wu-sheng commented on code in PR #12730:
URL: https://github.com/apache/skywalking/pull/12730#discussion_r1822086084
##########
docs/en/api/metrics-query-expression.md:
##########
@@ -237,7 +237,12 @@ top_n(<metric_name>, <top_number>, <order>, <attrs>)
- `attrs` optional, attrs is the attributes of the metrics, could be used to
filter the topN results.
SkyWalking supports 6 attrs: `attr0`, `attr1`, `attr2`, `attr3`, `attr4`,
`attr5`.
The format is `attr0='value', attr1='value'...attr5='value5'`, could use
one or multiple attrs to filter the topN results.
- **Notice**: The `attrs` only support Service metrics for now and should be
added in the metrics first, see [Metrics Additional
Attributes](../concepts-and-designs/metrics-additional-attributes.md).
+ The attrs filter also supports not equal filter `!=`, the format is
`attr0='value'`.
+
+**Notice**:
+- The `attrs` only support Service metrics for now and should be added in the
metrics first, see [Metrics Additional
Attributes](../concepts-and-designs/metrics-additional-attributes.md).
+- When use not equal filter, for example `attr1!='value'`, if the storage is
using `MySQL` or other JDBC storage and `attr1 value not exists` in the
metrics,
+the result of `attr1!='value'` will always `false` due to SQL can't compare
`NULL`.
Review Comment:
```suggestion
the result of `attr1!='value'` will always `false` and would NOT include
this metric in the result due to SQL can't compare `NULL` with the `value`.
```
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