MYSQL: COUNT with GROUP BY, LEFT JOIN and WHERE clause doesn't return zero values

Question

Hi guys, thanks in advance for any help on this topic!

I'm sure this has a very simple answer, but I can't seem to find it (not sure what to search on!). A standard count / group by query may look like this:

SELECT COUNT(`t2`.`name`) 
FROM `table_1` `t1` 
    LEFT JOIN `table_2` `t2` ON `t1`.`key_id` = `t2`.`key_id` 
WHERE `t1`.`another_column` = 123 

and this works as expected, returning 0 if no rows are found. However:

SELECT COUNT(`t2`.`name`) 
FROM `table_1` `t1` 
    LEFT JOIN `table_2` `t2` ON `t1`.`key_id` = `t2`.`key_id` 
WHERE `t1`.`another_column` = 123 
GROUP BY `t1`.`any_col` 

only works if there is at least one row in table_1 and fails miserably returning an empty result set if there are zero rows. I would really like this to return 0! Anyone enlighten me on this? Beer can be provided in exchange if you are in London ;-)

Answer 1

The reason it returns zero rows is that you are grouping on a value in table_1. SInce there are no values in table_1, there are no rows to return. Said another way, if you returned t1.any_col in your query from the GROUP BY like so:

SELECT `t1`.`any_col`, COUNT(`t2`.`name`) 
FROM `table_1` `t1` 
    LEFT JOIN `table_2` `t2` ON `t1`.`key_id` = `t2`.`key_id` 
WHERE `t1`.`another_column` = 123 
GROUP BY `t1`.`any_col` 

What would display for t1.any_col when there were no rows? The only way to achieve what you want is to union your results with another query that checks for no rows in table_1. In this example, I'm using the INFORMATION_SCHEMA view simply to have something against which I can query.

SELECT COUNT(`t2`.`name`) 
FROM `table_1` `t1` 
    LEFT JOIN `table_2` `t2` ON `t1`.`key_id` = `t2`.`key_id` 
WHERE `t1`.`another_column` = 123 
GROUP BY `t1`.`any_col` 
UNION ALL
SELECT 0
FROM INFORMATION_SCHEMA.TABLES
Where Not Exists( Select 1 From `table_1` )
LIMIT 1

Answer 2

Here's the better solution:

SELECT COUNT(*) FROM
(SELECT `t2`.`name`
FROM `table_1` `t1` 
    LEFT JOIN `table_2` `t2` ON `t1`.`key_id` = `t2`.`key_id` 
WHERE `t1`.`another_column` = 123 
GROUP BY `t1`.`any_col`) tmp

You just need to wrap SELECT COUNT(*) around the query.

The problem is that COUNT() changes from meaning "give me a count of the rows in the result set" to an aggregate function meaning "give me a count of the rows for each group that match my criteria" when used with the GROUP BY clause. In your case, since there are no rows for the group where another_column = 123, nothing is returned.

If you wrap that in another SELECT COUNT(*), you're again saying, "give me a count of the rows in the result set".

In the first query, COUNT() is working with GROUP BY. But with no criteria specified, you're getting at least one row returned from table_1. Since there are no corresponding table_2 records, NULL evaluates to 0 for COUNT(). Don't use the first query. Though it returns what you're expecting, it's only by chance. For the query to be meaningful, you should specify criteria for t1.another_column in the WHERE clause.

The second query works because you're asking for a row count of the result set. Since there are no rows where t1.another_column = 123, there are 0 rows returned, and COUNT() evaluates to 0.