Add (#5409)

Author: jiangzhx

datafusion/expr/src/expr_fn.rs

@@ -248,6 +248,16 @@ pub fn scalar_subquery(subquery: Arc<LogicalPlan>) -> Expr {
     Expr::ScalarSubquery(Subquery { subquery })
 }
 
+/// Create an expression to represent the stddev() aggregate function
+pub fn stddev(expr: Expr) -> Expr {
+    Expr::AggregateFunction(AggregateFunction::new(
+        aggregate_function::AggregateFunction::Stddev,
+        vec![expr],
+        false,
+        None,
+    ))
+}
+
 /// Create a grouping set
 pub fn grouping_set(exprs: Vec<Vec<Expr>>) -> Expr {
     Expr::GroupingSet(GroupingSet::GroupingSets(exprs))