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Jack Ye
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OSS docs now own the full usage content for features that only run against an Enterprise deployment, since the client interface itself lives here; Enterprise docs keep only the parts specific to its own behavior. Sophon's overlay for these five pages is dropped since the OSS versions are now the complete reference. SQL Reference is restructured to match an object-type SQL manual (DDL by object, then DML, then DQL, then Expressions) instead of the old flat, loosely-ordered page list, absorbing the SQL client-connection guide and FTS-via-SQL examples into it.
77 lines
2.3 KiB
Plaintext
77 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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---
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<Badge color="red">Enterprise</Badge> [SQL runs on a deployment.](/sql)
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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. It can be [sampled](/sql/dql/sampling) like a base
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table:
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```sql
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SELECT * FROM (SELECT * FROM documents) d TABLESAMPLE (10)
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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 must return one row and one column. Returning more is an
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error rather than a silently-taken first row.
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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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`EXISTS` stops at the first matching row, so it is the cheaper test when you
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only care whether anything matches. `IN` builds the full set.
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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 — `d.id` above — is
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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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