使用 AQL 变量，例如用于计数（LET sum = sum + 1）

Question

根据https://www.arangodb.com/2014/07/13/arangodb-2-2-0-released，应该可以使用这样的语句：

LET sum = 0
FOR v IN values
  SORT v.year
  LET sum = sum + v.value
  RETURN { year: v.year, value: v.value, sum: sum }

我目前使用的是 2.4 版，但我无法使用它，例如在这样的声明中：

LET sum = 0
FOR i in memoryColl
    LET sum = sum + 1
    // sum = sum + 1
    RETURN { "i": i, "sum": sum }

我收到错误 [1511] 变量 'sum' 被赋值多次（解析时）

有人能告诉我这样的声明原则上是否有效，具体如何？

Answer 1

如 2.3 的 upgrading docs 中所述，无法再更新查询中的变量：

Previous versions of ArangoDB allowed the modification of variables inside AQL queries [...]

While this is admittedly a convenient feature, the new query optimizer design did not allow to keep it.

Additionally, updating variables inside a query would prevent a lot of optimizations to queries that we would like the optimizer to make. Additionally, updating variables in queries that run on different nodes in a cluster would like cause non-deterministic behavior because queries are not executed linearly.

要枚举文档，您可以这样做

LET range = 0..LENGTH(memoryColl)-1

FOR i IN range
    RETURN {i: i+1, doc: memoryColl[i]}

但我觉得这真是个坏主意。最好return文档，让客户列举。

如果你真的想统计文档的数量，你可以使用子查询：

LET result = (
    FOR doc IN memoryColl
        FILTER True // add some condition here for instance
        RETURN doc
)

RETURN LENGTH(result)

在2.4中，还可以更高效地计数：
http://jsteemann.github.io/blog/2014/12/12/aql-improvements-for-24/

Answer 2

在 2020 年的 arango 3.7 上，您可以执行描述的操作 here

LET values = [
    { year: 2019, value: 35 },
    { year: 2017, value: 8 },
    { year: 2018, value: 17 },
    { year: 2020, value: 84 }
]

LET sortedValues = (FOR v IN values SORT v.year RETURN v)
FOR i IN 0..LENGTH(sortedValues)-1
    LET v = sortedValues[i]
    LET sum = sortedValues[i].value + SUM(SLICE(sortedValues, 0, i)[*].value)
    RETURN {year:v.year,value:v.value,sum:sum}

这回了

[
  {
    "year": 2017,
    "value": 8,
    "sum": 8
  },
  {
    "year": 2018,
    "value": 17,
    "sum": 25
  },
  {
    "year": 2019,
    "value": 35,
    "sum": 60
  },
  {
    "year": 2020,
    "value": 84,
    "sum": 144
  }
]

使用 AQL 变量，例如用于计数（LET sum = sum + 1）

Use AQL Variables e.g. for counting (LET sum = sum + 1)

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