Learning MySQL - TRIGGERS 

If you are talking about multiple databases that are connected and sharing responsibility for the same data then that would be a cluster - dev.mysql.com/doc/mysql-cluster-excerpt/5.7/en/mysql-cluster-basics.html If you are talking about different databases storing different data then you need to use other tools / write your own code to manage that concurrently.

@@SteveGriffith-Prof3ssorSt3v3 I needed clustering. Thank you for clarifying.

Triggers are only running within the database. They cannot trigger things to happen outside the database with your code. They can cause changes within the data, like setting a flag value in a table. Your code can check the database to see if the flag is set within the data and then decide to do something.

Ok. Thanks.

WHERE clauses filter based on values in columns. They can't work if the column doesn't exist within the scope of the current query. The Trigger is like another query running on its own.

Triggers are tied to changes made to data in a table. If you add or delete or update data in a table, then that will cause your trigger to run. Triggers don't monitor values and compare the values against some other value to run some procedure. If you want things to happen on a schedule, like checking once per day if the value in a column called "expiration date" has reach some threshold, then you need to do that outside the database. Cron tasks and similar things can run a server-side script which looks at the data and decides what to do. I would probably create a VIEW of all the expiry dates and the row ids for each item and have that view sorted/indexed by the date. The VIEW will always reflect the latest data and is an easy thing to run queries against. The result of the query can be used to update whatever you want.

@@SteveGriffith-Prof3ssorSt3v3 thank you so much for your reply! I'll look into it; unfortunately I'm starting out with databases and such, do you happen to have any resources I may look into to get it done? Again, thank you for being such a great help!

@@camiloandreshernandez1234 This playlist is my only relational DB resource.

You might want to look into MySQL Scheduled Tasks. dev.mysql.com/doc/refman/8.0/en/events-overview.html I don't have a video on them but they might help you.

@@SteveGriffith-Prof3ssorSt3v3 thank you so much Steve! I'll be sure to check it out :D

A trigger is on every operation. Maybe consider a stored procedure

Oracle, msaccess, SQL server, mysql, etc. they all have minor differences in what characters they use and slight variations in their version of SQL. Sometime * is a wildcard. Sometimes % is the wildcard. Sometimes you need to use double quotes and sometimes not. Backticks work for some but not others.

Not from MySQL. No.

@@SteveGriffith-Prof3ssorSt3v3 Thanks sir for you quick reply, u have a fan here

triggers are more of a row-level thing. Delete a row and then do something with the deleted value or insert a row and do something with the inserted value - like check if it is a valid value. A trigger can access both old and new data in its own table. A trigger can also affect other tables, but it is not permitted to modify a table that is already being used (for reading or writing) by the statement that invoked the function or trigger. I would suggest creating a stored procedure that you can run from your server-side code each time you do an insert. OR Call the stored procedure from the Trigger. Here is a quick reference for doing this - www.mysqltutorial.org/mysql-triggers/mysql-call-stored-procedure-from-trigger/ OR Create an Event that runs on a schedule - dev.mysql.com/doc/refman/5.7/en/create-event.html - like a scheduled task that checks the size of the table once an hour and

@@SteveGriffith-Prof3ssorSt3v3 thanks for the quick reply, I went with the event scheduler. Didnt know this existed, thanks alot!

Triggers only run on the table where they were created. They can't cause a change in another table. However, if you call a stored procedure instead of just an UPDATE command, then you can use the trigger to set a value in a variable like in this example - dev.mysql.com/doc/refman/5.7/en/trigger-syntax.html - and then, in the Stored Procedure, run your next SQL command too update the other table.

@@SteveGriffith-Prof3ssorSt3v3 Wow! Thank you so much! You're awesome!

We are taking the NEW value that gets passed in and converting that NEW value to uppercase. We are not touching the OLD value. We don't care about it any more. We are going to be replacing it with the NEW value.

Yes. This stackoverflow answer has a good code sample stackoverflow.com/questions/22170758/mysql-timestamp-fields-created-modified

Did you create the employee table inside one of the system databases?

omg the aircrafts question, my class as well

It says that you have not created the function or stored procedure called `char_race` in your database.

Having an automatic new id is what setting your primary key column to AUTO_INCREMENT is for.

@@SteveGriffith-Prof3ssorSt3v3 Thank you! Keep making videos! I look forward to seeing more of them.

Do a trigger on insert that calls a stored procedure. The stored procedure could do things like check the number of records in the table. Here is a reference on combining triggers with stored procedures - www.mysqltutorial.org/mysql-triggers/mysql-call-stored-procedure-from-trigger/

If you are using PhpMyAdmin, open a table and you should see a tab at the top called "triggers".

@@SteveGriffith-Prof3ssorSt3v3 Thanks :)

I am a web developer. I have to be able to test what I build on multiple platforms.