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Jack Ye
734eb39dc3
Six groups were verbs -- Manage, Ingest, Train, Operate, Secure, Deploy -- and now name the thing rather than the action. Four more were sentence case. The page labels that were verb phrases follow: Add Computed Column becomes Computed Columns, Refresh Column becomes Column Refresh, Configuring Storage becomes Configuration, and Explain becomes Query Plans, which is also more accurate now that the page covers analyze as well. SQL statement pages take the keyword -- SELECT, DDL, DML, REFRESH, SHOW, EXPLAIN -- because a statement name is a noun and lowercase Select and Show read as instructions. Concept pages there keep noun phrases: Joins, Window Functions, Subqueries, Operators. Two duplicate labels are resolved on the way. Reranking sat in Semantic Search labelled Overview despite being the only page on its subject, and the Enterprise client page sat in SQL Reference labelled SQL next to the reference Overview; it is Client Connection now. Title case leaves prepositions and articles lowercase, so Needle in a Haystack and Hybrid Search on BEIR are already correct and unchanged.
101 lines
2.7 KiB
Plaintext
101 lines
2.7 KiB
Plaintext
---
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title: "Window functions"
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sidebarTitle: "Window Functions"
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description: "Compute over a set of rows related to the current one, without collapsing them."
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icon: chart-simple
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keywords: ["window", "over", "partition by", "row_number", "rank", "lag", "lead"]
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---
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<Badge color="red">Enterprise</Badge> [SQL runs on a deployment.](/sql)
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A window function computes across a set of rows related to the current one and
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returns a value **per row**. That is the difference from
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[aggregation](/sql/aggregation), which collapses the rows it covers.
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```sql
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SELECT
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title,
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score,
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row_number() OVER (PARTITION BY category ORDER BY score DESC) AS rank
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FROM documents
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```
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Every row survives; each one gains its rank within its category.
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## The window {#window}
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```sql
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OVER (
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[PARTITION BY expr [, ...]]
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[ORDER BY expr [, ...]]
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[ROWS | RANGE BETWEEN frame_start AND frame_end]
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)
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```
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`PARTITION BY` splits the rows into independent groups. `ORDER BY` orders them
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inside each partition -- required by the ranking functions, since a rank without
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an order is meaningless. The frame clause narrows the window further, which is
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how running totals and moving averages are expressed:
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```sql
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SELECT
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created_at,
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sum(amount) OVER (ORDER BY created_at ROWS UNBOUNDED PRECEDING) AS running_total,
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avg(amount) OVER (ORDER BY created_at ROWS BETWEEN 6 PRECEDING AND CURRENT ROW) AS trailing_7
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FROM transactions
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```
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A `WINDOW` clause names a definition when several functions share one:
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```sql
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SELECT
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row_number() OVER w,
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lag(score) OVER w
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FROM documents
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WINDOW w AS (PARTITION BY category ORDER BY score DESC)
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```
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## The functions {#functions}
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Registered in DataFusion 54.
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**Ranking**
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```text
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row_number rank dense_rank percent_rank cume_dist ntile
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```
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**Offset and positional**
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```text
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lag lead first_value last_value nth_value
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```
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**Aggregates**
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Any [aggregate function](/sql/aggregation#functions) used with `OVER` becomes a
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window function — that is how running totals and moving averages are written.
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## Deduplicating with a window {#dedup}
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The common practical use: keep one row per key, chosen by some ordering. A
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window ranks, and an outer query filters:
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```sql
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WITH ranked AS (
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SELECT *, row_number() OVER (PARTITION BY doc_id ORDER BY version DESC) AS rn
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FROM revisions
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)
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SELECT * FROM ranked WHERE rn = 1
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```
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The outer query is needed because a window function cannot appear in `WHERE` --
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windows are computed after filtering, so the predicate would have nothing to
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read.
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<Note>
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Window functions sort within each partition, so on a large table they are among
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the more expensive things you can ask for. Filter first, and check the cost with
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[Explain Analyze](/search/explain).
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</Note>
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