mirror of https://gitee.com/bigwinds/arangodb
47 lines
1.5 KiB
Plaintext
47 lines
1.5 KiB
Plaintext
!CHAPTER Advanced features
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!SUBSECTION Subqueries
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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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FOR u IN users
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LET recommendations = (
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FOR r IN recommendations
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FILTER u.id == r.userId
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SORT u.rank DESC
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LIMIT 10
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RETURN r
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)
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RETURN { "user" : u, "recommendations" : recommendations }
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FOR u IN users
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COLLECT city = u.city INTO g
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RETURN { "city" : city, "numUsers" : LENGTH(g), "maxRating": MAX(
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FOR r IN g
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RETURN r.user.rating
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) }
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Subqueries might also include other subqueries themselves.
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!SUBSECTION Variable expansion
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In order to access a named attribute from all elements in a list easily, AQL
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offers the shortcut operator `[*]` for variable expansion.
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Using the `[*]` operator with a variable will iterate over all elements in the
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variable thus allowing to access a particular attribute of each element. It is
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required that the expanded variable is a list. The result of the `[*]`
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operator is again a list.
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FOR u IN users
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RETURN { "user" : u, "friendNames" : u.friends[*].name }
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In the above example, the attribute `name` is accessed for each element in the
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list `u.friends`. The result is a flat list of friend names, made available as
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the attribute `friendNames`. |