Merge pull request #5103 from influxdata/jgm-null-tags

chore: explain null tags in primary keys

content/influxdb/cloud-serverless/get-started/_index.md

@@ -55,10 +55,13 @@ tags and fields.
       - **Tags**: Key-value pairs that provide metadata for each point--for example,
         something to identify the source or context of the data like host,
         location, station, etc.
+        Tag values may be null.
       - **Fields**: Key-value pairs with values that change over time--for example,
         temperature, pressure, stock price, etc.
+        Field values may be null, but at least one field value is not null on any given row.
       - **Timestamp**: Timestamp associated with the data.
         When stored on disk and queried, all data is ordered by time.
+        A timestamp is never null.
 
 <!-- _For detailed information and examples of the InfluxDB data model, see
 [Data elements](/influxdb/v2.5/reference/key-concepts/data-elements/)._ -->
@@ -78,6 +81,7 @@ The following definitions are important to understand when using InfluxDB:
 - **Series**: A group of points with the same _measurement, tag keys and values, and field key_.
 - **Primary key**: Columns used to uniquely identify each row in a table.
   Rows are uniquely identified by their _timestamp and tag set_.
+  A row's primary key _tag set_ does not include tags with null values.
 
 ##### Example InfluxDB query results
 

content/influxdb/cloud-serverless/reference/glossary.md

@@ -586,6 +586,7 @@ The local server's nanosecond timestamp.
 
 A data type that represents a missing or unknown value.
 Denoted by the `null` value.
+Values of [tags](#tag) and [fields](#field) may be `null`, but timestamp values are never `null`.
 
 ## O
 
@@ -667,6 +668,7 @@ Related entries:
 With the InfluxDB IOx storage engine, the primary key is the list of columns
 used to uniquely identify each row in a table.
 Rows are uniquely identified by their timestamp and tag set.
+A row's primary key tag set does not include tags with null values.
 
 ### precision
 
@@ -959,6 +961,7 @@ The collection of tag keys and tag values on a point.
 
 Related entries:
 [point](#point),
+[primary key](#primary-key),
 [series](#series),
 [tag](#tag),
 [tag key](#tag-key),

content/influxdb/cloud-serverless/write-data/best-practices/schema-design.md

@@ -47,16 +47,20 @@ tags and fields.
       - **Tags**: Key-value pairs that store metadata string values for each point--for example,
         a value that identifies or differentiates the data source or context--for example, host,
         location, station, etc.
+        Tag values may be null.
       - **Fields**: Key-value pairs that store data for each point--for example,
         temperature, pressure, stock price, etc.
+        Field values may be null, but at least one field value is not null on any given row.
       - **Timestamp**: Timestamp associated with the data.
         When stored on disk and queried, all data is ordered by time.
         In InfluxDB, a timestamp is a nanosecond-scale [unix timestamp](#unix-timestamp) in UTC.
+        A timestamp is never null.
 
 ### Primary keys
 
 In time series data, the primary key for a row of data is typically a combination of timestamp and other attributes that uniquely identify each data point.
 In InfluxDB, the primary key for a row is the combination of the point's timestamp and _tag set_ - the collection of [tag keys](/influxdb/cloud-serverless/reference/glossary/#tag-key) and [tag values](/influxdb/cloud-serverless/reference/glossary/#tag-value) on the point.
+A row's primary key _tag set_ does not include tags with null values.
 
 ### Tags versus fields