Conn.ExecContext in Go

package main

import "log"

func main() {
	// A *DB is a pool of connections. Call Conn to reserve a connection for
	// exclusive use.
	conn, err := db.Conn(ctx)
	if err != nil {
		log.Fatal(err)
	}
	defer conn.Close() // Return the connection to the pool.
	id := 41
	result, err := conn.ExecContext(ctx, `UPDATE balances SET balance = balance + 10 WHERE user_id = ?;`, id)
	if err != nil {
		log.Fatal(err)
	}
	rows, err := result.RowsAffected()
	if err != nil {
		log.Fatal(err)
	}
	if rows != 1 {
		log.Fatalf("expected single row affected, got %d rows affected", rows)
	}
}