oracle 提供相当于Oracle NVL的mysql函数
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mysql function to provide equivalent to Oracle NVL
提问by Stewart Robinson
I have a strange situation. I have to use NVL(columna, columnb) in Oracle and MySQL. I can't change the SQL as it is in a package I can't edit but it is the only thing that doesn't work in the application I have between MySQL and Oracle.
我有一个奇怪的情况。我必须在 Oracle 和 MySQL 中使用 NVL(columna, columnb)。我无法更改 SQL，因为它在我无法编辑的包中，但它是我在 MySQL 和 Oracle 之间的应用程序中唯一不起作用的东西。
How would I write NVL() in MySQL. I've looked here (http://dev.mysql.com/doc/refman/5.0/en/create-function-udf.html) and it looks like I have to write it in C and link it to MySQL.
我将如何在 MySQL 中编写 NVL()。我看过这里（http://dev.mysql.com/doc/refman/5.0/en/create-function-udf.html），看起来我必须用 C 编写它并将其链接到 MySQL。
However http://dev.mysql.com/doc/refman/5.0/en/create-procedure.htmlseems to say I can do it without compiling an add on.
但是http://dev.mysql.com/doc/refman/5.0/en/create-procedure.html似乎说我可以在不编译插件的情况下做到这一点。
I've tried this but it doesn't work. What am I doing wrong?
我试过这个，但它不起作用。我做错了什么？
CREATE FUNCTION NVL (first DATETIME, second DATETIME)
RETURNS DATETIME
begin
DECLARE first DATETIME;
DECLARE second DATETIME;
DECLARE return_value DATETIME;
SET return_value = IFNULL(first,second);
RETURN return_value;
end
回答by Quassnoi
Use COALESCE, it's much more simple:
使用COALESCE，就简单多了：
DELIMITER $$
CREATE FUNCTION NVL (first DATETIME, second DATETIME)
RETURNS DATETIME
BEGIN
RETURN COALESCE(first, second);
END
$$
DELIMITER ;
回答by Cody Caughlan
I think the COALESCE() method is what you need.
我认为 COALESCE() 方法是您所需要的。
http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#function_coalesce
http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#function_coalesce
