如何在Go中构建类型接口的varidics？

Question

I'm building mysql query dynamically ( due the mysql IN clause) so I need to pass a list of arguments as the sql.Query function requires. Any idea how can do that ? I don't really understand how the variadic type works or how I can 'generate' it. If I pass a slice the sql complains that there is only one parameter instead of len(in clause params) thus I can only assume that the function sees the slice as a parameter instead a list ( variadic ) of parameters.

My query / code

var carsa []string
q := `SELECT field1,field2 FROM cooltable WHERE nicecars IN(?` + strings.Repeat(",?", len(ha.cars)-1) + ")"
hs := strings.Join(carsa, "','")
    hs = "'" + hs + "'"

    rows, err := cx.Query(q, country, hs)

sql Query

func (db *DB) Query(query string, args ...interface{}) (*Rows, error) {

Edit: The actual code I'm trying to run fails with error sql: converting Exec argument #0's type: unsupported type []interface {}, a slice

package main

import (
    "database/sql"
    "fmt"
    _ "github.com/go-sql-driver/mysql"
)

func main() {

    argsS := []string{"apple", "greph", "3rd"}

    db, err := sql.Open("mysql", dbUser+":"+dbPass+"@tcp("+dbHost+":3306)/"+dbName)
    if err != nil {
        return
    }
    query := `SELECT id,label FROM fruits WHERE country=? AND stack IN(?` + strings.Repeat(",?", len(args)-1) + ")"

    args := make([]interface{}, len(ha))
    args["country"] = interface{}("France")

    for i, v := range query {
        args[i] = interface{}(v)
    }

    rows, err := db.Query(q, args...)
    if err != nil {
        return
    }
    var id, label string

    for rows.Next() {
        if err = rows.Scan(&id, label); err != nil {
            fmt.Printf("%v", err)
        }
    }
}

Answer 1

With the given function, you can call it with a variable argument list as:

var query string
var args []interface{}
// initialise variables
cx.Query(query, args...)

This is equivalent to calling Query with each of the elements in the args slice passed as separate arguments, as described in the language specification . Note also that the type of the slice needs to match the variadic function definition (so in this case a slice of interface{} .

Note also that the slice needs to represent the entire variadic argument list. So it would be an error to call cx.Query(q, country, hs...) , for example. In cases like that, you will need to construct a slice that holds all the arguments you want to pass.

---

Following on from the question's update, to combine the fixed arguments with your variable ones you want your code to look something like this:

stacks := []string{"apple", "greph", "3rd"}

query := `SELECT id,label FROM fruits WHERE country=? AND stack IN(?` + strings.Repeat(",?", len(stacks)-1) + ")"

args := make([]interface{}, len(stacks)+1)
args[0] = "France"
for i := range stacks {
    args[i+1] = stacks[i]
}

rows, err := cx.Query(query, args...)