Can Go's Rows.Scan ignore fields from SQL query

Question

The Rows.Scan method takes as many parameters as there are columns in the SQL query.

As the query being executed is SHOW COLUMNS FROM my_table I cannot omit any column which I don't require (or can I?).

Is there any way to ignore some fields from the query result set which is not required?

Below is my code:

rows, err := db.Query("SHOW COLUMNS FROM " + r.Name)
DieIf(err)
//var field, dataType, ignoreMe1, ignoreMe2, ignoreMe3 string
var field, dataType string
for rows.Next() {
                    //This Place
                    //   |
                    //   V
    if err := rows.Scan(&field, &dataType); err != nil {
        DieIf(err)
    }
    r.Attributes[field] = Attribute{
        Name:       field,
        DataType:   dataType,
        Constraint: false,
    }
}

error: sql: expected 5 destination arguments in Scan, not 2

Aishaaisle

you can not ignore, if the selected fields count 5 then you have to pass 5 variable to scan in scan() , if you want scan only two values then you can write query like select field, type from <tableName> – Undistinguished

I cannot pass field names in SHOW COLUMNS QUERY. – Aishaaisle

so you can use `[]interface' to get that values and use according to your requirement. – Undistinguished

i've attached code below, try this one, hope you will get your point.Thanks. – Undistinguished

Answer 1

So here I'm with one solution for you, try this one to get field and type from query.

package main

import (
    "fmt"
    _ "github.com/lib/pq"
    "database/sql"
)

func main() {

    db, _ := sql.Open(
        "postgres",
        "user=postgres dbname=demo password=123456")

    rows, _ := db.Query("SELECT * FROM tableName;")

    columns, _ := rows.Columns()
    count := len(columns)
    values := make([]interface{}, count)
    valuePtr := make([]interface{}, count)

    for rows.Next() {
        for i, _ := range columns {
            valuePtr[i] = &values[i]
        }

        rows.Scan(valuePtr...)

        for i, col := range columns {
            var v interface{}

            val := values[i]

            b, ok := val.([]byte)
            if (ok) {
                v = string(b)
            } else {
                v = val
            }

            fmt.Println(col, v)
        }
    }
}

Undistinguished

why fetch all rows when you need only 1? use SELECT * FROM tableName LIMIT 1; instead ;) – Hydrated

Answer 2

Like this answer suggested, sqlx could be a good choice.

I personally use db.Unsafe() to ignore unwanted fields.

type MyTable struct {
    // list the fields you want here
    Name   string `db:"name"`
    Field1 string `db:"field1"`
}

db := try(sqlx.ConnectContext(context.Background(), "mysql",
    fmt.Sprintf("root:@tcp(%s:%d)/?charset=utf8mb4&timeout=3s", host, port)))
db = db.Unsafe()

rows := try(db.Queryx("select * from mytable"))
for rows.Next() {
    myTable := MyTable{}
    rows.StructScan(&myTable)
}

Incandesce