Source: placeholder.go in package github.com/Masterminds/squirrel

package squirrel

import (
	"bytes"
	"fmt"
	"strings"
)

PlaceholderFormat is the interface that wraps the ReplacePlaceholders method. ReplacePlaceholders takes a SQL statement and replaces each question mark placeholder with a (possibly different) SQL placeholder.

type PlaceholderFormat interface {
	ReplacePlaceholders(sql string) (string, error)
}

type placeholderDebugger interface {
	debugPlaceholder() string
}

Question is a PlaceholderFormat instance that leaves placeholders as question marks.

	Question = questionFormat{}

Dollar is a PlaceholderFormat instance that replaces placeholders with dollar-prefixed positional placeholders (e.g. $1, $2, $3).

	Dollar = dollarFormat{}

Colon is a PlaceholderFormat instance that replaces placeholders with colon-prefixed positional placeholders (e.g. :1, :2, :3).

	Colon = colonFormat{}

AtP is a PlaceholderFormat instance that replaces placeholders with "@p"-prefixed positional placeholders (e.g. @p1, @p2, @p3).

	AtP = atpFormat{}
)

type questionFormat struct{}

func (questionFormat) ReplacePlaceholders(sql string) (string, error) {
	return sql, nil
}

func (questionFormat) debugPlaceholder() string {
	return "?"
}

type dollarFormat struct{}

func (dollarFormat) ReplacePlaceholders(sql string) (string, error) {
	return replacePositionalPlaceholders(sql, "$")
}

func (dollarFormat) debugPlaceholder() string {
	return "$"
}

type colonFormat struct{}

func (colonFormat) ReplacePlaceholders(sql string) (string, error) {
	return replacePositionalPlaceholders(sql, ":")
}

func (colonFormat) debugPlaceholder() string {
	return ":"
}

type atpFormat struct{}

func (atpFormat) ReplacePlaceholders(sql string) (string, error) {
	return replacePositionalPlaceholders(sql, "@p")
}

func (atpFormat) debugPlaceholder() string {
	return "@p"
}

Placeholders returns a string with count ? placeholders joined with commas.

func Placeholders(count int) string {
	if count < 1 {
		return ""
	}

	return strings.Repeat(",?", count)[1:]
}

func replacePositionalPlaceholders(sql, prefix string) (string, error) {
	buf := &bytes.Buffer{}
	i := 0
	for {
		p := strings.Index(sql, "?")
		if p == -1 {
			break
		}

		if len(sql[p:]) > 1 && sql[p:p+2] == "??" { // escape ?? => ?
			buf.WriteString(sql[:p])
			buf.WriteString("?")
			if len(sql[p:]) == 1 {
				break
			}
			sql = sql[p+2:]
		} else {
			i++
			buf.WriteString(sql[:p])
			fmt.Fprintf(buf, "%s%d", prefix, i)
			sql = sql[p+1:]
		}
	}

	buf.WriteString(sql)
	return buf.String(), nil