Limit a LEFT JOIN Subquery to 1 result

Question

The query below seems to LIMIT all the results when it's being LEFT JOINed, so the total in the subquery is just 1. How can I make it LIMIT so that I get a 1:1 match between projects rows and the last date stored in projects_hours_archive which stores records of projects.projected_hours that are collected on a cron job once per week?

projected_hours_archive has columns: id , project_id , hours and datetime .

SELECT
    GROUP_CONCAT( projected_hours_last.date, '|', projected_hours_last.number ) AS 'projected_last_info'
FROM
projects


LEFT JOIN (
    SELECT *
    FROM
    projected_hours_archive
    ORDER BY date DESC
    LIMIT 1
) AS projected_hours_last ON ( projected_hours_last.project_id = projects.id )

WHERE projected_hours > 0

GROUP BY projects.id

I tried to adopt using MySQL Limit LEFT JOIN Subquery after joining but wasn't successful. If I remove the LIMIT in the subquery I get too many results.

Answer 1

use group by in the sub query and get maximum date per project.

EDIT: as per the OP comment, adding second max date.

The trick from this mysql how to get 2nd highest value with group by and in a left join is used.

SELECT
    GROUP_CONCAT( projected_hours_last.secondMaxDate, '|', projected_hours_last.number ) AS 'projected_last_info'
FROM
projects


LEFT JOIN (
    SELECT project_id, max(date) as maxDate,
           substring_index(substring_index(group_concat(date order by date desc), ',', 2), ',', -1
                            ) as secondMaxDate
    FROM
    projected_hours_archive
    group by project_id
) AS projected_hours_last ON ( projected_hours_last.project_id = projects.id )

WHERE projected_hours > 0

GROUP BY projects.id

Answer 2

I had the same question.

I suppose your field projects.project_id is unique (so there are no duplicates in that table ). Otherwise, you can use DISTINCT(projects.project_id) or GROUP BY projects.project_id as you've used before.

Solution #1 (using GROUP BY while joining):

        SELECT  a1.project_id,
                GROUP_CONCAT(a2.date, '|', a2.number ) AS 'projected_last_info'
        FROM projects a1
        LEFT JOIN (
            SELECT b1.project_id, b1.date, b1.number
            FROM projected_hours_archive b1 
            ORDER BY b1.date DESC
            GROUP BY b1.project_id
        ) a2 ON a2.project_id = a1.project_id
        WHERE a1.projected_hours > 0

Solution #2 (using LIMIT directly in subquery in SELECT statement):

        SELECT  a1.project_id,
                (SELECT GROUP_CONCAT(a2.date, '|', a2.number )
                    FROM projected_hours_archive b1
                    WHERE b1.project_id = a1.project_id
                    ORDER BY b1.date DESC
                ) AS 'projected_last_info'
        FROM projects a1
        WHERE a1.projected_hours > 0