SELECT max(x)返回null; 怎么能让它返回0？

Question

运行以下命令时,如何返回0而不是null:

SELECT MAX(X) AS MaxX
FROM tbl
WHERE XID = 1

(假设没有XID = 1的行)

Answer 1

要么:

SELECT coalesce(MAX(X), 0) AS MaxX
FROM tbl
WHERE XID = 1

Answer 2

在SQL 2005/2008中:

SELECT ISNULL(MAX(X), 0) AS MaxX
FROM tbl WHERE XID = 1

Answer 3

像这样(对于MySQL):

SELECT IFNULL(MAX(X), 0) AS MaxX
FROM tbl
WHERE XID = 1

对于MSSQL替换IFNULL用ISNULL或用于Oracle使用NVL

Answer 4

你也可以使用COALESCE(表达式[,... n]) - 首先返回非null,如:

SELECT COALESCE(MAX(X),0) AS MaxX
FROM tbl
WHERE XID = 1

Answer 5

甲骨文会的

SELECT NVL(MAX(X), 0) AS MaxX
FROM tbl
WHERE XID = 1;