fix: Fix invalid MSSQL SQL when DISTINCT and FETCH are combined

prqlc/prqlc/src/sql/gen_query.rs

@@ -218,10 +218,19 @@ fn translate_select_pipeline(
             })
         }
         if order_by.is_empty() {
+            // When DISTINCT is used, MSSQL requires ORDER BY items to appear
+            // in the SELECT list. Use the first column from the projection
+            // instead of (SELECT NULL).
+            let order_expr = is_distinct
+                .then(|| first_expr_from_projection(&projection))
+                .flatten()
+                .unwrap_or_else(|| {
+                    sql_ast::Expr::Value(
+                        sql_ast::Value::Placeholder("(SELECT NULL)".to_string()).into(),
+                    )
+                });
             order_by.push(sql_ast::OrderByExpr {
-                expr: sql_ast::Expr::Value(
-                    sql_ast::Value::Placeholder("(SELECT NULL)".to_string()).into(),
-                ),
+                expr: order_expr,
                 options: sqlparser::ast::OrderByOptions {
                     asc: None,
                     nulls_first: None,
@@ -726,6 +735,22 @@ fn count_tables(transforms: &[Transform]) -> usize {
 
     count
 }
+
+/// Extract the first expression from a projection for use in ORDER BY.
+/// Returns None if the projection is empty or only contains wildcards.
+fn first_expr_from_projection(projection: &[SelectItem]) -> Option<sql_ast::Expr> {
+    for item in projection {
+        match item {
+            SelectItem::UnnamedExpr(expr) => return Some(expr.clone()),
+            SelectItem::ExprWithAlias { alias, .. } => {
+                return Some(sql_ast::Expr::Identifier(alias.clone()));
+            }
+            SelectItem::Wildcard(_) | SelectItem::QualifiedWildcard(_, _) => continue,
+        }
+    }
+    None
+}
+
 #[cfg(test)]
 mod test {
     use insta::assert_snapshot;

prqlc/prqlc/tests/integration/sql.rs

@@ -2365,6 +2365,29 @@ fn test_take_mssql() {
     ");
 }
 
+#[test]
+fn test_mssql_distinct_fetch() {
+    // Issue #5628: MSSQL requires ORDER BY items to appear in SELECT list when DISTINCT is used.
+    // Using (SELECT NULL) for ORDER BY with DISTINCT is invalid in MSSQL.
+    assert_snapshot!((compile(r#"
+    prql target:sql.mssql
+
+    from t
+    take 100
+    group {this.`District`} (take 1)
+    select {this.`District`}
+    "#).unwrap()), @r###"
+    SELECT
+      DISTINCT "District"
+    FROM
+      t
+    ORDER BY
+      "District" OFFSET 0 ROWS
+    FETCH FIRST
+      100 ROWS ONLY
+    "###);
+}
+
 #[test]
 fn test_distinct_01() {
     // window functions cannot materialize into where statement: CTE is needed