Add aggregate expression for column collapse with separator

[romankonz]

The library exposes per-driver SQL builders for CONCAT, LEAST, SUBSTRING etc., but not the row-collapsing aggregate (GROUP_CONCAT / STRING_AGG). Add it.

API

public function getGroupConcatExpression(
    string $column,
    string $separator,
    bool $escapeValues = false,
): string;

Per-driver SQL

• MySQL: GROUP_CONCAT(<col> SEPARATOR '<sep>')
• Postgres: STRING_AGG(<col>::text, '<sep>')
• SQLite: GROUP_CONCAT(<col>, '<sep>')

The separator is embedded as a SQL string literal (single quotes doubled) — it cannot be a bind parameter because MySQL's SEPARATOR clause requires a literal at parse time.

Value escaping ($escapeValues = true)

Raw GROUP_CONCAT is lossy when any value contains the separator — the result is unsplittable. When $escapeValues = true, wrap the column in two nested REPLACE() calls so backslashes in values become \\ and separator occurrences become \<sep>:

• MySQL: GROUP_CONCAT(REPLACE(REPLACE(<col>, '\\', '\\\\'), '<sep>', '\\<sep>') SEPARATOR '<sep>')
• Postgres: STRING_AGG(REPLACE(REPLACE(<col>::text, E'\\', E'\\\\'), '<sep>', E'\\<sep>'), '<sep>')
• SQLite: same shape as MySQL (no E-string syntax)

Order matters: backslashes must be escaped before the separator, otherwise backslashes introduced by separator-escaping would be doubled.

Default is false — REPLACE on every row isn't free, and the simple form is fine when callers know their data won't contain the separator.

Decoding the escaped output is left to the consumer (walk the string, treat \X as literal X, split on unescaped <sep>).