Trigger raise error

[sinva]

I have never written a trigger before and now am seeing the light. Is
there a way to write a trigger so that if a user changes on a column A in
a single row on table A then the trigger could check if other table B has a column X equals to table A column B and table B column Y has a value equals to "A" will raise an error. Thanks,

Create table A
(
A varchar(10),
B varchar(10)
)


Create table B
(
X varchar(10),
Y varchar(10)
)