COALESCE two dates returns binary?

问题

Why does COALESCE(date1,date2) return Blob(binary)? Both columns are of type DATETIME.

Here's the complete sql query:

SELECT COALESCE( last_modified, date_purchased )As LastModifiedOrPurchased
FROM Orders
ORDER BY COALESCE( last_modified, date_purchased )DESC
LIMIT 1;

I'm normally using SQL-Server as rdbms but need to write a synchronization tool. Now i'm trying to select one of two dates. If last_modified is null date_purchased should be returned.

But even if there are multiple records where both dates are not null, the query returns just Blob. How to get the correct last date?

Edit: on SQL-Fiddle it is not reproducable with MySql 5.1.61 (i'm on 5.0.51).

回答1:

This is probably a bug that has been fixed: bug-35662

[28 Mar 2008 22:44] Patrick Crews

Description:
COALESCE(date, date) returns varbinary on 5.0.51a for Windows XP 32bit (only tested this flavor)

COALESCE(date,date) returns date on Mac OS

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[31 Mar 2008 17:41] Omer BarNir
The bug was fixed in 5.0.52 and was not observed when testing 5.0.56 - closing

回答2:

I still don't know why this happens in MySql Workbench (and also MS Visual Studio). But i have found a workaround. I just cast the result to DATETIME and it works as expected:

SELECT CAST(COALESCE( last_modified, date_purchased )AS DATETIME)As LastModifiedOrPurchased
FROM Orders
ORDER BY COALESCE( last_modified, date_purchased )DESC
LIMIT 1;

Edit: as Jack has commented, IFNULL works also:

SELECT IFNULL( last_modified, date_purchased )As LastModifiedOrPurchased
FROM Orders
ORDER BY IFNULL( last_modified, date_purchased )DESC
LIMIT 1;

来源：https://stackoverflow.com/questions/13761960/coalesce-two-dates-returns-binary