diff --git a/expression.go b/expression.go
index 4144075..06134da 100644
--- a/expression.go
+++ b/expression.go
@@ -82,6 +82,8 @@ type StringExpression interface {
 	Concat(other interface{}) StringExpression
 	IfEmpty(altValue interface{}) StringExpression
 	IsEmpty() BooleanExpression
+	Lower() StringExpression
+	Upper() StringExpression
 }
 
 // UnknownExpression is the interface of an SQL expression with unknown value.
@@ -108,6 +110,8 @@ type UnknownExpression interface {
 	Concat(other interface{}) StringExpression
 	IfEmpty(altValue interface{}) StringExpression
 	IsEmpty() BooleanExpression
+	Lower() StringExpression
+	Upper() StringExpression
 }
 
 type expression struct {
@@ -215,6 +219,14 @@ func (e expression) IsEmpty() BooleanExpression {
 	return e.Equals("")
 }
 
+func (e expression) Lower() StringExpression {
+	return function("LOWER", e)
+}
+
+func (e expression) Upper() StringExpression {
+	return function("UPPER", e)
+}
+
 func (e expression) GetSQL(scope scope) (string, error) {
 	if e.sql != "" {
 		return e.sql, nil
diff --git a/expression_test.go b/expression_test.go
index 9fb24e5..ef6d36f 100644
--- a/expression_test.go
+++ b/expression_test.go
@@ -115,6 +115,8 @@ func TestFunc(t *testing.T) {
 	assertValue(t, e.IfNull(3), "IFNULL(<>, 3)")
 	assertValue(t, e.IfEmpty(3), "IF(<> <> '', <>, 3)")
 	assertValue(t, e.IsEmpty(), "<> = ''")
+	assertValue(t, e.Lower(), "LOWER(<>)")
+	assertValue(t, e.Upper(), "UPPER(<>)")
 
 	e5 := expression{
 		builder: func(scope scope) (string, error) {