Implement DISTINCT clause

query_executors/memory_executor/memory_executor.go

@@ -35,6 +35,18 @@ func Execute(query parsers.SelectStmt, data []RowType) []RowType {
 		ctx.orderBy(query.OrderExpressions, result)
 	}
 
+	// Apply select
+	selectedData := make([]RowType, 0)
+	for _, row := range result {
+		selectedData = append(selectedData, ctx.selectRow(query.SelectItems, row))
+	}
+	result = selectedData
+
+	// Apply distinct
+	if query.Distinct {
+		result = deduplicate(result)
+	}
+
 	// Apply result limit
 	if query.Count > 0 {
 		count := func() int {
@@ -46,13 +58,7 @@ func Execute(query parsers.SelectStmt, data []RowType) []RowType {
 		result = result[:count]
 	}
 
-	// Apply select
-	selectedData := make([]RowType, 0)
-	for _, row := range result {
-		selectedData = append(selectedData, ctx.selectRow(query.SelectItems, row))
-	}
-
-	return selectedData
+	return result
 }
 
 func (c memoryExecutorContext) selectRow(selectItems []parsers.SelectItem, row RowType) interface{} {
@@ -349,3 +355,23 @@ func compareValues(val1, val2 interface{}) int {
 		return 1
 	}
 }
+
+func deduplicate(slice []RowType) []RowType {
+	var result []RowType
+
+	for i := 0; i < len(slice); i++ {
+		unique := true
+		for j := 0; j < len(result); j++ {
+			if compareValues(slice[i], result[j]) == 0 {
+				unique = false
+				break
+			}
+		}
+
+		if unique {
+			result = append(result, slice[i])
+		}
+	}
+
+	return result
+}

query_executors/memory_executor/select_test.go

@@ -35,6 +35,24 @@ func Test_Execute_Select(t *testing.T) {
 		)
 	})
 
+	t.Run("Should execute SELECT DISTINCT", func(t *testing.T) {
+		testQueryExecute(
+			t,
+			parsers.SelectStmt{
+				SelectItems: []parsers.SelectItem{
+					{Path: []string{"c", "pk"}},
+				},
+				Table:    parsers.Table{Value: "c"},
+				Distinct: true,
+			},
+			mockData,
+			[]memoryexecutor.RowType{
+				map[string]interface{}{"pk": 123},
+				map[string]interface{}{"pk": 456},
+			},
+		)
+	})
+
 	t.Run("Should execute SELECT TOP", func(t *testing.T) {
 		testQueryExecute(
 			t,