mirror of https://gitee.com/bigwinds/arangodb
65 lines
1.2 KiB
Markdown
65 lines
1.2 KiB
Markdown
---
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layout: default
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description: Wherever an expression is allowed in AQL, a subquery can be placed
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---
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Combining queries
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=================
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Subqueries
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----------
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Wherever an expression is allowed in AQL, a subquery can be placed. A subquery
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is a query part that can introduce its own local variables without affecting
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variables and values in its outer scope(s).
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It is required that subqueries be put inside parentheses *(* and *)* to
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explicitly mark their start and end points:
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```js
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FOR p IN persons
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LET recommendations = (
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FOR r IN recommendations
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FILTER p.id == r.personId
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SORT p.rank DESC
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LIMIT 10
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RETURN r
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)
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RETURN { person : p, recommendations : recommendations }
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```
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```js
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FOR p IN persons
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COLLECT city = p.city INTO g
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RETURN {
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city : city,
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numPersons : LENGTH(g),
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maxRating: MAX(
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FOR r IN g
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RETURN r.p.rating
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)}
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```
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Subqueries may also include other subqueries.
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Note that subqueries always return a result **array**, even if there is only
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a single return value:
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```js
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RETURN ( RETURN 1 )
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```
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```json
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[ [ 1 ] ]
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```
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To avoid such a nested data structure, [FIRST()](functions-array.html#first)
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can be used for example:
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```js
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RETURN FIRST( RETURN 1 )
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```
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```json
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[ 1 ]
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```
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