Multi-trigger creation in MySQL and its advantages and disadvantages
For a table with the same event and action time, you will learn how to create Multi-Trigger in this blog.
This guide applies to MySQL versions 5.7.2 and beyond. The statements in the tutorial won’t function if you’re using an earlier version of MySQL.
Prior to MySQL 5.7.2, you were only able to construct one trigger for each event in a table, such as the BEFORE UPDATE or AFTER UPDATE event. This restriction was removed in MySQL 5.7.2+, enabling the creation of numerous triggers with the same event and action time for a single table. When an event occurs, each of these triggers will turn on in turn.
The syntax for defining a trigger that will react to the same event and action time before or after an existing trigger is as follows:
DELIMITER $$ CREATE TRIGGER trigger_name {BEFORE|AFTER}{INSERT|UPDATE|DELETE} ON table_name FOR EACH ROW {FOLLOWS|PRECEDES} existing_trigger_name BEGIN -- statements END$$ DELIMITER ;
The FOLLOWS or PRECEDES in this syntax indicate whether the new trigger should be used before or after an already-invoked trigger.
- The FOLLOWS command enables a new trigger to go off following an already-active trigger.
- The PRECEDES clause permits the new trigger to go off before an already-active trigger.
What is Trigger in MySQL?
A trigger in MySQL is a stored program that is automatically run in response to a table-related event, such as an insert, update, or deletion. You could, for instance, create a trigger that runs automatically before a new row is added to a table.
The INSERT, UPDATE or DELETE event can be used to trigger triggers in MySQL.
Row-level triggers and statement-level triggers are the two kinds of triggers that the SQL standard specifies.
- Each time a row is added, changed or removed, a row-level trigger is set off. For instance, the trigger is automatically triggered 100 times for the 100 rows affected if a table has 100 rows that have been added, changed, or removed.
- No matter how many rows are added, changed, or removed, a statement-level trigger only ever runs once for each transaction.
In MySQL, only row-level triggers are supported. It does not support triggers at the statement level.
Advantages of MySQL trigger
- Triggers offer an additional method for verifying the accuracy of data. Errors from the database layer are handled through triggers.
- An alternate technique to execute scheduled tasks is through triggers. By using triggers, you can avoid waiting for scheduled events to complete because they are automatically triggered before or after a change is made to a table’s data.
- The auditing of data changes in tables can be done with the use of triggers.
Disadvantages of MySQL trigger
- Only extended validations, not all validations, can be provided via triggers. Use the NOT NULL, UNIQUE, CHECK, and FOREIGN KEY constraints for straightforward validations.
- Because triggers run automatically in the database, which may not be transparent to the client apps, they might be challenging to diagnose.
- The MySQL Server’s overhead may grow as a result of triggers.
In charge of MySQL trigger
- Create triggers – describe the procedures involved in creating a MySQL trigger.
- Create a BEFORE INSERT trigger – to maintain a summary table from another table, I’ll demonstrate how to establish a BEFORE INSERT trigger.
- Create an AFTER INSERT trigger – explain how to design an AFTER INSERT trigger so that data is inserted into one table after being inserted into another.
- Create a BEFORE UPDATE trigger – Find out how to construct a BEFORE UPDATE trigger that verifies information before it is updated in the table.
- Create an AFTER UPDATE trigger – demonstrate to you the process of generating an AFTER UPDATE trigger to record changes to data in a table.
- Create a BEFORE DELETE trigger – exemplify the BEFORE DELETE trigger creation process.
- Create an AFTER DELETE trigger – explain the creation of an AFTER DELETE
- Create multiple triggers for a table with the same trigger event and time – MySQL 8.0 lets you establish multiple triggers for a table with the same trigger event and time.
- Show triggers – table triggers in a database according to particular patterns
How to Create Trigger in MySQL?
In this lesson, you will discover how to build a trigger in a database using the MySQL CREATE TRIGGER statement. A description of the CREATE TRIGGER statement in a new trigger is created by the CREATE TRIGGER statement.
The statement’s basic syntax for CREATE TRIGGER is as follows:
CREATE TRIGGER trigger_name {BEFORE | AFTER} {INSERT | UPDATE| DELETE } ON table_name FOR EACH ROW trigger_body;
In this syntax:
- After the CREATE TRIGGER keywords, provide the trigger you want to create its name. Keep in mind that a database’s trigger name needs to be distinct.
- The trigger is then triggered before or after each row is modified, depending on the trigger action time, which can be either BEFORE or AFTER.
- The next step is to identify the trigger’s activating action, which can be either an INSERT, UPDATE, or DELETE.
- After the ON keyword, indicate the name of the table that the trigger belongs to.
- Finally, state the statement that will be carried out when the trigger is triggered. The BEGIN END compound statement is used to execute multiple statements.
The trigger body has access to the values of the column that the DML statement is affecting.
The NEW and OLD modifiers are used to differentiate between the values of the columns BEFORE and AFTER the DML has fired.
For instance, if you change the description of a column, you may access both the old value (OLD.description) and the new value (NEW.description) in the trigger body.
MySQL DROP TRIGGER
In this tutorial, you will discover how to remove a trigger from a database using the MySQL DROP TRIGGER statement. A description of the DROP TRIGGER statement in MySQL. A trigger is removed from the database using the DROP TRIGGER statement.
The basic syntax for the DROP TRIGGER statement is as follows:
DROP TRIGGER [IF EXISTS] [schema_name.]trigger_name;
In this syntax:
- After the DROP TRIGGER keywords, first, indicate the name of the trigger that you wish to remove.
- The name of the schema to which the trigger belongs should then be specified. The statement will remove the trigger from the current database if the schema name is omitted.
- Third, you can conditionally drop the trigger if it already exists by using the IF EXISTS option. Optional: the IF EXISTS clause.
MySQL generates an error if you drop a trigger that doesn’t exist without utilizing the IF EXISTS clause. However, MySQL generates a NOTE if the IF EXISTS clause is used.
The TRIGGER privilege for the trigger’s related table is necessary for the DROP TRIGGER operation.
It should be noted that if you delete a table, MySQL will also delete all associated triggers.