Golang, Pointers, Functions

Question

I'm new to Golang and to compiled, statically-typed programming in general. All my prior experience has been with Python.

This paradigm shift has been both frustrating (programs rarely compile) and rewarding as I'm finally getting my head around many concepts that were previously foreign to me (garbage collection, pointers, scope).

Can someone explain to me on a conceptual level why this program won't compile and the syntax to fix it? I'm simply trying to query a DB and print the results:

package main

import (
    "database/sql"
    "log"

    _ "github.com/denisenkom/go-mssqldb"
)

func main() {

    db, err := sql.Open("sqlserver", "odbc:server=myServer;user id=myName;password=myPassword;")
    if err != nil {
        log.Fatal(err)
    }
    defer db.Close()

    q()
}

func q() {

    var (
        id   int
        name string
    )

    rows, err := db.Query("SELECT id, name FROM myTable")
    if err != nil {
        log.Fatal(err)
    }
    defer rows.Close()

    for rows.Next() {
        err := rows.Scan(&id, &name)

        if err != nil {
            log.Fatal(err)
        }
        log.Println(id, name)
    }
    err = rows.Err()
    if err != nil {
        log.Fatal(err)
    }

}

The error I'm getting is:

undefined: db in db.Query

The query works when I put the logic inside q() in the main function - I'm assuming this is because functions have "local" scope (is that the correct terminology?) and I need to define the db object as I have in the main function.

If this is the case - how do I run the q() function without repeating myself in establishing the db connection? Is this where "pointers" come in? Also, I'm still uncertain as to what the ampersands are doing here:

 err := rows.Scan(&id, &name)

Thanks

how about passing db as an argument to q()? that would work. — Not_a_Golfer
– Not_a_Golfer, Commented Feb 16, 2017 at 22:26
This wouldn't work with python scoping rules either, you defined db in main, but are trying to reference it in q. — Mr_Pink
– Mr_Pink, Commented Feb 16, 2017 at 22:31
Your assumption as well as the terminology is perfectly right and the same thing won't work in python too. — hashcode55
– hashcode55, Commented Feb 16, 2017 at 23:21
I think you need to install plugin first to checking error of your code. there are tremendous editor and plugin out there. — Gujarat Santana
– Gujarat Santana, Commented Feb 17, 2017 at 14:47

Tarsis Azevedo · Accepted Answer · 2017-02-20 22:26:06Z

3

db var is defined at main func scope, so your q func don't "see" it. You have to pass as pointer to q method. Something like that:

func q(db *sql.DB) {
   ...
}

Your db var in main is already a pointer to sql.DB struct, so you just have to do q(db) call in main func and it'll work.

edited Feb 20, 2017 at 22:26

answered Feb 16, 2017 at 22:33

Tarsis Azevedo

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2 Comments

Tarsis Azevedo Over a year ago

Keep going! You are doing well! Go is new world for you, so it's normal to get confused at firsts tries

kostix Over a year ago

Please drop the "reference" word: there are no references in Go.

deLta · Accepted Answer · 2017-02-16 23:54:28Z

As others mentioned, the problem is the variable db is declared in function main and you are trying to access that variable in q function. You have two options to solve this problem:

1) Declare the variable db as a global variable outside the main function like this:

As others mentioned, the problem is the variable db is declared in function main and you are trying to access that variable in q function. You have two options to solve this problem:

1) Declare the variable db as a global variable outside the main function like this:

package main

import (
"database/sql"
"log"
_ "github.com/denisenkom/go-mssqldb"
)

var db *sql.DB

func main() {

db, err = sql.Open("sqlserver", "odbc:server=myServer;user id=myName;password=myPassword;")
.
.
.
.

The important thing to notice is the assignment (= instead of :=) operator used to assign value to the global variable. Because if you use the short variable declaration := then it would create a local db variable.

2) You can pass the db pointer as a parameter to the function q like this:

package main

import (
    "database/sql"
    "log"
    _ "github.com/denisenkom/go-mssqldb"
)
func main() {

    db, err := sql.Open("sqlserver", "odbc:server=myServer;user id=myName;password=myPassword;")
    if err != nil {
      log.Fatal(err)
    }
    defer db.Close()

    q(db)
}

func q(db *sql.DB) {

    var (
        id   int
        name string
    )
.
.
.
.

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