add QMarks(num int) helper function

[jmhodges]

It's a common pattern to have to break up JOINs that are too expensive for the database into separate queries. But doing that naively causes the n+1 query problem. So, one gathers data in one query for the left side with whatever LIMITs or munging was needed, batches that data up with an IN operator in one or a few more queries, and goes home.

But using that IN operator means having to write this function (or similar) in every project that does so:

func QMarks(num int) string {
  if num == 0 {
    return "()"
  }
  return "(?" + strings.Repeat(",?", num-1) + ")"
}

This gets repetitive. It would be nice if, since slices are disallowed from being used in the driver, this function could live in the driver so folks can avoid having to reproduce this each time they need to solve a problem with the IN operator.

[mbyio]

For this use case, I've found I usually want something more generic, like

func joinRepeated(s, sep string, n int) string {
	var buf strings.Builder
	for i := 0; i < n; i++ {
		if i > 0 {
			buf.WriteString(sep)
		}
		buf.WriteString(s)
	}
	return buf.String()
}

Basically, it's like strings.Join except it also does strings.Repeat.

Then you can use it like

fmt.Sprintf(
  "SELECT * FROM my_table IN (%s)",
  joinRepeated("(?,?)", ",", len(myTuples)),
)

If you use strings.Repeat as demonstrated, you'll do unnecessary allocations/copies, and it isn't as flexible.

I mean honestly IMO it would be even better if the driver could just handle this for us if interpolateParams is enabled. But that's obviously a much more complicated feature.