SQL语句中格式化处理null值

SELECT *
FROM Post_index i LEFT OUTER JOIN
          (SELECT Index_Id, CASE WHEN COUNT(*) IS NULL THEN '0' ELSE COUNT(*)
               END AS num
         FROM [Post_Reply]
         GROUP BY Index_Id) R ON i.ID = R.Index_Id
ORDER BY i.ID DESC

一直是这么写的，可sql语句一执行就发现原来是错的。找了找原因，原来是因为LEFT JOIN的原因，右边的数据记录更本就不存在，改成：

SELECT i.*, ISNULL(R.num, 0) AS Expr1
FROM Post_index i LEFT OUTER JOIN
          (SELECT Index_Id, COUNT(*) AS num
         FROM [Post_Reply]
         GROUP BY Index_Id) R ON i.ID = R.Index_Id
ORDER BY i.ID DESC