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Brendan Clement
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This PR is part of the documentation revamp. Once the full set of PRs is merged, we’ll publish the combined site to staging for review before updating docs.lancedb.com. The curation, SQL and operations pages contain outdated claims and examples that do not reliably work as written. This corrects those pages against the reviewed SDK and local server evidence, including prepared statements, background jobs, cleanup and monitoring. Integration guides are grouped with their examples in #4533. Function and Secret-object documentation stays unpublished until its release. The two Function pages are listed in `.mintignore` and left out of the navigation, and the sections about them on other pages are MDX comments. Stacked on #4533. Each MDX file has one PR owner. Rebase onto main after #4533 is squash-merged. Local assembly, Mint validation and link checks pass, and the assembled site shows no Function or Secret-object content. A local release rehearsal that uncomments the drafts and restores the navigation also passes. Hold publication until this PR and lancedb/sophon#8688 are on `main`.
137 lines
3.7 KiB
Plaintext
137 lines
3.7 KiB
Plaintext
---
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title: "SELECT"
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sidebarTitle: "SELECT"
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description: "Projection, filtering, ordering, and composing queries out of other queries."
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icon: table-columns
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keywords: ["select", "where", "order by", "limit", "cte", "union", "distinct"]
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---
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<Badge color="red">Enterprise</Badge> [SQL runs in LanceDB Enterprise.](/sql)
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```sql
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[WITH cte AS (...)]
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SELECT [ALL | DISTINCT] select_expr [, ...]
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FROM from_item [[JOIN ...] ON ...]
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[WHERE predicate]
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[GROUP BY expr [, ...]] [HAVING predicate]
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[WINDOW name AS (...)]
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[ORDER BY expr [ASC | DESC] [NULLS FIRST | LAST] [, ...]]
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[LIMIT n] [OFFSET n]
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```
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## Projection {#projection}
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<CodeGroup>
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```sql SQL
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SELECT id, title, identifier FROM documents
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```
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```python Python icon="python"
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tbl.search().select(["id", "title", "identifier"]).to_arrow()
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```
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```typescript TypeScript icon="square-js"
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await tbl.query().select(["id", "title", "identifier"]).toArrow();
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```
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</CodeGroup>
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Name the columns you need instead of using `SELECT *`, particularly for
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tables containing images, video or large vectors. Projection lets the engine
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avoid loading unnecessary columns, although columns used by filters, joins or
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other expressions may still need to be read.
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Row metadata is available as [pseudo-columns](/sql/data-types#pseudo-columns)
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(`_rowid`, `_rowaddr`, `_rowoffset`) but only when named, so they stay out of
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`SELECT *`.
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`DISTINCT` deduplicates the projected rows.
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## Filtering {#filtering}
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<CodeGroup>
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```sql SQL
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SELECT * FROM documents
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WHERE identifier > 0 AND identifier < 1000000
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```
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```python Python icon="python"
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tbl.search().where("identifier > 0 AND identifier < 1000000").to_arrow()
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```
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```typescript TypeScript icon="square-js"
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await tbl.query().where("identifier > 0 AND identifier < 1000000").toArrow();
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```
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</CodeGroup>
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The SDK `where` takes the same expression language, so a predicate moves between
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them unchanged. A supported predicate can use a scalar index; confirm the chosen plan with
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[`explain_plan`](/search/explain). See
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[Filtering](/search/filtering) for the operators and their pushdown behaviour.
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## Ordering and limiting {#order-limit}
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```sql
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SELECT id, title FROM documents
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ORDER BY created_at DESC NULLS LAST
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LIMIT 100 OFFSET 200
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```
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`ORDER BY` on a large table is a sort over the whole result, so pair it with a
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`LIMIT` unless you genuinely need every row ordered.
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## Set operations {#set-operations}
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`UNION`, `UNION ALL`, `INTERSECT` and `EXCEPT` combine whole queries:
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```sql
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SELECT id FROM documents WHERE category = 'film'
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UNION ALL
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SELECT id FROM archived_documents WHERE category = 'film'
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```
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`UNION` deduplicates and `UNION ALL` does not, which makes `UNION ALL` the
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cheaper one when you already know the inputs are disjoint.
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## Common table expressions {#ctes}
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```sql
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WITH recent AS (
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SELECT * FROM documents WHERE created_at > now() - INTERVAL '7 days'
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)
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SELECT category, count(*) FROM recent GROUP BY category
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```
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A CTE is scoped to the statement. One you want to keep is a
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[view](/management/view); one whose rows you want stored is a
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[materialized view](/management/materialized-view).
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## Subqueries {#subqueries}
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Subqueries are allowed anywhere an expression or a relation is -- in `FROM`, in
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`WHERE` with `IN` or `EXISTS`, and as a scalar in the select list:
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```sql
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SELECT title
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FROM documents
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WHERE category_id IN (SELECT id FROM categories WHERE active)
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```
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## Namespaced tables {#namespaces}
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A table inside a [namespace](/management/namespace) is written as a dotted path:
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```sql
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SELECT * FROM prod.events.clicks
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```
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## Reading the plan {#plans}
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`EXPLAIN` and `EXPLAIN ANALYZE` work as DataFusion defines them, and produce the
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same operators the SDK methods do:
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```sql
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EXPLAIN ANALYZE SELECT * FROM documents WHERE identifier < 1000
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```
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See [Explain](/search/explain) and [Explain Analyze](/search/explain).
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