When are MYSQL delimiters useful? - mysql

Question

I am writing SQL to create a trigger for a table in my database. This code should be in a script we can run whenever we spawn a new database.
I have been reading the SQL doc to understand how to do this and eventually came up with a query of this type:
DELIMITER $$
CREATE TRIGGER calendar_event_after_insert AFTER INSERT
ON calendar_event
FOR EACH ROW
BEGIN
IF #log_calendar_event_id = 'YES'
THEN
...
END IF;
IF #log_calendar_event_name = 'YES'
THEN
...
END IF;
...
END
$$
DELIMITER ;
Now I am getting an error on my use of DELIMITER, and I admit I don't understand much of how that works. My basic understanding is that since there are multiple queries inside this big one that all end with a ; I need to tell sql when this query ends using another delimiter.
After reading a bit about delimiters this seems like the right syntax to me, and on top of that i have been running this query in sequel pro and it works at times and at times it does not (weird, I know).
So eventually I decided to remove the delimiters part and everything works smoothly. I am a bit worried this will have side effects tho.
So my question is: when are delimiters used? are they useful in my case? if so, what am I doing wrong here?
EDIT as requested in comments
This is the error I get when running the query:
You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use
near 'DELIMITER $$ CREATE TRIGGER calendar_event_after_insert AFTER
INSERT ON calenda' at line 1
FYI we have an old version of MYSQL installed on our dev machines -- MYSQL 5.1.73

Answer 1

DELIMITER is not part of the SQL language. It's a command of the official command-line tool:
If you use the mysql client program to define a stored program containing semicolon characters, a problem arises. By default, mysql itself recognizes the semicolon as a statement delimiter, so you must redefine the delimiter temporarily to cause mysql to pass the entire stored program definition to the server.
To redefine the mysql delimiter, use the delimiter command.
There're some third-party programs that also implement a delimiter command in order to be able to run complex MySQL scripts (e.g. HeidiSQL).
If you are running your script in a context that doesn't allow multiple statements (e.g. a typical PHP application) the command will neither exist nor be needed at all.