diff --git a/docs/index.rst b/docs/index.rst
index 0386a8175f..af212430ec 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -63,6 +63,7 @@ code ever again.
    reference/datatypes.md
    reference/query-annotations.md
    reference/language-support.rst
+   reference/environment-variables.md
 
 .. toctree::
    :maxdepth: 2
diff --git a/docs/reference/environment-variables.md b/docs/reference/environment-variables.md
new file mode 100644
index 0000000000..359bebcc3f
--- /dev/null
+++ b/docs/reference/environment-variables.md
@@ -0,0 +1,112 @@
+# Environment variables
+
+## SQLCDEBUG
+
+The `SQLCDEBUG` variable controls debugging variables within the runtime. It is
+a comma-separated list of name=val pairs settings.
+
+### dumpast
+
+The `dumpast` command shows the SQL AST that was generated by the parser. Note
+that this is the generic SQL AST, not the engine-specific SQL AST.
+
+```
+SQLCDEBUG=dumpast=1
+```
+
+```
+([]interface {}) (len=1 cap=1) {
+ (*catalog.Catalog)(0xc0004f48c0)({
+  Comment: (string) "",
+  DefaultSchema: (string) (len=6) "public",
+  Name: (string) "",
+  Schemas: ([]*catalog.Schema) (len=3 cap=4) {
+   (*catalog.Schema)(0xc0004f4930)({
+    Name: (string) (len=6) "public",
+    Tables: ([]*catalog.Table) (len=1 cap=1) {
+     (*catalog.Table)(0xc00052ff20)({
+      Rel: (*ast.TableName)(0xc00052fda0)({
+       Catalog: (string) "",
+       Schema: (string) "",
+       Name: (string) (len=7) "authors"
+      }),
+```
+
+### dumpcatalog
+
+The `dumpcatalog` command outputs the entire catalog. If you're using MySQL or
+PostgreSQL, this can be a bit overwhelming. Expect this output to change in
+future versions.
+
+```
+SQLCDEBUG=dumpcatalog=1
+```
+
+```
+([]interface {}) (len=1 cap=1) {
+ (*catalog.Catalog)(0xc00050d1f0)({
+  Comment: (string) "",
+  DefaultSchema: (string) (len=6) "public",
+  Name: (string) "",
+  Schemas: ([]*catalog.Schema) (len=3 cap=4) {
+   (*catalog.Schema)(0xc00050d260)({
+    Name: (string) (len=6) "public",
+    Tables: ([]*catalog.Table) (len=1 cap=1) {
+     (*catalog.Table)(0xc0000c0840)({
+      Rel: (*ast.TableName)(0xc0000c06c0)({
+       Catalog: (string) "",
+       Schema: (string) "",
+       Name: (string) (len=7) "authors"
+      }),
+```
+
+### trace
+
+The `trace` command is helpful for tracking down performance issues.
+
+`SQLCDEBUG=trace=1`
+
+By default, the trace output is written to `trace.out` in the current working
+directory. You can configure a different path if needed.
+
+`SQLCDEBUG=trace=name.out`
+
+View the execution trace using the Go `trace` tool.
+
+```
+go tool trace trace.out
+```
+
+There's a ton of different views for the trace output, but here's an example
+log showing the execution time for each package.
+
+```
+0.000043897 	 .         	1 	task sqlc (id 1, parent 0) created
+0.000144923 	 .   101026 	1 	region generate started (duration: 47.619781ms)
+0.001048975 	 .   904052 	1 	region package started (duration: 14.588456ms)
+0.001054616 	 .     5641 	1 	name=authors dir=/Users/kyle/projects/sqlc/examples/python language=python
+0.001071257 	 .    16641 	1 	region parse started (duration: 7.966549ms)
+0.009043960 	 .  7972703 	1 	region codegen started (duration: 6.587086ms)
+0.009171704 	 .   127744 	1 	new goroutine 35: text/template/parse.lex·dwrap·1
+0.010361654 	 .  1189950 	1 	new goroutine 36: text/template/parse.lex·dwrap·1
+0.015641815 	 .  5280161 	1 	region package started (duration: 10.904938ms)
+0.015644943 	 .     3128 	1 	name=booktest dir=/Users/kyle/projects/sqlc/examples/python language=python
+0.015647431 	 .     2488 	1 	region parse started (duration: 4.207749ms)
+0.019860308 	 .  4212877 	1 	region codegen started (duration: 6.681624ms)
+0.020028488 	 .   168180 	1 	new goroutine 37: text/template/parse.lex·dwrap·1
+0.021020310 	 .   991822 	1 	new goroutine 8: text/template/parse.lex·dwrap·1
+0.026551163 	 .  5530853 	1 	region package started (duration: 9.217294ms)
+0.026554368 	 .     3205 	1 	name=jets dir=/Users/kyle/projects/sqlc/examples/python language=python
+0.026556804 	 .     2436 	1 	region parse started (duration: 3.491005ms)
+0.030051911 	 .  3495107 	1 	region codegen started (duration: 5.711931ms)
+0.030213937 	 .   162026 	1 	new goroutine 20: text/template/parse.lex·dwrap·1
+0.031099938 	 .   886001 	1 	new goroutine 38: text/template/parse.lex·dwrap·1
+0.035772637 	 .  4672699 	1 	region package started (duration: 10.267039ms)
+0.035775688 	 .     3051 	1 	name=ondeck dir=/Users/kyle/projects/sqlc/examples/python language=python
+0.035778150 	 .     2462 	1 	region parse started (duration: 4.094518ms)
+0.039877181 	 .  4099031 	1 	region codegen started (duration: 6.156341ms)
+0.040010771 	 .   133590 	1 	new goroutine 39: text/template/parse.lex·dwrap·1
+0.040894567 	 .   883796 	1 	new goroutine 40: text/template/parse.lex·dwrap·1
+0.046042779 	 .  5148212 	1 	region writefiles started (duration: 1.718259ms)
+0.047767781 	 .  1725002 	1 	task end
+```
diff --git a/internal/compiler/output_columns.go b/internal/compiler/output_columns.go
index 0dbb084549..b6a3ce077e 100644
--- a/internal/compiler/output_columns.go
+++ b/internal/compiler/output_columns.go
@@ -196,7 +196,7 @@ 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"})
 			}
diff --git a/internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/go/db.go b/internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/go/db.go
new file mode 100644
index 0000000000..aafbb25321
--- /dev/null
+++ b/internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/go/db.go
@@ -0,0 +1,29 @@
+// Code generated by sqlc. DO NOT EDIT.
