Update documentation of Window function and order by

crates/gitql-engine/src/engine_executor.rs

@@ -296,12 +296,6 @@ fn execute_where_statement(
         return Ok(());
     }
 
-    if gitql_object.len() > 1 {
-        gitql_object.flat()
-    }
-
-    // Perform where command only on the first group
-    // because group by command not executed yet
     apply_filter_operation(
         env,
         &statement.condition,

docs/expression/call.md

@@ -0,0 +1,36 @@
+### Call expression
+
+## 1. Standard Function Calls
+
+Standard functions call operate on individual rows and return a single value per row, with syntax similar to most programming languages for example
+
+```sql
+LEN(name)
+LOWER(author_name)
+```
+
+## 2. Aggregate Function Calls
+
+Aggregate functions call has the same syntax like standard function call but it's operate on a set of rows (a group) and return a single value for the entire group. They are often used with the `GROUP BY` clause, the value of aggregation function can be used only after group by statement.
+
+```sql
+SELECT author_name, COUNT(author_name) AS commit_num FROM commits GROUP BY author_name, author_email ORDER BY commit_num DESC LIMIT 10
+```
+
+## 3. Window functions
+
+Window functions perform calculations across a set of rows that are related to the current row. Unlike aggregate functions with GROUP BY, window functions do not collapse rows into a single output row. Instead, they return a value for each input row based on a "window" of related rows,
+in window function call you must to explicit define `OVER` clauses even if it empty, also you can use aggregation function as window function.
+
+```sql
+SELECT emp_name, dep_name, ROW_NUMBER() OVER(PARTITION BY dep_name) AS row_number FROM emp_salaries
+
+SELECT emp_name,
+       dep_name,
+       ROW_NUMBER() OVER partition_dep_order_salary_des AS row_number_per_department,
+       MIN(salary) OVER partition_dep_order_salary_des AS min_salary_per_department,
+       MAX(salary) OVER partition_dep_order_salary_des AS max_salary_per_department
+FROM emp_salaries
+WINDOW partition_dep_order_salary_des AS (PARTITION BY dep_name ORDER BY salary DESC)
+ORDER BY dep_name ASC NULLS LAST;
+```

docs/expression/index.md

@@ -5,4 +5,5 @@ GitQL Expressions
 - [Case expression](case.md).
 - [Cast expression](cast.md)
 - [Array expression](array.md).
-- [Access Member](access.md).
+- [Access Member](access.md).
+- [Call expression](call.md)

docs/statement/order_by.md

@@ -13,4 +13,11 @@ The `ORDER BY` Statement with `USING <operator>` syntax inspired by PostgreSQL
 ```sql
 SELECT author_name, author_email FROM commits ORDER BY author_email, commit_id USING <
 SELECT author_name, author_email FROM commits ORDER BY author_name USING >
+```
+
+You can define nulls order policy to set if you want null value to be first or last in the order
+
+```sql
+SELECT author_name, author_email FROM commits ORDER BY author_email NULLS FIRST
+SELECT author_name, author_email FROM commits ORDER BY author_name NULLS LAST
 ```