Finding rows for a specified date range

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).

It seems that the following code solves my problem:

SELECT DISTINCT groupid
FROM pupilgroupteacher x
WHERE teacherid=@teacher AND startdatex.startdate AND startdate

(Here @month is the date of the first day of the specified month and @teacher is the teacher ID.)

But:

• is this code error-free? Please help to check it.

• can it be optimized to use fewer server resources?

StackExchange Database Administrators Q#46343, answer score: 3