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Jack Ye
52684d50b9
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.
72 lines
2.2 KiB
Plaintext
72 lines
2.2 KiB
Plaintext
---
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title: "Joins"
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sidebarTitle: "Joins"
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description: "Combining rows from more than one table, including across namespaces."
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icon: link
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keywords: ["join", "inner", "left", "outer", "cross", "namespace"]
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---
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<Badge color="red">Enterprise</Badge> [SQL runs on a deployment.](/sql)
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Joining is the other thing the query builders cannot do. A question spanning two
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tables is a SQL question.
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## The join types {#types}
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```sql
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SELECT d.title, c.name
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FROM documents AS d
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JOIN categories AS c ON c.id = d.category_id
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```
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| | What it keeps |
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| --- | --- |
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| `[INNER] JOIN` | rows matching on both sides |
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| `LEFT [OUTER] JOIN` | every left row, nulls where the right has no match |
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| `RIGHT [OUTER] JOIN` | every right row |
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| `FULL [OUTER] JOIN` | every row from both |
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| `CROSS JOIN` | every combination -- the product of the two |
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`USING (col)` is shorthand for an equality join on a shared column name, and
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avoids repeating it in the select list.
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## Across namespaces {#namespaces}
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Tables in different [namespaces](/management/namespace) join by their dotted paths -- the
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catalog is one catalog, so there is nothing to federate:
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```sql
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SELECT c.id, u.email
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FROM prod.events.clicks AS c
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JOIN prod.reference.users AS u ON u.id = c.user_id
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```
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## Joining a search result {#search}
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[`fts()`](/sql/functions#fts) is a table function, so its result is a relation
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and joins like any other:
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```sql
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SELECT c.name, count(*) AS hits
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FROM fts('my_docs', 'puppy') AS m
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JOIN categories AS c ON c.id = m.category_id
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GROUP BY c.name
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ORDER BY hits DESC
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```
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That composition is the reason full-text search is a relation rather than a
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predicate -- a `WHERE` clause has nowhere to put the relevance score.
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## Performance {#performance}
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A join key with a [scalar index](/indexing/scalar-index) on it lets the planner
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look rows up rather than scan for them. Check with
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[`explain_plan`](/search/explain): an indexed key shows a `ScalarIndexQuery`,
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an unindexed one shows a `LanceScan` feeding the join.
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<Warning>
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Avoid selecting blob or vector columns you do not need on either side of a join.
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The join multiplies rows before projection is applied in some plans, so a wide
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column can be materialized many more times than the result has rows.
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</Warning>
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