Using the golang example below, how can I query (JOIN) multiple databases.
For example, I want to have the relation db1.username.id = db2.comments.username_id.
id := 123
var username string
err := db.QueryRow("SELECT username FROM users WHERE id=?", id).Scan(&username)
switch {
case err == sql.ErrNoRows:
log.Printf("No user with that ID.")
case err != nil:
log.Fatal(err)
default:
fmt.Printf("Username is %s\n", username)
}
As you are using MySQL, you can select fields across databases. See this related question for
details. For example you should be able to do this:
err := db.QueryRow(`
SELECT
db1.users.username
FROM
db1.users
JOIN
db2.comments
ON db1.users.id = db2.comments.username_id
`).Scan(&username)
You can of course simply fetch all entries from db2.comments using a second database connection and use the values in a query to db1.users. This is, of course, not recommended as it is the job of the database server which it can, most likely, do better than you.
Related
I have this MySQL database where I need to add records with a go program and need to retrieve the id of the last added record, to add the id to another table.
When i run insert INSERT INTO table1 values("test",1); SELECT LAST_INSERT_ID() in MySQL Workbench, it returns the last id, which is auto incremented, with no issues.
If I run my go code however, it always prints 0. The code:
_, err := db_client.DBClient.Query("insert into table1 values(?,?)", name, 1)
var id string
err = db_client.DBClient.QueryRow("SELECT LAST_INSERT_ID()").Scan(&id)
if err != nil {
panic(err.Error())
}
fmt.Println("id: ", id)
I tried this variation to try to narrow down the problem scope further: err = db_client.DBClient.QueryRow("SELECT id from table1 where name=\"pleasejustwork\";").Scan(&id), which works perfectly fine; go returns the actual id.
Why is it not working with the LAST_INSERT_ID()?
I'm a newbie in go so please do not go hard on me if i'm making stupid go mistakes that lead to this error :D
Thank you in advance.
The MySQL protocol returns LAST_INSERT_ID() values in its response to INSERT statements. And, the golang driver exposes that returned value. So, you don't need the extra round trip to get it. These ID values are usually unsigned 64-bit integers.
Try something like this.
res, err := db_client.DBClient.Exec("insert into table1 values(?,?)", name, 1)
if err != nil {
panic (err.Error())
}
id, err := res.LastInsertId()
if err != nil {
panic (err.Error())
}
fmt.Println("id: ", id)
I confess I'm not sure why your code didn't work. Whenever you successfully issue a single-row INSERT statement, the next statement on the same database connection always has access to a useful LAST_INSERT_ID() value. This is true whether or not you use explicit transactions.
But if your INSERT is not successful, you must treat the last insert ID value as unpredictable. (That's a technical term for "garbage", trash, rubbish, basura, etc.)
I am developing a system to enable patient registration with incremental queue number. I am using Go, GORM, and MySQL.
An issue happens when more than one patients are registering at the same time, they tend to get the same queue number which it should not happen.
I attempted using transactions and hooks to achieve that but I still got duplicate queue number. I have not found any resource about how to lock the database when a transaction is happening.
func (r repository) CreatePatient(pat *model.Patient) error {
tx := r.db.Begin()
defer func() {
if r := recover(); r != nil {
tx.Rollback()
}
}()
err := tx.Error
if err != nil {
return err
}
// 1. get latest queue number and assign it to patient object
var queueNum int64
err = tx.Model(&model.Patient{}).Where("registration_id", pat.RegistrationID).Select("queue_number").Order("created_at desc").First(&queueNum).Error
if err != nil && err != gorm.ErrRecordNotFound {
tx.Rollback()
return err
}
pat.QueueNumber = queueNum + 1
// 2. write patient data into the db
err = tx.Create(pat).Error
if err != nil {
tx.Rollback()
return err
}
return tx.Commit().Error
}
As stated by #O. Jones, transactions don't save you here because you're extracting the largest value of a column, incrementing it outside the db and then saving that new value. From the database's point of view the updated value has no dependence on the queried value.