+
+package aggregate_functions
+
+import (
+	"context"
+	"database/sql"
+)
+
+type DBTX interface {
+	ExecContext(context.Context, string, ...interface{}) (sql.Result, error)
+	PrepareContext(context.Context, string) (*sql.Stmt, error)
+	QueryContext(context.Context, string, ...interface{}) (*sql.Rows, error)
+	QueryRowContext(context.Context, string, ...interface{}) *sql.Row
+}
+
+func New(db DBTX) *Queries {
+	return &Queries{db: db}
+}
+
+type Queries struct {
+	db DBTX
+}
+
+func (q *Queries) WithTx(tx *sql.Tx) *Queries {
+	return &Queries{
+		db: tx,
+	}
+}
diff --git a/internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/go/group_concat.sql.go b/internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/go/group_concat.sql.go
new file mode 100644
index 0000000000..6bf897f3ce
--- /dev/null
+++ b/internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/go/group_concat.sql.go
@@ -0,0 +1,78 @@
+// Code generated by sqlc. DO NOT EDIT.
+// source: group_concat.sql
+
+package aggregate_functions
+
+import (
+	"context"
+	"database/sql"
+)
+
+const groupConcat = `-- name: GroupConcat :many
+SELECT student_name, GROUP_CONCAT(test_score)
+FROM student
+GROUP BY student_name
+`
+
+type GroupConcatRow struct {
+	StudentName sql.NullString
+	GroupConcat sql.NullString
+}
+
+func (q *Queries) GroupConcat(ctx context.Context) ([]GroupConcatRow, error) {
+	rows, err := q.db.QueryContext(ctx, groupConcat)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []GroupConcatRow
+	for rows.Next() {
+		var i GroupConcatRow
+		if err := rows.Scan(&i.StudentName, &i.GroupConcat); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Close(); err != nil {
+		return nil, err
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
+const groupConcatOrderBy = `-- name: GroupConcatOrderBy :many
+SELECT student_name,
+    GROUP_CONCAT(DISTINCT test_score ORDER BY test_score DESC SEPARATOR ' ')
+FROM student
+GROUP BY student_name
+`
+
+type GroupConcatOrderByRow struct {
+	StudentName sql.NullString
+	GroupConcat sql.NullString
+}
+
+func (q *Queries) GroupConcatOrderBy(ctx context.Context) ([]GroupConcatOrderByRow, error) {
+	rows, err := q.db.QueryContext(ctx, groupConcatOrderBy)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []GroupConcatOrderByRow
+	for rows.Next() {
+		var i GroupConcatOrderByRow
+		if err := rows.Scan(&i.StudentName, &i.GroupConcat); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Close(); err != nil {
+		return nil, err
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
diff --git a/internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/go/models.go b/internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/go/models.go
new file mode 100644
index 0000000000..358a98b131
--- /dev/null
+++ b/internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/go/models.go
@@ -0,0 +1,12 @@
+// Code generated by sqlc. DO NOT EDIT.
+
+package aggregate_functions
+
+import (
+	"database/sql"
+)
+
+type Student struct {
+	StudentName sql.NullString
+	Score       sql.NullFloat64
+}
diff --git a/internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/group_concat.sql b/internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/group_concat.sql
new file mode 100644
index 0000000000..f1858e42fd
--- /dev/null
+++ b/internal/endtoend/testdata/mysql_reference_manual/aggregate_functions/group_concat.sql
@@ -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;
diff --git a/internal/endtoend/testdata/mysql_reference_manual/sqlc.json b/internal/endtoend/testdata/mysql_reference_manual/sqlc.json
index 31e209605f..26e8643999 100644
--- a/internal/endtoend/testdata/mysql_reference_manual/sqlc.json
+++ b/internal/endtoend/testdata/mysql_reference_manual/sqlc.json
@@ -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"
     }
   ]
 }
diff --git a/internal/engine/dolphin/stdlib.go b/internal/engine/dolphin/stdlib.go
index 9744fea435..7fa567729d 100644
--- a/internal/engine/dolphin/stdlib.go
+++ b/internal/engine/dolphin/stdlib.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",
diff --git a/internal/sql/catalog/catalog.go b/internal/sql/catalog/catalog.go
index d81ab57008..e7964cf16f 100644
--- a/internal/sql/catalog/catalog.go
+++ b/internal/sql/catalog/catalog.go
@@ -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 {