Creates a temporary [Merge](../../engines/table-engines/special/merge.md) table. The table structure is taken from the first table encountered that matches the regular expression.
-`db_name` — Database name or a regular expression to match the DB names. You can use a constant expression that returns a string, for example, `currentDatabase()`.
-`tables_regexp` — A regular expression to match the table names in the specified DB or DBs.