如果没有匹配项，MySQL返回默认记录（无LIMIT）

Question

I have a table like this:

-----------------------------------------
|  ID   |  ACTIVE  |       MESSAGE      |
-----------------------------------------
|   0   |     0    |   Default message  |
-----------------------------------------
|   1   |     1    |   Message one      |
-----------------------------------------
|   2   |     1    |   String two       |
-----------------------------------------
|   3   |     0    |   Text three       |
-----------------------------------------

I have been looking for quite a few hours for a MySQL query showing me the record with ID = 0 as default result if no message is 'active'. If there are one or more active message(s) I like to get these records.

SELECT * FROM table WHERE ACTIVE = 1;

Also, an option would be to add the default record manually in the query by a UNION or similar idea. Adding the default record always, ORDER and then LIMIT 1 doesn't work for me, as I need to get back all active messages if existing, not one only.

One more constraint: My table and the WHERE condition are more complex in real world, dealing with start and end timestamp comparison.

Answer 1

What about this (untested):

SELECT * FROM table WHERE id = 0 AND NOT EXISTS (SELECT * FROM table WHERE active = 1)
UNION SELECT * FROM table WHERE ACTIVE = 1;

That's probably not very efficient, but it depends on the number of records.
An index on active could help, in case.

It should fit your question: if there are active = 1 rows, they're returned from the second query of the union, while the first part resolves to the empty set (as the EXISTS subquery would fail). If there are not, the second query returns nothing while the first part returns the row with id = 0 , as the EXISTS subquery now will succeed.

Answer 2

Save the default message (id = 0) in your actual application code, just query it once and save it. Use the query you've wrote up there, if there are no results, then use the default message that you've saved in your application code.

What you're trying to do isn't very clean, there's ways to do it all within MySQL, as watery's answer demonstrates, but it's not efficient and not the best way to handle your issue.