Merge pull request #478 from influxdata/guide/exists-operator

Added guide for using the 'exists' operator

content/v2.0/query-data/guides/exists.md

@@ -0,0 +1,69 @@
+---
+title: Check if a value exists
+seotitle: Use Flux to check if a value exists
+description: >
+  Use the Flux `exists` operator to check if an object contains a key or if that
+  key's value is `null`.
+v2.0/tags: [exists]
+menu:
+  v2_0:
+    name: Check if a value exists
+    parent: How-to guides
+weight: 209
+---
+
+Use the Flux `exists` operator to check if an object contains a key or if that
+key's value is `null`.
+
+```js
+p = {firstName: "John", lastName: "Doe", age: 42}
+
+exists p.firstName
+// Returns true
+
+exists p.height
+// Returns false
+```
+
+Use `exists` with row functions (
+[`filter()`](/v2.0/reference/flux/stdlib/built-in/transformations/filter/),
+[`map()`](/v2.0/reference/flux/stdlib/built-in/transformations/map/),
+[`reduce()`](/v2.0/reference/flux/stdlib/built-in/transformations/aggregates/reduce/))
+to check if a row includes a column or if the value for that column is `null`.
+
+#### Filter out null values
+```js
+from(bucket: "example-bucket")
+  |> range(start: -5m)
+  |> filter(fn: (r) => exists r._value)
+```
+
+#### Map values based on existence
+```js
+from(bucket: "default")
+  |> range(start: -30s)
+  |> map(fn: (r) => ({
+      r with
+      human_readable:
+        if exists r._value then "${r._field} is ${string(v:r._value)}."
+        else "${r._field} has no value."
+  }))
+```
+
+#### Ignore null values in a custom aggregate function
+```js
+customSumProduct = (tables=<-) =>
+  tables
+    |> reduce(
+      identity: {sum: 0.0, product: 1.0},
+      fn: (r, accumulator) => ({
+        r with
+        sum:
+          if exists r._value then r._value + accumulator.sum
+          else accumulator.sum,
+        product:
+          if exists r._value then r.value * accumulator.product
+          else accumulator.product
+      })
+    )
+```

content/v2.0/reference/flux/stdlib/built-in/transformations/aggregates/reduce.md

@@ -10,6 +10,7 @@ menu:
     name: reduce
     parent: built-in-aggregates
 weight: 501
+v2.0/tags: [exists]
 ---
 
 The `reduce()` function aggregates records in each table according to the reducer,
@@ -96,7 +97,7 @@ creates a new column if it doesn't exist, and includes all existing columns in
 the output table.
 
 ```js
-recduce(fn: (r) => ({ r with newColumn: r._value * 2 }))
+reduce(fn: (r) => ({ r with newColumn: r._value * 2 }))
 ```
 
 

content/v2.0/reference/flux/stdlib/built-in/transformations/filter.md

@@ -9,6 +9,7 @@ menu:
     name: filter
     parent: built-in-transformations
 weight: 401
+v2.0/tags: [exists]
 ---
 
 The `filter()` function filters data based on conditions defined in a predicate function ([`fn`](#fn)).
@@ -42,6 +43,7 @@ Objects evaluated in `fn` functions are represented by `r`, short for "record" o
 
 ## Examples
 
+##### Filter based on measurement, field, and tag
 ```js
 from(bucket:"example-bucket")
   |> range(start:-1h)
@@ -52,6 +54,20 @@ from(bucket:"example-bucket")
   )
 ```
 
+##### Filter out null values
+```js
+from(bucket:"example-bucket")
+  |> range(start:-1h)
+  |> filter(fn: (r) => exists r._value )
+```
+
+##### Filter values based on thresholds
+```js
+from(bucket:"example-bucket")
+  |> range(start:-1h)
+  |> filter(fn: (r) => r._value > 50.0 and r._value < 65.0 )
+```
+
 <hr style="margin-top:4rem"/>
 
 ##### Related InfluxQL functions and statements:

content/v2.0/reference/flux/stdlib/built-in/transformations/map.md

@@ -9,6 +9,7 @@ menu:
     name: map
     parent: built-in-transformations
 weight: 401
+v2.0/tags: [exists]
 ---
 
 The `map()` function applies a function to each record in the input tables.