[QUOTE][i]Originally posted by lesstjm [/i]
[B]Try a before insert trigger. Check if there are two message text in there already. If there is don't insert. [/B][/QUOTE]
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In the Before Insert Triger:

IF NoOfMessage >= 2 THEN
delete ones which has not the latest date on it;
insert into the new message;
ELSE
insert into the new message;
END IF;