lancedb/docs/web/sql/dql/subqueries.mdx

---
title: "Subqueries"
sidebarTitle: "Subqueries"
description: "Queries inside queries: scalar, IN, EXISTS, correlated, and derived tables."
icon: code-branch
keywords: ["subquery", "exists", "in", "correlated", "derived table", "lateral"]
---

<Badge color="red">Enterprise</Badge> [SQL runs on a deployment.](/sql)

A subquery is allowed anywhere an expression or a relation is.

## As a relation {#derived}

```sql
SELECT * FROM (
    SELECT category, count(*) AS n FROM documents GROUP BY category
) AS counts
WHERE n > 100
```

A derived table needs an alias. It can be [sampled](/sql/dql/sampling) like a base
table:

```sql
SELECT * FROM (SELECT * FROM documents) d TABLESAMPLE (10)
```

## As a scalar {#scalar}

```sql
SELECT title, (SELECT count(*) FROM revisions r WHERE r.doc_id = d.id) AS revisions
FROM documents AS d
```

A scalar subquery must return one row and one column. Returning more is an
error rather than a silently-taken first row.

## IN and EXISTS {#in-exists}

```sql
SELECT title FROM documents
WHERE category_id IN (SELECT id FROM categories WHERE active)

SELECT title FROM documents AS d
WHERE EXISTS (SELECT 1 FROM revisions r WHERE r.doc_id = d.id)
```

`EXISTS` stops at the first matching row, so it is the cheaper test when you
only care whether anything matches. `IN` builds the full set.

`NOT IN` and `NOT EXISTS` differ on nulls: `NOT IN` against a set containing a
null yields no rows, because the comparison is unknown rather than false. Prefer
`NOT EXISTS` unless you have ruled nulls out.

## Correlated subqueries {#correlated}

A subquery referencing a column from the enclosing query — `d.id` above — is
correlated, and is conceptually evaluated per outer row. The planner usually
rewrites it into a join.

<Note>
Check with [`EXPLAIN`](/sql/dql/explain) when a correlated subquery is slow. A plan
showing a join has been optimized; one showing a repeated scan has not, and
rewriting it as an explicit [join](/sql/dql/join) is the fix.
</Note>

## CTEs as an alternative {#ctes}

A [CTE](/sql/dql/select#ctes) is a named subquery, and is usually more readable when
the same subquery appears twice or when nesting gets deep.

```sql
WITH active AS (SELECT id FROM categories WHERE active)
SELECT title FROM documents WHERE category_id IN (SELECT id FROM active)
```