Multiple-row functions和Single-row functions有什么区别解决办法

Multiple-row functions和Single-row functions有什么区别
求高手各举个例子说明

------解决方案--------------------
单行函数：
每一行都有一个结果
SQL> SELECT last_name,TRUNC((SYSDATE-hire_date)/7,0) AS weeks
2 FROM employees
3 WHERE department_id = 90;

LAST_NAME WEEKS
-------------------------------------------------- ----------
King 1267
Kochhar 1148
De Haan 976

多行函数只有一个结果
SQL> SELECT count(distinct salary) FROM employees ;

COUNT(DISTINCTSALARY)
---------------------
57