There are several ways to simply count the number of rows in two tables using single query.
select ( select count(*) from Table1 ) + ( select count(*) from Table2 ) as total_rows from my_one_row_table
select sum(rows) as total_rows from ( select count(*) as rows from Table1 union all select count(*) as rows from Table2 ) as u
UNION ALL is necessary here, not UNION, to guard against the case when both tables have the same number of rows, because UNION would discard one of the duplicate counts! Note also that no GROUP BY is necessary in the main query, because all the rows (produced by the UNION ALL subquery) are considered one group.
- We could also use a CROSS JOIN:
select t1.rows + t2.rows as total_rows from ( select count(*) as rows from Table1 ) as t1 cross join ( select count(*) as rows from Table2 ) as t2
The cross join works because each derived table has only one row.