diff --git a/.github/workflows/go.yml b/.github/workflows/go.yml
index d34a5425..c8197beb 100644
--- a/.github/workflows/go.yml
+++ b/.github/workflows/go.yml
@@ -11,10 +11,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-    - name: Set up Go 1.19
-      uses: actions/setup-go@v5
+    - name: Set up Go 1.20
+      uses: actions/setup-go@v4
       with:
-        go-version: 1.19
+        go-version: "1.20"
       id: go
 
     - name: Check out code into the Go module directory
diff --git a/ast/pos.go b/ast/pos.go
index 98431a5d..c817c775 100644
--- a/ast/pos.go
+++ b/ast/pos.go
@@ -4,6 +4,51 @@ import (
 	"github.com/cloudspannerecosystem/memefish/token"
 )
 
+// ================================================================================
+//
+// Helper functions for Pos(), End()
+// These functions are intended for use within this file only.
+//
+// ================================================================================
+
+// lastNode returns last element of Node slice.
+// This function corresponds to NodeSliceVar[$] in ast.go.
+func lastNode[T Node](s []T) T {
+	return s[len(s)-1]
+}
+
+// firstValidEnd returns the first valid Pos() in argument.
+// "valid" means the node is not nil and Pos().Invalid() is not true.
+// This function corresponds to "(n0 ?? n1 ?? ...).End()"
+func firstValidEnd(ns ...Node) token.Pos {
+	for _, n := range ns {
+		if n != nil && !n.End().Invalid() {
+			return n.End()
+		}
+	}
+	return token.InvalidPos
+}
+
+// firstPos returns the Pos() of the first node.
+// If argument is an empty slice, this function returns token.InvalidPos.
+// This function corresponds to NodeSliceVar[0].pos in ast.go.
+func firstPos[T Node](s []T) token.Pos {
+	if len(s) == 0 {
+		return token.InvalidPos
+	}
+	return s[0].Pos()
+}
+
+// lastEnd returns the End() of the last node.
+// If argument is an empty slice, this function returns token.InvalidPos.
+// This function corresponds to NodeSliceVar[$].end in ast.go.
+func lastEnd[T Node](s []T) token.Pos {
+	if len(s) == 0 {
+		return token.InvalidPos
+	}
+	return lastNode(s).End()
+}
+
 // ================================================================================
 //
 // SELECT
@@ -39,25 +84,7 @@ func (c *CTE) End() token.Pos { return c.Rparen + 1 }
 func (s *Select) Pos() token.Pos { return s.Select }
 
 func (s *Select) End() token.Pos {
-	if s.Limit != nil {
-		return s.Limit.End()
-	}
-	if s.OrderBy != nil {
-		return s.OrderBy.End()
-	}
-	if s.Having != nil {
-		return s.Having.End()
-	}
-	if s.GroupBy != nil {
-		return s.GroupBy.End()
-	}
-	if s.Where != nil {
-		return s.Where.End()
-	}
-	if s.From != nil {
-		return s.From.End()
-	}
-	return s.Results[len(s.Results)-1].End()
+	return firstValidEnd(s.Limit, s.OrderBy, s.Having, s.GroupBy, s.Where, s.From, lastNode(s.Results))
 }
 
 func (c *CompoundQuery) Pos() token.Pos {
@@ -376,8 +403,8 @@ func (p *Param) End() token.Pos { return p.Atmark + 1 + token.Pos(len(p.Name)) }
 func (i *Ident) Pos() token.Pos { return i.NamePos }
 func (i *Ident) End() token.Pos { return i.NameEnd }
 
-func (p *Path) Pos() token.Pos { return p.Idents[0].Pos() }
-func (p *Path) End() token.Pos { return p.Idents[len(p.Idents)-1].End() }
+func (p *Path) Pos() token.Pos { return firstPos(p.Idents) }
+func (p *Path) End() token.Pos { return lastEnd(p.Idents) }
 
 func (a *ArrayLiteral) Pos() token.Pos {
 	if !a.Array.Invalid() {
diff --git a/ast/sql.go b/ast/sql.go
index 71889a1e..2cb2c744 100644
--- a/ast/sql.go
+++ b/ast/sql.go
@@ -2,8 +2,59 @@ package ast
 
 import (
 	"github.com/cloudspannerecosystem/memefish/token"
+	"strings"
 )
 