You could try doing the update in a single query, which would make the dependence obvious:
UPDATE patient AS p
JOIN (
SELECT max(queue_number) AS queue_number FROM patient WHERE registration_id = ?
) maxp
SET p.queue_number = maxp.queue_number + 1
WHERE id = ?
In gorm you can't run a complex update like this, so you'll need to make use of Exec.
I'm not 100% certain the above will work because I'm less familiar with MySQL transaction isolation guarantees.
A cleaner way
Overall, it'd be cleaner to keep a table of queues (by reference_id) with a counter that you update atomically:
Start a transaction, then
SELECT queue_number FROM queues WHERE registration_id = ? FOR UPDATE;
Increment the queue number in your app code, then
UPDATE queues SET queue_number = ? WHERE registration_id = ?;
Now you can use the incremented queue number in your patient creation/update before transaction commit.
I want to execute a query something like this (using MySql):
select * from user where id = 5
Ex 1. This returns err = sql.ErrNoRows:
err := db.QueryRow("select * from user where ? = ?", f, v).Scan(&user.Id, etc...)
Ex 2. Since the above doesn't work, I am doing it this way, which works but doesn't feel right:
err := db.QueryRow("select * from user where "+f+" = ?", v).Scan(&user.Id, etc...)
What is wrong in Ex. 1? Is Ex 2. an acceptable way to do this?
EDIT
From the link in the comments I can do it a third way.
Ex 3:
q := fmt.Sprintf("select * from user where %s = ?", f)
err := db.QueryRow(q, v).Scan(&user.Id, etc...)
You need to provide column names directly in sql query. '?' sign is a placeholder for values you provide after sql string.
Your Ex3. is correct, but it is not a good solution, because it is open for SQL-injection attacks.
If you need a lightweight solution you can create a map with known column names for each table like: userColumns := map[string]bool {"id": true, ...} and then just use next check:
if userColumns[f] {
...you able to use f in sql...
} else {
...provided value is not one of known columns, it is an error...
}
I have a problem with getting database table list (SHOW TABLES) in Go.
I use this packages
database/sql
gopkg.in/gorp.v1
github.com/ziutek/mymysql/godrv
and connect to MYSQL by this code:
db, err := sql.Open(
"mymysql",
"tcp:127.0.0.1:3306*test/root/root")
if err != nil {
panic(err)
}
dbmap := &DbMap{Conn:&gorp.DbMap{Db: db}}
And I use this code to get list of tables
result, _ := dbmap.Exec("SHOW TABLES")
But result is empty!
I use classic go-sql-driver/mysql:
db, _ := sql.Open("mysql", "root:qwerty#/dbname")
res, _ := db.Query("SHOW TABLES")
var table string
for res.Next() {
res.Scan(&table)
fmt.Println(table)
}
PS don't ignore errors! This is only an example
I'm trying this code and work successfully. I create a list of string and use Select query to get list of database tables.
tables := []string{}
dbmap.Select(&tables, "SHOW TABLES")
fmt.Println(tables)
Suppose that I've the following structures that it's the mapped tables.
type Publisher struct{
ID int `db:"id"`
Name string `db:"name"`
Books []*Book
}
type Book struct {
ID int `db:"id"`
Name string `db:"name"`
PublisherID `db:"publisher_id"`
}
So, What if I wanna retrieve all the Publisher with all related Books I would like to get a JSON like this:
[ //Publisher 1
{
"id" : "10001",
"name":"Publisher1",
"books" : [
{ "id":321,"name": "Book1"},
{ "id":333,"name": "Book2"}
]
},
//Publisher 2
{
"id" : "10002",
"name":"Slytherin Publisher",
"books" : [
{ "id":4021,"name": "Harry Potter and the Chamber of Secrets"},
{ "id":433,"name": "Harry Potter and the Order of the Phoenix"}
]
},
]
So I've the following structure that I use to retrieve all kind of query related with Publisher
type PublisherRepository struct{
Connection *sql.DB
}
// GetEbooks return all the books related with a publisher
func (r *PublisherRepository) GetBooks(idPublisher int) []*Book {
bs := make([]Book,0)
sql := "SELECT * FROM books b WHERE b.publisher_id =$1 "
row, err := r.Connection.Query(sql,idPublisher)
if err != nil {
//log
}
for rows.Next() {
b := &Book{}
rows.Scan(&b.ID, &b.Name, &b.PublisherID)
bs := append(bs,b)
}
return bs
}
func (r *PublisherRepository) GetAllPublishers() []*Publisher {
sql := "SELECT * FROM publishers"
ps := make([]Publisher,0)
rows, err := r.Connection.Query(sql)
if err != nil {
// log
}
for rows.Next() {
p := &Publisher{}
rows.Scan(&p.ID,&p.Name)
// Is this the best way?
