MYSQL concurrency select and update - mysql

Question

I can't understand how to select and then update table with several goroutines. In the documentation for db and stmt it says: "is safe for concurrent use by multiple goroutines." Also I use transatcions but without success. I want to start 7 goroutines and take every row.
Data
+--------+-----------+---------------------+
| idTest | someValue | date |
+--------+-----------+---------------------+
| 1 | 1 | 2019-06-11 11:29:42 |
| 2 | 2 | 2019-06-11 11:29:42 |
| 3 | 3 | NULL |
| 4 | 4 | NULL |
| 5 | 5 | NULL |
| 6 | 6 | NULL |
| 7 | 7 | NULL |
+--------+-----------+---------------------+
current code
db, err := sql.Open("mysql", strConn)
if err != nil {
fmt.Printf("Troubles in connaction! %s", err)
}
var idTest int
var someValue string
stmt, err := db.Prepare("select idTest,someValue from test where date is null limit 1")
CheckError(err)
defer stmt.Close()
rows, err := stmt.Query()
CheckError(err)
defer rows.Close()
for rows.Next() {
rows.Scan(&idTest, &someValue)
stmt, err = db.Prepare("update test set date = now() where idTest= ?")
CheckError(err)
_, err = stmt.Exec(idTest)
CheckError(err)
}
Every goroutine have db.conn and sometimes trying to select and update table.
func main() {
for i := 0; i < 7; i++ {
dbConn := "blabla"
go ChildBot(dbConn)
}
var input string
fmt.Scanln(&input)
}

Answer 1

you should read the data and then run a goroutine.
for rows.Next() {
rows.Scan(&idTest, &someValue)
go func(idTest int) {
stmt, err = db.Prepare("update test set date = now() where idTest= ?")
CheckError(err)
_, err = stmt.Exec(idTest)
CheckError(err)
}(idTest)
}
it's done this way because you have to read the data first before doing anything with it. Otherwise, the next read could override your previous value from time to time.