Select random row if value is not found

[aviddv1]

Hey there,

I'm trying to write a query that will select 1 random row from a table only if the value 0 does not exist in a field called random.  If 0 does exist then i want it to return that row instead of randomly selecting one.

thanks,
ward

[fenway]

You can probably cheat with a clever UNION statement and a LIMIT clause, but why not just do this with multiple queries?

[shoz]

[quote author=aviddv1 link=topic=109771.msg442759#msg442759 date=1159400126]
Hey there,

I'm trying to write a query that will select 1 random row from a table only if the value 0 does not exist in a field called random.  If 0 does exist then i want it to return that row instead of randomly selecting one.

thanks,
ward
[/quote]

You should also be able to use the following
[code]
SELECT * FROM tablename ORDER BY IF(random = 0, -1, RAND()) ASC LIMIT 1
[/code]

[fenway]

[quote author=shoz link=topic=109771.msg442823#msg442823 date=1159410902]
You should also be able to use the following
[code]
SELECT * FROM tablename ORDER BY IF(random = 0, -1, RAND()) ASC LIMIT 1
[/code]
[/quote]
How would that work? Which records would it examine?  And I didn't that that you use an expression in an ORDER BY clause.

[shoz]

[quote author=fenway link=topic=109771.msg443097#msg443097 date=1159454723]
[quote author=shoz link=topic=109771.msg442823#msg442823 date=1159410902]
You should also be able to use the following
[code]
SELECT * FROM tablename ORDER BY IF(random = 0, -1, RAND()) ASC LIMIT 1
[/code]
[/quote]
How would that work? Which records would it examine?  And I didn't that that you use an expression in an ORDER BY clause.
[/quote]

Just as when you "ORDER BY RAND()" each row is ordered based on the evaluation of the expression. In this case the return value of RAND().

[code]
ORDER BY IF(random = 0, -1, RAND())
[/code]
When MYSQL encounters a row whose "random" field contains the value 0 it will be ordered based on the value -1. Otherwise, the row is ordered based on the value returned by RAND().

Since -1 will be lower than any RAND() (between 0 and 1) value, if a row in the table has a "random" value of 0 it will be first in the result. If not, one of the other fields will be the first in the result and that will be based on the RAND() value.

The LIMIT 1 of course gives us only the first record.

[fenway]

Of course it can use an expression... my brain wasn't working.  Very clever trick!