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Jack Ye
ddb318567a
## Summary - Complete the SQL reference: `ATTACH DATABASE`, the `CHANGES` clause, and `dedupe()`/`duplicate_pairs()` (verified against current sophon source; these were recently renamed from `vector_dedup`/`vector_duplicate_pairs`). Fix the blob-column default-projection claim and add the previously undocumented `blob_descriptor()`. - Enrich the gated Functions draft (still `.mintignore`'d, pre-release): chained Functions over blob outputs, GPU-produced blobs, Secrets, multi-input/output binding, `code=[module]` packaging. - Upgrade the materialized-view and computed-column auto-refresh drafts with precise, verified catch-up semantics, and add the missing `ALTER MATERIALIZED VIEW` / `CREATE ... WITH(...)` sections (also gated — the automatic-refresh scheduler is still dev/staging-only). - Thread one running example (video frames → GPU extract/caption → dedup → auto-refreshing materialized view) through Functions, materialized views, and the LeRobotDataset integration guide instead of a separate tutorial. - Rename "LanceDB OSS"/"OSS" to "LanceDB Embedded"/"Embedded" throughout (52 files); activate the existing availability-comparison mechanism on `basics/offerings.mdx` for a full feature-by-feature comparison table.
79 lines
2.3 KiB
Plaintext
79 lines
2.3 KiB
Plaintext
---
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title: "Subqueries"
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sidebarTitle: "Subqueries"
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description: "Queries inside queries: scalar, IN, EXISTS, correlated, and derived tables."
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icon: code-branch
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keywords: ["subquery", "exists", "in", "correlated", "derived table", "lateral"]
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availability:
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embedded: unavailable
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enterprise: available
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---
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A subquery is allowed anywhere an expression or a relation is.
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## As a relation {#derived}
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```sql
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SELECT * FROM (
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SELECT category, count(*) AS n FROM documents GROUP BY category
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) AS counts
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WHERE n > 100
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```
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A derived table needs an alias. Its result cannot be
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[sampled](/sql/dql/sampling); sample the table inside it instead:
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```sql
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SELECT * FROM (SELECT * FROM documents TABLESAMPLE (10)) d
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```
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## As a scalar {#scalar}
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```sql
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SELECT title, (SELECT count(*) FROM revisions r WHERE r.doc_id = d.id) AS revisions
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FROM documents AS d
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```
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A scalar subquery returns one column and at most one row. No rows produces
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`NULL`; more than one row is an error.
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## IN and EXISTS {#in-exists}
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```sql
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SELECT title FROM documents
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WHERE category_id IN (SELECT id FROM categories WHERE active)
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SELECT title FROM documents AS d
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WHERE EXISTS (SELECT 1 FROM revisions r WHERE r.doc_id = d.id)
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```
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Use `EXISTS` to test whether a matching row exists and `IN` to test membership
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in a set of values. The optimizer may rewrite either into a join; inspect
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`EXPLAIN` before choosing one for performance reasons.
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`NOT IN` and `NOT EXISTS` differ on nulls: `NOT IN` against a set containing a
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null yields no rows, because the comparison is unknown rather than false. Prefer
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`NOT EXISTS` unless you have ruled nulls out.
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## Correlated subqueries {#correlated}
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A subquery referencing a column from the enclosing query, such as `d.id` above,
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is correlated, and is conceptually evaluated per outer row. The planner usually
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rewrites it into a join.
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<Note>
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Check with [`EXPLAIN`](/sql/dql/explain) when a correlated subquery is slow. A plan
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showing a join has been optimized; one showing a repeated scan has not, and
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rewriting it as an explicit [join](/sql/dql/join) is the fix.
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</Note>
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## CTEs as an alternative {#ctes}
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A [CTE](/sql/dql/select#ctes) is a named subquery, and is usually more readable when
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the same subquery appears twice or when nesting gets deep.
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```sql
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WITH active AS (SELECT id FROM categories WHERE active)
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SELECT title FROM documents WHERE category_id IN (SELECT id FROM active)
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```
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