How to Extract Column Values from a Go SQL Rows Instance into a []string?

Extracting Column Values into []string using Go SQL Rows Instance

Challenge:

Many programming tasks require extracting row data from database tables into a structured format. In Go, the database/sql package provides Rows instances for iterating through table rows. However, the Scan method of Rows expects pointers to target variables with known types, which can be challenging when the number and types of columns are unknown in advance.

Solution:

To read columns from a Rows instance into a []string, you can use the following approach:

1. Obtain Column Names: Call Rows.Columns() to get a slice of column names.
2. Create Interface Slice: Create an []interface{} slice called readCols with the same length as the columns slice. This slice will hold the addresses of string variables.
3. Create String Slice: Create an empty []string slice called writeCols with the same length as the columns slice. This slice will store the string values.
4. Populate Addresses: Iterate over the indices of writeCols and assign each index in readCols to the address of the corresponding element in writeCols.
5. Scan Rows: Iterate over the Rows instance using Next() and Scan(). Pass readCols as an argument to Scan(), which will populate the string addresses in writeCols.
6. Append to Result: Append the values in writeCols to your desired data structure, such as a CSV writer.

Example:

package main

import (
    "fmt"
    "io"

    "database/sql"
)

func dumpTable(rows *sql.Rows, out io.Writer) error {
    colNames, err := rows.Columns()
    if err != nil {
        return err
    }

    writer := csv.NewWriter(out)
    writer.Comma = '\t'

    readCols := make([]interface{}, len(colNames))
    writeCols := make([]string, len(colNames))
    for i, _ := range writeCols {
        readCols[i] = &writeCols[i]
    }

    for rows.Next() {
        err := rows.Scan(readCols...)
        if err != nil {
            return err
        }
        writer.Write(writeCols)
    }

    if err = rows.Err(); err != nil {
        return err
    }

    writer.Flush()
    return nil
}

This code allows you to read columns from a Rows instance and convert them into a []string for further processing.

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