使用NVL顯示'none'而不是0

Question

我無法弄清楚如何用'none'替換0。我在這裏做錯了什麼？

SELECT s.student_id, s.first_name, s.last_name, NVL(TO_CHAR(COUNT(e.student_id)), 'none') AS enrollments 
FROM student s 
LEFT OUTER JOIN enrollment e 
ON s.student_id = e.student_id 
WHERE s.phone LIKE '%617%' 
GROUP BY s.student_id, s.first_name, s.last_name 
ORDER BY s.last_name, s.first_name; 

Answer 1

替換單個語句：

DECODE(COUNT(e.student_id), 0, 'none', COUNT(e.student_id)) AS enrollments

它所做的是它首先檢查計數;如果沒有適用的值，則COUNT將返回0.然後，它會將其解碼爲您的字符串。如果有記錄，它將返回COUNT函數。