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docs: explain missing properties in SQL comparisons #21877

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11 changes: 11 additions & 0 deletions contents/docs/sql/expressions.mdx
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Expand Up @@ -71,6 +71,17 @@ Expressions can use operators to filter and combine data. These include:
- Logical operators like `AND`, `OR`, `IS` or `NOT`
- Arithmetic operators like `+`, `-`, `*`, `/`

HogQL treats missing properties as `NULL`. A filter such as `properties.plan != ''` also matches events where `plan` is missing.

To keep only events with a non-empty `plan` string, check for both `NULL` and an empty string:

```sql
SELECT event
FROM events
WHERE properties.plan IS NOT NULL
AND properties.plan != ''
```

## Functions and aggregations

You can filter, modify, or aggregate accessed data with [supported ClickHouse functions](/docs/sql/clickhouse-functions) like `dateDiff()` and `concat()` and [aggregations](/docs/hogql/aggregations) like `sumIf()` and `count()`.
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