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Brendan Clement
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The existing docs have 254 findings from our shared Vale rules. Contributors would otherwise inherit these when editing the same lines. Clean up the prose across 80 pages: correct product names and a repeated word, simplify a few phrases, and replace dashes with punctuation that fits each sentence. The pinned Vale scan now reports zero findings, with no suppressions.
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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