How to join array of strings into single comma-separated string in GROQ?

Great question! Yes, GROQ has a built-in function for exactly this: array::join().

You can modify your query like this:

*[_type == "technologyProduct"]{
   "sectors": array::join(targetIndustries[]->title, ", "),
}

The array::join() function takes two parameters:

1. An array of strings, numbers, or booleans
2. A separator string (in your case ", " for comma-separated)

It returns a single concatenated string.

According to the GROQ Functions Reference, the syntax is:

array::join(source <array[string|number|boolean]>, separator<string>) <string>

So in your case, targetIndustries[]->title produces an array of strings, and array::join() combines them into a single comma-separated string.

Note that if any element in the array isn't a string, number, or boolean, the function will return null. Also, if the input itself isn't an array (like if it's a single value), it will return null as well.

You can find more examples in the Query Cheat Sheet which shows examples like array::join(["a", "b", "c"], ".") returning "a.b.c".