Go's `*string` vs. `sql.NullString`: When Should I Use Which?

Understanding the Difference between *string and sql.NullString in Go

In Go, dealing with SQL NULL values can be challenging. Recently, discussions emerged around the usage of *string (string pointer) and sql.NullString for handling null values. This article aims to clarify the distinctions between these two types.

SQL Null Values vs. Go Null Values

SQL and Go have different representations for null values. In SQL, NULL is a special value that indicates the absence of a database entry, while in Go, nil represents a pointer with no value.

sql.NullString

To represent SQL NULL values, Go provides the sql.NullString type. It consists of two fields:

• String: Represents the string value (if not null)
• Valid: A boolean indicating if the String field contains a valid value (not null)

By design, sql.NullString does not represent nil in Go; instead, it represents SQL NULL.

*string

A *string is a pointer to a string value. It can be set to nil to explicitly indicate the absence of a value. Unlike sql.NullString, it does not relate to SQL NULL semantics but to the general concept of nil values in Go.

Usage Differences

• JSON Unmarshalling: sql.NullString is commonly used when unmarshalling JSON objects containing nullable string fields. It allows distinguishing between an empty string and a null value.
• Database Interactions: sql.NullString is often employed when working with databases that support SQL NULL, such as MySQL or PostgreSQL. It allows correctly handling null values retrieved from the database.
• Nil Checking: Both sql.NullString.Valid and *string != nil can be used for checking if the value is null. However, the former is specific to sql.NullString, while the latter is more general and can be applied to any pointer type.

Conclusion

sql.NullString and string are distinct types with different purposes. sql.NullString represents SQL NULL values and is used in conjunction with databases that support them. string, on the other hand, represents nil pointers and is more general for handling null values in Go applications. Understanding their differences is crucial for effectively managing null values in Go programs.

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