Implement select_columns

minisql/src/main.rs

@@ -136,8 +136,17 @@ type Rows =
     // insert(id, value)
 
 fn select_columns(row: &Row, columns: &Vec<ColumnPosition>) -> Row {
-    // row.column_position
-    todo!()
+    // If the index from `columns` is non-existant in `row`, it will just ignore it.
+    let mut subrow: Row = vec![];
+    for column_position in columns {
+        match row.get(*column_position) {
+            Some(value) => {
+                subrow.push(value.clone())
+            },
+            None => {}
+        }
+    }
+    subrow
 }
 
 // ==============Interpreter================