Added docs

docs/en/sql-reference/statements/select/index.md

@@ -21,6 +21,8 @@ SELECT [DISTINCT [ON (column1, column2, ...)]] expr_list
 [WHERE expr]
 [GROUP BY expr_list] [WITH ROLLUP|WITH CUBE] [WITH TOTALS]
 [HAVING expr]
+[WINDOW window_expr_list]
+[QUALIFY expr]
 [ORDER BY expr_list] [WITH FILL] [FROM expr] [TO expr] [STEP expr] [INTERPOLATE [(expr_list)]]
 [LIMIT [offset_value, ]n BY columns]
 [LIMIT [n, ]m] [WITH TIES]
@@ -45,6 +47,7 @@ Specifics of each optional clause are covered in separate sections, which are li
 - [GROUP BY clause](../../../sql-reference/statements/select/group-by.md)
 - [LIMIT BY clause](../../../sql-reference/statements/select/limit-by.md)
 - [HAVING clause](../../../sql-reference/statements/select/having.md)
+- [QUALIFY clause](../../../sql-reference/statements/select/qualify.md)
 - [LIMIT clause](../../../sql-reference/statements/select/limit.md)
 - [OFFSET clause](../../../sql-reference/statements/select/offset.md)
 - [UNION clause](../../../sql-reference/statements/select/union.md)

docs/en/sql-reference/statements/select/qualify.md

@@ -0,0 +1,14 @@
+---
+slug: /en/sql-reference/statements/select/qualify
+sidebar_label: QUALIFY
+---
+
+# QUALIFY Clause
+
+Allows filtering window functions results. It is similar to the [WHERE](../../../sql-reference/statements/select/where.md) clause, but the difference is that `WHERE` is performed before window functions evaluation, while `QUALIFY` is performed after it.
+
+It is possible to reference window functions results from `SELECT` clause in `QUALIFY` clause by their alias. Alternatively, `QUALIFY` clause can filter on results of additional window functions that are not returned in query results.
+
+## Limitations
+
+`QUALIFY` can’t be used if there are no window functions to evaluate. Use `WHERE` instead.