alamb commented on code in PR #10260:
URL: https://github.com/apache/datafusion/pull/10260#discussion_r1583694523
##########
datafusion/expr/src/expr_fn.rs:
##########
@@ -215,6 +215,18 @@ pub fn count(expr: Expr) -> Expr {
))
}
+/// Create an expression to represent the COUNT(IS NULL x) aggregate function
Review Comment:
I found this confusing as first because it isn't `COUNT(x IS NULL)` -- I
think a more accurate description is `COUNT(x) FILTER (WHERE x IS NULL)` or
something like that
```suggestion
/// Create an aggregate that returns the number `NULL` values in a column
```
##########
datafusion/core/src/dataframe/mod.rs:
##########
@@ -534,7 +534,7 @@ impl DataFrame {
vec![],
original_schema_fields
.clone()
- .map(|f| count(is_null(col(f.name()))).alias(f.name()))
+ .map(|f| count_null(col(f.name())).alias(f.name()))
Review Comment:
Got it. Make sense to me. Another classic trick is to use a CASE expression,
something like
```sql
SELECT SUM(CASE WHEN x IS NULL THEN 1 ELSE 0 END) as null_count ...
```
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