sql: scan row(s) with unknown number of columns (select * from ...)

Question

I have a table t containing a lot of columns, and my sql is like this: select * from t. Now I only want to scan one column or two from the wide returned row set. However, the sql.Scan accepts dest ...interface{} as arguments. Does it mean I have to scan everything and use only the column I needed?

I know I could change the sql from select * to select my_favorite_rows, however, in this case, I have no way to change the sql.

Answer 1

You can make use of Rows.Columns, e.g.

package main

import (
    "database/sql"
    "fmt"

    "github.com/lib/pq"
)

type Vehicle struct {
    Id     int
    Name   string
    Wheels int
}

// VehicleCol returns a reference for a column of a Vehicle
func VehicleCol(colname string, vh *Vehicle) interface{} {
    switch colname {
    case "id":
        return &vh.Id
    case "name":
        return &vh.Name
    case "wheels":
        return &vh.Wheels
    default:
        panic("unknown column " + colname)
    }
}

func panicOnErr(err error) {
    if err != nil {
        panic(err.Error())
    }
}

func main() {
    conn, err := pq.ParseURL(`postgres://docker:[email protected]:5432/pgsqltest?schema=public`)
    panicOnErr(err)

    var db *sql.DB
    db, err = sql.Open("postgres", conn)
    panicOnErr(err)

    var rows *sql.Rows
    rows, err = db.Query("select * from vehicle")
    panicOnErr(err)

    // get the column names from the query
    var columns []string
    columns, err = rows.Columns()
    panicOnErr(err)

    colNum := len(columns)

    all := []Vehicle{}

    for rows.Next() {
        vh := Vehicle{}

        // make references for the cols with the aid of VehicleCol
        cols := make([]interface{}, colNum)
        for i := 0; i < colNum; i++ {
            cols[i] = VehicleCol(columns[i], &vh)
        }

        err = rows.Scan(cols...)
        panicOnErr(err)

        all = append(all, vh)
    }

    fmt.Printf("%#v\n", all)
}

Answer 2

For unknown length of columns but if you're sure about their type,

    cols, err := rows.Columns()
    if err != nil {
        log.Fatal(err.Error())
    }

    colLen := len(cols)
    vals := make([]interface{}, colLen)
    for rows.Next() {
        for i := 0; i < len(colLen); i++ {
            vals[i] = new(string)
        }
        err := rows.Scan(vals...)
        if err != nil {
            log.Fatal(err.Error()) // if wrong type
        }

        fmt.Printf("Column 1: %s\n", *(vals[0].(*string))) // will panic if wrong type
    }

PS: Not recommended for prod