* string和sql.NullString之间的区别

Question

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 ?

Answer 1

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.