database/sql: add Null[T]

Generic version of NullString, NullInt64, etc.

Fixes #60370

Change-Id: I166a05a6126e8b8571db5cbb026303bb6551d56b
GitHub-Last-Rev: 3c8d2d5141c36f034d2124e19ee090620363ba24
GitHub-Pull-Request: golang/go#60677
Reviewed-on: https://go-review.googlesource.com/c/go/+/501700
Reviewed-by: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 33 insertions, 0 deletions

diff --git a/src/database/sql/sql.go b/src/database/sql/sql.go
index 836fe83e2e..7a57c7de36 100644
--- a/src/database/sql/sql.go
+++ b/src/database/sql/sql.go
@@ -391,6 +391,39 @@ func (n NullTime) Value() (driver.Value, error) {
 	return n.Time, nil
 }
 
+// Null represents a value that may be null.
+// Null implements the Scanner interface so
+// it can be used as a scan destination:
+//
+//	var s Null[string]
+//	err := db.QueryRow("SELECT name FROM foo WHERE id=?", id).Scan(&s)
+//	...
+//	if s.Valid {
+//	   // use s.V
+//	} else {
+//	   // NULL value
+//	}
+type Null[T any] struct {
+	V     T
+	Valid bool
+}
+
+func (n *Null[T]) Scan(value any) error {
+	if value == nil {
+		n.V, n.Valid = *new(T), false
+		return nil
+	}
+	n.Valid = true
+	return convertAssign(&n.V, value)
+}
+
+func (n Null[T]) Value() (driver.Value, error) {
+	if !n.Valid {
+		return nil, nil
+	}
+	return n.V, nil
+}
+
 // Scanner is an interface used by Scan.
 type Scanner interface {
 	// Scan assigns a value from a database driver.