Question

I have two tables that needs to be linked together, I want to count the number of instances of the category in the name table

SELECT category.name, COUNT(name.category) as availability FROM
        category LEFT JOIN name ON name.category = category.name
        GROUP BY name.category

This is great but I want the query to only count the names that is not yet over now. So I add a WHERE
condition time > now()

SELECT category.name, COUNT(name.category) as availability FROM
        category LEFT JOIN name ON name.category = category.name WHERE
        time > now()
        GROUP BY name.category

So far so good, this query returns only the rows that have the time that is farther from the future.
But it isn’t returning every category from the category table. This query returns only those categories that have instances from the name table that have set time farther from the future.

What I want is to return all the categories from the category table.. If the category don’t have an instance in the name table, I still want it to return but the availability will be 0

Answer 1

If time is a column from the name table, then all conditions that should be applied to the left joined table must be done in the ON clause. Otherwise, the WHERE clause will transform it back into an INNER JOIN:

SELECT category.name, COUNT(name.category) as availability FROM
    category LEFT JOIN name ON name.category = category.name AND
    time > now()
    GROUP BY category.name

Basically, the WHERE clause applies to the entire (potential) result set that has been generated from your FROM clause (and all of your JOINs). The conditions in the where clause must be met by a row in this result set in order for further processing to occur. But in the case of a LEFT JOIN, if the join hasn’t succeeded, the columns from that table will all be NULL. So unless your WHERE clause allows those columns to be NULL, they will not proceed further in the query.