77 lines
2.8 KiB
Markdown
77 lines
2.8 KiB
Markdown
---
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title: Query fields and tags
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seotitle: Query fields and tags in InfluxDB using Flux
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description: >
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Use `filter()` to query data based on fields, tags, or any other column value.
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`filter()` performs operations similar to the `SELECT` statement and the `WHERE`
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clause in InfluxQL and other SQL-like query languages.
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weight: 201
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menu:
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influxdb_v2:
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parent: Query with Flux
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influxdb/v2/tags: [query, select, where]
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related:
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- /flux/v0/stdlib/universe/filter/
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- /influxdb/v2/query-data/flux/conditional-logic/
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- /influxdb/v2/query-data/flux/regular-expressions/
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list_code_example: |
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```js
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from(bucket: "example-bucket")
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|> range(start: -1h)
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|> filter(fn: (r) => r._measurement == "example-measurement" and r.tag == "example-tag")
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|> filter(fn: (r) => r._field == "example-field")
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```
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---
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Use [`filter()`](/flux/v0/stdlib/universe/filter/)
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to query data based on fields, tags, or any other column value.
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`filter()` performs operations similar to the `SELECT` statement and the `WHERE`
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clause in InfluxQL and other SQL-like query languages.
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## The filter() function
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`filter()` has an `fn` parameter that expects a [predicate function](/influxdb/v2/reference/glossary/#predicate-function),
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an anonymous function comprised of one or more [predicate expressions](/influxdb/v2/reference/glossary/#predicate-expression).
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The predicate function evaluates each input row.
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Rows that evaluate to `true` are **included** in the output data.
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Rows that evaluate to `false` are **excluded** from the output data.
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```js
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// ...
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|> filter(fn: (r) => r._measurement == "example-measurement-name" )
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```
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The `fn` predicate function requires an `r` argument, which represents each row
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as `filter()` iterates over input data.
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Key-value pairs in the row record represent columns and their values.
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Use [dot notation or bracket notation](/flux/v0/data-types/composite/record/#reference-values-in-a-record)
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to reference specific column values in the predicate function.
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Use [logical operators](/flux/v0/spec/operators/#logical-operators)
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to chain multiple predicate expressions together.
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```js
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// Row record
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r = {foo: "bar", baz: "quz"}
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// Example predicate function
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(r) => r.foo == "bar" and r["baz"] == "quz"
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// Evaluation results
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(r) => true and true
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```
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## Filter by fields and tags
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The combination of [`from()`](/flux/v0/stdlib/influxdata/influxdb/from),
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[`range()`](/flux/v0/stdlib/universe/range),
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and `filter()` represent the most basic Flux query:
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1. Use `from()` to define your [bucket](/influxdb/v2/reference/glossary/#bucket).
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2. Use `range()` to limit query results by time.
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3. Use `filter()` to identify what rows of data to output.
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```js
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from(bucket: "example-bucket")
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|> range(start: -1h)
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|> filter(fn: (r) => r._measurement == "example-measurement-name" and r.mytagname == "example-tag-value")
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|> filter(fn: (r) => r._field == "example-field-name")
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```
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