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Jack Ye
5b5ed930ac
Every group now answers exactly one question. Get started and Concepts cover what the system is; the verb groups cover doing something with it; Operate, Secure and Deploy cover running it; SQL reference is lookup. Maintenance becomes Operate, Governance becomes Secure, and the rest take verbs to match. Three new pages in Concepts and Get started: Guarantees states what a write promises when it returns, which version a reader sees, what survives a failure, and -- the half usually missing -- what is not guaranteed: no multi-table transactions, no read-your-writes across processes without an interval, unenforced primary keys that do not enforce, no background compaction. Limits collects bounds that were stated in passing across a dozen pages, with the numbers verified against the code rather than inferred: name lengths, job event caps, SHOW ERRORS bounds, the sampling range that is refused rather than clamped. Architecture illustrates embedded against distributed as the one deployment decision, and is written to cover both rather than only Enterprise. Indexing splits by question. Choosing an index is a Query decision and moves there; building, repairing and GPU-building it stay in Operate. SQL becomes a top-level reference of fifteen pages instead of a sub-group of Query. Auditing sophon-sql turned up a large undocumented surface: ALTER TABLE ADD COLUMN in its plain, computed and function forms with FILTER, IGNORE NULLS and OVER modifiers; SET UNENFORCED PRIMARY KEY; SET and UNSET WAL with sharding; CREATE FUNCTION with its dependency options; REFRESH COLUMN and REFRESH MATERIALIZED VIEW; and the SHOW family. None of it was written down.
77 lines
2.2 KiB
Plaintext
77 lines
2.2 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/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/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/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/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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