I've been strugguling with SQL NULL values in Golang recently. After trying to unsuccessfully unmarshal JSON objects using Decode() and sql.NullString, I came to this answer on StackOverflow :
Assigning null to JSON fields instead of empty strings in Golang
The solution using a string pointer seems to work perfectly with Decode() and nil values. So what is the difference between sql.NullString and *string ?
Is it just about nil checking ?
SQL has different null values than Golang.
If you look at the definition of sql.NullString then this is what you get:
type NullString struct {
String string
Valid bool // Valid is true if String is not NULL
}
As you can see, sql.NullString is a way to represent null string coming from SQL (which correspond to "NULL").
On the other hand, a nil *string is a pointer to a string which is nil, so the two are different.
sql.NullString over using *string?
There's no effective difference. We thought people might want to use NullString because it is so common and perhaps expresses the intent more clearly than *string. But either will work.
nil has no type, sql.NullString does, so it's better to use sql.NullString, but people don't want to do the extra typing so they just defaulted to *string, which IMHO is an ugly hack.
Making @sepehr's comment in the previous answer an answer of it's own, and adding some conversion functions I find helpful.
From Russ Cox
There's no effective difference. We thought people might want to use NullString because it is so common and perhaps expresses the intent more clearly than *string. But either will work
In fact, a few generic functions make it easy to convert from *string and sql.NullString with a minimum of fuss:
func DerefOrEmpty[T any](val *T) T {
if val == nil {
var empty T
return empty
}
return *val
}
func IsNotNil[T any](val *T) bool {
return val != nil
}
Usage:
func main() {
var comment string = "hello"
commentPtr := &comment
sqlStrComment := sql.NullString{
String: DerefOrEmpty(commentPtr),
Valid: IsNotNil(commentPtr),
}
fmt.Printf("%#v\n", sqlStrComment)
commentPtr = nil
sqlStrComment = sql.NullString{
String: DerefOrEmpty(commentPtr),
Valid: IsNotNil(commentPtr),
}
fmt.Printf("%#v\n", sqlStrComment)
}
Playground link at https://go.dev/play/p/KFNHe2iJmGU
