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DELIMITER // What is the of use of it?
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A delimiter is a sequence of one or more characters for specifying the boundary between separate, independent regions in plain text, mathematical expressions or other data streams.
A delimiter is a simple or compound symbol that has a special meaning to PL/SQL. For example, you use delimiters to represent arithmetic operations such as addition and subtraction.
You define a DELIMITER to tell the mysql client to treat the statements, functions, stored procedures or triggers as an entire statement. Normally in a . sql file you set a different DELIMITER like $$. The DELIMITER command is used to change the standard delimiter of MySQL commands (i.e. ;).
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.
It changes the statement delimiter from ; to //. This is so you can write ; in your trigger definition without the MySQL client misinterpreting that as meaning you're done with it.
Note that when changing back, it's DELIMITER ;, not DELIMITER; as I've seen people try to do.
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In SQL you close each statement with a delimiter, which is by default a semicolon (;). In a trigger you need to write multiple statements, each ending in a semicolon. To tell MySQL that those semicolons are not the end of your trigger statement, you temporarily change the delimiter from ; to //, so MySQL will know that the trigger statement only ends when it econunters a //.
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