Introduction to mysql triggers and how to create and delete triggers

What is a mysql trigger

Requires MySQL 5 Support for triggers was added in MySQL 5. Therefore, the content in this chapter applies to MySQL 5 or later versions.

MySQL statements are executed when needed, as are stored procedures. But what if you want a certain statement (or some statements) to execute automatically when an event occurs? For example:

1. Whenever a customer is added to a database table, check whether the phone number format is correct and whether the state abbreviation is capitalized;

2. Whenever an order is placed When a product is deleted, the ordered quantity is subtracted from the inventory quantity;

3. Whenever a row is deleted, a copy is retained in an archive table.

What all these examples have in common is that they all need to automatically handle when a certain table changes. This is literally the trigger. A trigger is a MySQL statement (or a group of statements between the BEGIN and END statements) that MySQL automatically executes in response to any of the following statements:

1.DELETE;

2.INSERT;

3.UPDATE.

Other MySQL statements do not support triggers.

Create mysql trigger

When creating a trigger, 4 pieces of information need to be given:

1. The unique trigger name;

2. The table associated with the trigger;

3. The activity the trigger should respond to (DELETE, INSERT or UPDATE);

4. When does the trigger execute (before or after processing).

Keep trigger names unique per database In MySQL 5, trigger names must be unique in each table, but not in each database. This means that two tables in the same database can have triggers with the same name. This is not allowed in other DBMSs where each database trigger name must be unique, and future versions of MySQL are likely to make the naming rules more strict. Therefore, it is now best to use unique trigger names within the database scope.

Triggers are created with the CREATE TRIGGER statement. The following is a simple example:

Input:

create trigger newproduct after insert on proucts for each row select 'product added';

Analysis: CREATE TRIGGER is used to create a new trigger named newproduct. Triggers can be executed before or after an operation occurs, here AFTER INSERT is given, so this trigger will execute after the INSERT statement executes successfully. This trigger also specifies FOR EACH ROW, so the code executes for each inserted row. In this example, the text Product added will appear once for each inserted row. To test this trigger, use an INSERT statement to add one or more rows to products

. You will see a Product added message displayed for each successful insert.

Only supports tables. Only tables support triggers, views do not support them (temporary tables do not support them either).

Triggers are defined for each event of each table. Only one trigger is allowed for each event of each table. Therefore, each table supports up to 6 triggers (before and after each INSERT , UPDATE and DELETE ). A single trigger cannot be associated with multiple events or multiple tables, so if you need a trigger that executes on INSERT and UPDATE operations, you should define

two triggers.

Trigger Failure If the BEFORE trigger fails, MySQL will not perform the requested operation. Additionally, if the BEFORE trigger or the statement itself fails, MySQL will not execute the AFTER trigger (if any).

Delete mysql trigger

Now, the syntax for deleting a trigger should be obvious. To delete a trigger, use the DROP TRIGGER statement as follows:

Input:

drop trigger newproduct ;

Analysis: Triggers cannot be updated or overwritten. In order to modify a trigger, it must be deleted and then re-created.

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