+// ================================================================================
+//
+// Helper functions for SQL()
+// These functions are intended for use within this file only.
+//
+// ================================================================================
+
+// sqlOpt outputs:
+//
+//	when node != nil: left + node.SQL() + right
+//	else            : empty string
+//
+// This function corresponds to sqlOpt in ast.go
+func sqlOpt[T interface {
+	Node
+	comparable
+}](left string, node T, right string) string {
+	var zero T
+	if node == zero {
+		return ""
+	}
+	return left + node.SQL() + right
+}
+
+// strOpt outputs:
+//
+//	when pred == true: s
+//	else            : empty string
+//
+// This function corresponds to {{if pred}}s{{end}} in ast.go
+func strOpt(pred bool, s string) string {
+	if pred {
+		return s
+	}
+	return ""
+}
+
+// sqlJoin outputs joined string of SQL() of all elems by sep.
+// This function corresponds to sqlJoin in ast.go
+func sqlJoin[T Node](elems []T, sep string) string {
+	var b strings.Builder
+	for i, r := range elems {
+		if i > 0 {
+			b.WriteString(sep)
+		}
+		b.WriteString(r.SQL())
+	}
+	return b.String()
+}
+
 type prec int
 
 const (
@@ -116,36 +167,16 @@ func (c *CTE) SQL() string {
 }
 
 func (s *Select) SQL() string {
-	sql := "SELECT "
-	if s.Distinct {
-		sql += "DISTINCT "
-	}
-	if s.AsStruct {
-		sql += "AS STRUCT "
-	}
-	sql += s.Results[0].SQL()
-	for _, r := range s.Results[1:] {
-		sql += ", " + r.SQL()
-	}
-	if s.From != nil {
-		sql += " " + s.From.SQL()
-	}
-	if s.Where != nil {
-		sql += " " + s.Where.SQL()
-	}
-	if s.GroupBy != nil {
-		sql += " " + s.GroupBy.SQL()
-	}
-	if s.Having != nil {
-		sql += " " + s.Having.SQL()
-	}
-	if s.OrderBy != nil {
-		sql += " " + s.OrderBy.SQL()
-	}
-	if s.Limit != nil {
-		sql += " " + s.Limit.SQL()
-	}
-	return sql
+	return "SELECT " +
+		strOpt(s.Distinct, "DISTINCT ") +
+		strOpt(s.AsStruct, "AS STRUCT ") +
+		sqlJoin(s.Results, ", ") +
+		sqlOpt(" ", s.From, "") +
+		sqlOpt(" ", s.Where, "") +
+		sqlOpt(" ", s.GroupBy, "") +
+		sqlOpt(" ", s.Having, "") +
+		sqlOpt(" ", s.OrderBy, "") +
+		sqlOpt(" ", s.Limit, "")
 }
 
 func (c *CompoundQuery) SQL() string {
@@ -464,27 +495,11 @@ func (i *IndexExpr) SQL() string {
 }
 
 func (c *CallExpr) SQL() string {
-	sql := c.Func.SQL() + "("
-	if c.Distinct {
-		sql += "DISTINCT "
-	}
-	for i, a := range c.Args {
-		if i != 0 {
-			sql += ", "
-		}
-		sql += a.SQL()
-	}
-	if len(c.Args) > 0 && len(c.NamedArgs) > 0 {
-		sql += ", "
-	}
-	for i, v := range c.NamedArgs {
-		if i != 0 {
-			sql += ", "
-		}
-		sql += v.SQL()
-	}
-	sql += ")"
-	return sql
+	return c.Func.SQL() + "(" + strOpt(c.Distinct, "DISTINCT ") +
+		sqlJoin(c.Args, ", ") +
+		strOpt(len(c.Args) > 0 && len(c.NamedArgs) > 0, ", ") +
+		sqlJoin(c.NamedArgs, ", ") +
+		")"
 }
 
 func (n *NamedArg) SQL() string { return n.Name.SQL() + " => " + n.Value.SQL() }
@@ -595,11 +610,7 @@ func (i *Ident) SQL() string {
 }
 
 func (p *Path) SQL() string {
-	sql := p.Idents[0].SQL()
-	for _, id := range p.Idents[1:] {
-		sql += "." + id.SQL()
-	}
-	return sql
+	return sqlJoin(p.Idents, ".")
 }
 
 func (a *ArrayLiteral) SQL() string {
diff --git a/go.mod b/go.mod
index 0defff2e..e65f1d20 100644
--- a/go.mod
+++ b/go.mod
@@ -1,6 +1,6 @@
 module github.com/cloudspannerecosystem/memefish
 
-go 1.19
+go 1.20
 
 require (
 	github.com/MakeNowJust/heredoc/v2 v2.0.1