Fix GROUP_CONCAT function signature (#1294)

* docs: Add environment variables

* fix(mysql): Fix the function signature for GROUP_CONCAT

Author: kyleconroy

internal/compiler/output_columns.go

@@ -210,7 +210,7 @@ func outputColumns(qc *QueryCatalog, node ast.Node) ([]*Column, error) {
 			}
 			fun, err := qc.catalog.ResolveFuncCall(n)
 			if err == nil {
-				cols = append(cols, &Column{Name: name, DataType: dataType(fun.ReturnType), NotNull: true})
+				cols = append(cols, &Column{Name: name, DataType: dataType(fun.ReturnType), NotNull: !fun.ReturnTypeNullable})
 			} else {
 				cols = append(cols, &Column{Name: name, DataType: "any"})
 			}

internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/go/db.go

@@ -0,0 +1,15 @@
+CREATE TABLE student (
+    student_name VARCHAR(255),
+    score DOUBLE
+);
+
+-- name: GroupConcat :many
+SELECT student_name, GROUP_CONCAT(test_score)
+FROM student
+GROUP BY student_name;
+
+-- name: GroupConcatOrderBy :many
+SELECT student_name,
+    GROUP_CONCAT(DISTINCT test_score ORDER BY test_score DESC SEPARATOR ' ')
+FROM student
+GROUP BY student_name;

internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/go/group_concat.sql.go

@@ -7,6 +7,13 @@
       "schema": "date_and_time_functions",
       "queries": "date_and_time_functions",
       "engine": "mysql"
+    },
+    {
+      "name": "aggregate_functions",
+      "path": "aggregate_functions/go",
+      "schema": "aggregate_functions",
+      "queries": "aggregate_functions",
+      "engine": "mysql"
     }
   ]
 }

internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/go/models.go

@@ -1069,8 +1069,13 @@ func defaultSchema(name string) *catalog.Schema {
 				{
 					Type: &ast.TypeName{Name: "any"},
 				},
+				{
+					Type: &ast.TypeName{Name: "any"},
+					Mode: ast.FuncParamVariadic,
+				},
 			},
-			ReturnType: &ast.TypeName{Name: "any"},
+			ReturnType:         &ast.TypeName{Name: "text"},
+			ReturnTypeNullable: true,
 		},
 		{
 			Name: "GTID_SUBSET",

internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/group_concat.sql

@@ -228,11 +228,12 @@ func (ct *CompositeType) SetComment(c string) {
 }
 
 type Function struct {
-	Name       string
-	Args       []*Argument
-	ReturnType *ast.TypeName
-	Comment    string
-	Desc       string
+	Name               string
+	Args               []*Argument
+	ReturnType         *ast.TypeName
+	Comment            string
+	Desc               string
+	ReturnTypeNullable bool
 }
 
 func (f *Function) InArgs() []*Argument {