Filtering down specific criteria for all rows and return only one row for each column with the same value

Question

I have house_leases and house_lease_terms (see table schemas below). A house_lease can have multiple house_lease_terms however there can only be one "active" term at a time.

Table Definitions:

CREATE TABLE `house_leases` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `house_id` int(10) unsigned NOT NULL,
  PRIMARY KEY (`id`)
);

CREATE TABLE `house_lease_terms` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `house_lease_id` int(10) unsigned NOT NULL,
  `date_start` datetime NOT NULL,
  `date_end` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `house_lease_terms_house_lease_id_foreign` (`house_lease_id`),
  CONSTRAINT `house_lease_terms_house_lease_id_foreign` FOREIGN KEY (`house_lease_id`) REFERENCES `house_leases` (`id`) ON DELETE CASCADE
);

As you can see the house_lease_terms.house_lease_id corresponds to a specific house_lease, however there can be multiple rows with the same house_lease_id.

The rules for determining the "active" terms are:

date_start <= NOW() AND date_end > NOW() OR date_end IS NULL

If no rows are returned then the "active" terms must be in the future, so then the rules change to be:

date_start > NOW()

We order by date_start DESC if the terms are not in the future since multiple rows could be returned we want the latest date_start at the top of the results. Otherwise we sort by date_start ASC because we want the closest date_start to now to be at the top.

I then limit by 1 to get only one result and that row is considered the "active" terms. If no results come back, then there are no "active" terms.

I have a SQL statement that has this logic for getting a specific house_lease_id. That looks like this:

SELECT * FROM house_lease_terms
WHERE 
CASE 
    WHEN 
        (SELECT COUNT(*) FROM house_lease_terms WHERE date_start <= NOW() AND (date_end > NOW() OR date_end IS NULL) AND house_lease_id = 1)
    THEN 
        date_start <= NOW() AND (date_end > NOW() OR date_end IS NULL)
    ELSE
        date_start > NOW()
END
AND house_lease_id = 1
ORDER BY
    IF(
        (SELECT COUNT(*) FROM house_lease_terms WHERE date_start <= NOW() AND (date_end > NOW() OR date_end IS NULL) AND house_lease_id = 1), 
        unix_timestamp(date_start), 
        -unix_timestamp(date_start)
    ) DESC
LIMIT 1;

This statement works, but I wish there was a better way (more efficient) of fetching the "active" terms for a specific house_lease_id (If you know a better solution please share).

Now I need to have a query that will fetch the "active" terms for all the different house_lease_id's.

I don't want any type of custom MySQL function or stored procedure to do this. I don't know where to start to create this query. I figure I can use the query from above in some sub select or join, but am not sure how I would do so.

Any help will be appreciated!

SQLFiddle with data: http://sqlfiddle.com/#!9/cab159/2/0

Answer 1

Must be missing something, but isn't this simply what you want?

SELECT house_lease_id, MIN(date_start)
FROM house_lease_terms
WHERE date_end > NOW() OR date_end IS NULL
GROUP BY house_lease_id

The earliest term that hasn't ended yet.

Answer 2

Answer credit goes to Akina over at StackOverflow

SELECT *
FROM (
SELECT *,
       CASE WHEN @var2=house_lease_id 
            THEN @var1:=@var1+1 
            ELSE @var1:=1
            END row_number_in_house_lease_id,
       @var2:=house_lease_id
FROM house_lease_terms, (SELECT @var1:=0, @var2:=0) variables 
WHERE (
        (date_start <= NOW() AND (date_end > NOW() OR date_end IS NULL))
        OR (date_start > NOW())
      )
ORDER BY 
  house_lease_id,
  CASE WHEN (date_start <= NOW() AND (date_end > NOW() OR date_end IS NULL))
  THEN DATEDIFF(NOW(), date_start)
  ELSE DATEDIFF(date_start, NOW()) + 1000000
  END,
  id DESC
) AS cte
WHERE row_number_in_house_lease_id = 1;