Joe Celko's Thinking in Sets: Auxiliary, Temporal, and Virtual Tables in SQL

13.2. Methods of Attack

This is a simple problem, but the replies to it illustrate approaches to solutions in any SQL. The problem is a single-column table that holds the time in seconds from some starting time (time zero or t₀) of some event. For simplicity, the event times are captured in even seconds from a time zero.

CREATE TABLE Events
(event_time DECIMAL(4,1) NOT NULL PRIMARY KEY
 CHECK(event_time  =  ((10 * event_time)/ 10))
 -- whole seconds);
INSERT INTO Events
VALUES (500), (505), (510), (535),
       (910), (939), (944), (977);

I need to assign a group number to the above values, based on a time interval of 30 seconds. The numbering does not matter, just as long as the numbering increases with the event times. For example, this is a correct result ...