books := r.GetBooks(p.ID)
p.Books = books
}
return ps
}
So , here my questions
What is the best approach to retrieve all the publisher with the best performance, because a for inside a for is not the best solution, what if I've 200 publisher and in the average of each publisher has 100 books.
Is in GoLang idiomatic PublisherRepository or is there another way to create something to manage the transactions of an entity with pure sql?
1) Bad about this would be the sql request per iteration. So here a solution that does not make an extra request per Publisher:
func (r *PublisherRepository) GetAllPublishers() []*Publisher {
sql := "SELECT * FROM publishers"
ps := make(map[int]*Publisher)
rows, err := connection.Query(sql)
if err != nil {
// log
}
for rows.Next() {
p := &Publisher{}
rows.Scan(&p.ID,&p.Name)
ps[p.ID] = p
}
sql = "SELECT * FROM books"
rows, err := connection.Query(sql)
if err != nil {
//log
}
for rows.Next() {
b := &Book{}
rows.Scan(&b.ID, &b.Name, &b.PublisherID)
ps[b.PublisherID].Books = append(ps[b.PublisherID].Books, b)
}
// you might choose to keep the map as a return value, but otherwise:
// preallocate memory for the slice
publishers := make([]*Publisher, 0, len(ps))
for _, p := range ps {
publishers = append(publishers, p)
}
return publishers
}
2) Unless you create the PublisherRepository only once, this might be a bad idea creating and closing loads of connections. Depending also on your sql client implementation I would suggest (and also have seen it for many other go database clients) to have one connection for the entire server. Pooling is done internally by many of the sql clients, that is why you should check your sql client.
If your sql client library does pooling internally use a global variable for the "connection" (it's not really one connection if pooling is done internally):
connection *sql.DB
func New () *PublisherRepository {
repo := &PublisherRepository{}
return repo.connect()
}
type PublisherRepository struct{
}
func (r *PublisherRepository) connect() *PublisherRepository {
// open new connection if connection is nil
// or not open (if there is such a state)
// you can also check "once.Do" if that suits your needs better
if connection == nil {
// ...
}
return r
}
So each time you create a new PublisherRepository, it will only check if connection already exists. If you use once.Do, go will only create the "connection" once and you are done with it.
If you have other structs that will use the connection as well, you need a global place for your connection variable or (even better) you write a little wrapper package for your sql client, that is in turn used in all your structs.
In your case, the simplest way would be to use json_agg in the query. Like here http://sqlfiddle.com/#!15/97c41/4 (sqlfiddle is slow so here is screenshot http://i.imgur.com/hxMPkUa.png) Not very Go friendly (you need to unmarshal query result data if you want to do something with the books) but all books in one query as you wanted without for loops.
As #TehSphinX said it is better to have single global db connection.
But before implementing strange queries I really suggest you to think: why do you need to return the full list of publishers and their books in one API query? I can't imagine the situation in web or mobile app where your idea might be a good decision. Usually, you just show users list of publishers then users chooses one and you show him the list of books by this publisher. This is "win-win" situation for you and your users - you can make simple queries and your users just get small sets of data that they actually need without paying for unnecessary traffic/wasting browser memory. As you said there can be 200 publishers with 100 books and I'm sure your users don't need 20000 books loaded in one request. Of course, if you are not trying to make your API more data theft friendly.
Even if you have something like a short preview-like list of books for each publisher you should think about pagination for publishers and/or denormalisation of books data for this case (add a column to publishers table with the short list of books in JSON format).