The rough idea is we have some sql like this which is written for postgres:

SELECT

STRING_AGG(u.firstname || ' ' || u.lastname, ',') creators

FROM {user}

This sql could be in code somewhere, or in a customsql report. But it's gonna break when we run it on mysql or oracle. The current correct way to fix it is to rip the sql apart and call

$sqlchunk = sql_group_concat(u.firstname || ' ' || u.lastname, ',')

$sql = '....' $sqlchunk . '....';

I'm just imagining a better world where we can do this all in "sql" and the dml libraries internally translate it into the right sql:

SELECT

group_concat(u.firstname || ' ' || u.lastname, ',') creators

FROM {user}

the exact syntax doesn't matter, it might need to have some sort of tokens to make it all work: 

SELECT

{{group_concat(u.firstname || ' ' || u.lastname, ',')}} creators

FROM {user}

This would make a lot of code much easier and more natural to write. This would apply to a bunch of the sql_* helpers functions