php error happened

Technical information
Error type256
Error descriptionQuery execution was interrupted, maximum statement execution time exceeded
URLclasses.amarillo.lk/booking/class_booking_list.php?
Error fileC:\Inetpub\vhosts\amarillo.lk\classes.amarillo.lk\booking\connections\Connection.php
Error line626
SQL querySELECT `Day` AS `alxlg6vyc5` FROM ( SELECT `time-slots`.`ID`, `time-slots`.`Day`, `time-slots`.`Instructor`, `time-slots`.`Time_Slot`, `instructors`.`Instrument`, GROUP_CONCAT(instructors.Instrument SEPARATOR ', ') as `Instruments`, `reservation`.`Availability`, `reservation`.`Extra_Notes` FROM `time-slots` LEFT OUTER JOIN `reservation` ON (`time-slots`.`Instructor` = reservation.Teacher) AND (`time-slots`.`Time_Slot` = reservation.`Time`) AND (`time-slots`.`Day` = reservation.`Day`) LEFT OUTER JOIN `instructors` ON `time-slots`.`Instructor` = instructors.Instructor GROUP BY `time-slots`.`Day`,`time-slots`.`Instructor`,`time-slots`.`Time_Slot`,`reservation`.`Availability`,`reservation`.`Extra_Notes` ) a WHERE ( NOT ( `Day` is null or `Day` = '' ) ) GROUP BY `Day`

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