GO：从SQL查询返回映射

Question

I am querying a mysql database in a GO function and want to return key value pairs in a map but can't quite figure out how to accomplish this. So far I have this function:

func GetData(callIds []string) map[string]Records {
    //db insert
    db, err := sql.Open("mysql", mySql)
    if err != nil {
        fmt.Printf(err.Error())
    }
    defer db.Close()
    //db query
    var foo string
    err = db.QueryRow("select foo from bardata where callId = %v", 1).Scan(&foo)
    if err != nil {
        fmt.Printf(err.Error())
    }
    fmt.Println(foo)

    return nil

I want to return a map with the key being callId and value being foo for each row returned from the query.

Answer 1

First, you need to build up your query. As it is, you're not even using your function input. Since we have a variable number of arguments, we need to do a little work to construct the right number of placeholders:

query := `select callid, foo from bardata where callid in (` +
    strings.Repeat(`?,`, len(callIds) - 1) + `?)`

then, execute with the values passed in:

rows, err := db.Query(query, callIds...)
if err != nil {
    // handle it
}
defer rows.Close()

then collect the results:

ret := map[string]string{}
for rows.Next() {
    var callid, foo string
    err = rows.Scan(&callid, &foo)
    if err != nil {
        // handle it
    }
    ret[callid] = foo
}

return ret

Caveats:

1. This will cause a placeholder mismatch error if callIds is an empty slice. If that's possible, then you need to detect it and handle it separately (maybe by returning an error or an empty map — querying the DB shouldn't be necessary).

2. This returns a map[string]string where the values are whatever "foo" is. In your question you have the function returning a map[string]Records but there's no information about what a Records might be or how to fetch one.

3. You might want to handle sql.ErrNoRows differently from other errors.