MySQL SELECT COUNT> 0作为布尔值

Question

I'm trying to get a true/false value in a column in a select statement which indicates if a count is >0 or not.

I read these docs: http://dev.mysql.com/doc/refman/5.1/en/expressions.html

Which I don't fully understand but they seemed to indicate I could use '>>' in my expression.

So I tried this:

SELECT COUNT(*)>>0 AS my_bool FROM table GROUP BY id;

It runs and doesnt generate errors or warnings, however the my_bool column just contains the results of the COUNT(*) . Is what I'm trying to do possible, if so how?

PS Yes I know I could just test for x>y in my code that handles the results, I want to know if it is something that can be done in MySQL alone (feel free to explain why it's not advisable, by all means)

Answer 1

In your example you are not using the greater than operator, but a bit shift (>>) . If you use

SELECT COUNT(*) > 0 AS my_bool FROM table GROUP BY id;

my_bool will contain 0 or 1 as boolean value.

Answer 2

Use the if condition to check and return the true/false.

Example:

SELECT if(COUNT(*)>0,'true','false') AS my_bool FROM table GROUP BY id;

If will check that if the count value is greater than 0 then return true else return false.

Answer 3

Try the following:

SELECT (COUNT(*) <> 0) AS my_bool 
FROM table 
GROUP BY id;

Answer 4

While I think Gene's answer is the better one, you could also use a subquery to do this:

SELECT (
    SELECT COUNT(*) AS my_bool FROM table GROUP BY id
 ) != 0;

If COUNT(*) is zero, the query returns 0, otherwise 1.

Answer 5

You could do this -

SET @countValue = (SELECT COUNT(*) FROM table GROUP BY id);  
if @countValue > 0
    return 1;
else
    return 0;