Firmino Changani

Go/SQL - Fixing the error - Converting NULL to string is unsupported error

26 Jun 2022
Converting NULL to string is unsupported error

The error above is thrown whenever a Go program tries to scan a query result that contains at least one column whose value is NULL to a struct defined with primitive types instead of sql.NullString type.

To solve this issue at the query level, the SQL function COALESCE comes in handy.

package demo

type User struct {
    Id string
    FirstName string
    LastName string

func findUserByEmail(ctx context.Context, email string) {
    user := &User{}
	row := DbConn.QueryRowContext(ctx, `
		SELECT id, COALESCE(first_name, '') as first_name, COALESCE(last_name, '') as last_name
		FROM users
		WHERE email = $1
	`, email)
	err := row.Scan(&user.Id, &user.FirstName, &user.LastName)

    // ...

When the query above is executed, the function COALESCE will replace the NULL value of the columns first_name and last_name returned by the database for the value passed as second argument, which is an empty string.