Use case statements in large sql query instead of many small ones for counts

From http://www.dbapool.com/downloads/whitepapers/tipsforesql.pdf

FROM emp
WHERE sal < 2000;
SELECT COUNT
FROM emp
WHERE sal BETWEEN 2000 AND 4000;
SELECT COUNT
FROM emp
WHERE sal>4000;
However, it is more efficient to run the entire query in a single statement. Each number is
calculated as one column. The count uses a filter with the CASE statement to count only the
rows where the condition is valid. For example:
SELECT COUNT (CASE WHEN sal < 2000
THEN 1 ELSE null END) count1,
COUNT (CASE WHEN sal BETWEEN 2001 AND 4000
THEN 1 ELSE null END) count2,
COUNT (CASE WHEN sal > 4000
THEN 1 ELSE null END) count3
FROM emp;