
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```text
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array_agg string_agg
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```
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The approximate functions are the ones worth reaching for on a large table.
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`approx_distinct` answers "roughly how many distinct values" for a fraction of
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what `count(DISTINCT ...)` costs, which matters most where it is most expensive
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— a high-cardinality column.
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`DISTINCT` works inside an aggregate: `count(DISTINCT category)`. `FILTER`
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narrows one aggregate without narrowing the others:
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```sql
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SELECT
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count(*) AS total,
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count(*) FILTER (WHERE score > 0.9) AS confident
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FROM documents
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```
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## Aggregating a sample {#sampling}
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For an estimate over a very large table, aggregate a
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[sample](/sql/sampling) rather than the whole thing:
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```sql
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SELECT category, count(*) * 100 AS estimated
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FROM documents TABLESAMPLE (1)
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GROUP BY category
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```
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Scale the result by the inverse of the sample fraction, and treat it as an
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estimate -- a 1% sample of a skewed column will miss rare categories entirely.
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## What aggregation does not do {#not}
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Aggregates run over rows the query already produced. They cannot combine with
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[vector search](/search/vector-search) in one statement, because similarity
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search has no SQL form. Run the search in an SDK and aggregate its result, or
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materialize the search output first.
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