I have a table which stores which teacher (teacherid
) works for which group (groupid
) of pupils starting from the date startdate
:
CREATE TABLE `pupilgroupteacher` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT, `groupid` smallint(5) unsigned NOT NULL, `startdate` date NOT NULL, `teacherid` int(10) unsigned DEFAULT NULL, PRIMARY KEY (`id`), UNIQUE KEY `date` (`groupid`,`startdate`), KEY `teacher` (`teacherid`), KEY `group` (`groupid`), CONSTRAINT `fk_pupilgroupteacher_2` FOREIGN KEY (`groupid`) REFERENCES `pupilgroup` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION, CONSTRAINT `fk_pupilgroupteacher_1` FOREIGN KEY (`teacherid`) REFERENCES `employee` (`personid`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin
Having a teacher ID and a month, I need to find all groups for which this teacher worked (or is going to work) at least one day in the given month.
Note: For a given teacherid
there may be more than one startdate
(if, for example, a teacher worked for a group, was replaced with another teacher and then replaced back on a later startdate
).
Best Answer
Another way to get the result is this. It finds first all groups that the teacher has surely taught (or is going to) by checking that she has started within the month and then in another subquery it finds - for every group - the last teacher that started at the first day of the month or earlier.
With the unique index you have on the table, the second subquery should be quite efficient. The first subquery would benefit from an index on
(teacherid, startdate, groupid)
: