SQL If statement

I wanna ask...

How does a IF statement structure look like?

for example:

Select <column name> from <table name>
[Optional]
Where <column name> (= / < / >)

When would one person use a If statement properly in a enterprise programming software/database. If I would to use if statement in Triggers where would the error comes out if the "IF" statement have errors. Is there a try and catch in SQL statement?

NHK