MySQL触发器是一个存储程序(带有查询)，它自动执行以响应特定事件，例如表中发生的插入、更新或删除。
MySQL 中有 6 种不同类型的触发器：
1. 更新前触发：
顾名思义，它是在调用更新之前执行的触发器。如果我们写一个更新语句，那么触发器的动作会在更新执行之前执行。
例子：
考虑表：
create table customer (acc_no integer primary key,
cust_name varchar(20),
avail_balance decimal);
create table mini_statement (acc_no integer,
avail_balance decimal,
foreign key(acc_no) references customer(acc_no) on delete cascade); 在其中插入值：
insert into customer values (1000, "Fanny", 7000);
insert into customer values (1001, "Peter", 12000); 在更新客户记录/表中的任何记录之前触发将(旧)值插入 mini_statement 记录(包括帐号和可用余额作为参数)：
delimiter //
create trigger update_cus
-> before update on customer
-> for each row
-> begin
-> insert into mini_statement values (old.acc_no, old.avail_balance);
-> end; // 更新以调用触发器：
delimiter;
update customer set avail_balance = avail_balance + 3000 where acc_no = 1001;
update customer set avail_balance = avail_balance + 3000 where acc_no = 1000; 输出：
select *from mini_statement;
+--------+---------------+
| acc_no | avail_balance |
+--------+---------------+
| 1001 | 12000 |
| 1000 | 7000 |
+--------+---------------+
2 rows in set (0.0007 sec) 2. 更新触发器后：
顾名思义，此触发器在发生更新后调用。 (即，它在执行更新语句后实现。)。
例子：
我们创建另一个表：
create table micro_statement (acc_no integer,
avail_balance decimal,
foreign key(acc_no) references customer(acc_no) on delete cascade); 将另一个值插入到客户中：
insert into customer values (1002, "Janitor", 4500);
Query OK, 1 row affected (0.0786 sec) 在发生更新后触发将帐号和可用余额的(新)值插入到 micro_statement 记录中：
delimiter //
create trigger update_after
-> after update on customer
-> for each row
-> begin
-> insert into micro_statement values(new.acc_no, new.avail_balance);
-> end; // 更新以调用触发器：
delimiter ;
update customer set avail_balance = avail_balance + 1500 where acc_no = 1002; 输出：
select *from micro_statement;
+--------+---------------+
| acc_no | avail_balance |
+--------+---------------+
| 1002 | 6000 |
+--------+---------------+
1 row in set (0.0007 sec) 3. 插入触发器前：
顾名思义，此触发器在插入之前或执行插入语句之前调用。
例子：
考虑表：
create table contacts (contact_id INT (11) NOT NULL AUTO_INCREMENT,
last_name VARCHAR (30) NOT NULL, first_name VARCHAR (25),
->birthday DATE, created_date DATE,
created_by VARCHAR(30),
CONSTRAINT contacts_pk PRIMARY KEY (contact_id)); 在插入之前触发将联系人信息(例如姓名、生日和创建日期/用户)插入表联系人中：
delimiter //
create trigger contacts_before_insert
-> before insert
-> on contacts for each row
-> begin
-> DECLARE vUser varchar(50);
->
-> -- Find username of person performing INSERT into table
-> select USER() into vUser;
->
-> -- Update create_date field to current system date
-> SET NEW.created_date = SYSDATE();
->
-> -- Update created_by field to the username of the person performing the INSERT
-> SET NEW.created_by = vUser;
-> end; // 进行插入以调用触发器：
delimiter;
insert into contacts values (1, "Newton", "Enigma",
str_to_date ("19-08-1999", "%d-%m-%Y"),
str_to_date ("17-03-2018", "%d-%m-%Y"), "xyz"); 输出：
select *from contacts;
+------------+-----------+------------+------------+--------------+----------------+
| contact_id | last_name | first_name | birthday | created_date | created_by |
+------------+-----------+------------+------------+--------------+----------------+
| 1 | Newton | Enigma | 1999-08-19 | 2019-05-11 | root@localhost |
+------------+-----------+------------+------------+--------------+----------------+ 4.插入触发器后：
顾名思义，这个触发器在执行插入后被调用。
例子：
考虑表：
create table contacts (contact_id int (11) NOT NULL AUTO_INCREMENT,
last_name VARCHAR(30) NOT NULL,
first_name VARCHAR(25), birthday DATE,
->CONSTRAINT contacts_pk PRIMARY KEY (contact_id));
create table contacts_audit (contact_id integer,
created_date date,
created_by varchar (30)); 在插入发生后触发以将 contact_id 和联系人创建日期/用户信息插入到 contacts_audit 记录中：
delimiter //
create trigger contacts_after_insert
-> after insert
-> on contacts for each row
-> begin
-> DECLARE vUser varchar(50);
->
-> -- Find username of person performing the INSERT into table
-> SELECT USER() into vUser;
->
-> -- Insert record into audit table
-> INSERT into contacts_audit
-> ( contact_id,
-> created_date,
-> created_by)
-> VALUES
-> ( NEW.contact_id,
-> SYSDATE(),
-> vUser );
-> END; // 进行插入以调用触发器：
insert into contacts values (1, "Kumar", "Rupesh",
str_to_date("20-06-1999", "%d-%m-%Y")); 输出：
select *from contacts_audit;
+------------+--------------+----------------+
| contact_id | created_date | created_by |
+------------+--------------+----------------+
| 1 | 2019-05-11 | root@localhost |
+------------+--------------+----------------+
1 row in set (0.0006 sec) 5. 删除触发器前：
顾名思义，该触发器在删除发生之前或删除语句执行之前被调用。
例子：
考虑表：
create table contacts (contact_id int (11) NOT NULL AUTO_INCREMENT,
last_name VARCHAR (30) NOT NULL, first_name VARCHAR (25),
birthday DATE, created_date DATE, created_by VARCHAR(30),
CONSTRAINT contacts_pk PRIMARY KEY (contact_id));
create table contacts_audit (contact_id integer, deleted_date date, deleted_by varchar(20)); 在删除发生之前触发将 contact_id 和联系人删除日期/用户信息插入到contacts_audit 记录中：
delimiter //
create trigger contacts_before_delete
-> before delete
-> on contacts for each row
-> begin
->
-> DECLARE vUser varchar(50);
->
-> -- Find username of person performing the DELETE into table
-> SELECT USER() into vUser;
->
-> -- Insert record into audit table
-> INSERT into contacts_audit
-> ( contact_id,
-> deleted_date,
-> deleted_by)
-> VALUES
-> ( OLD.contact_id,
-> SYSDATE(),
-> vUser );
-> end; // 进行插入然后删除以调用触发器：
delimiter;
insert into contacts values (1, "Bond", "Ruskin",
str_to_date ("19-08-1995", "%d-%m-%Y"),
str_to_date ("27-04-2018", "%d-%m-%Y"), "xyz");
delete from contacts where last_name="Bond"; 输出：
select *from contacts_audit;
+------------+--------------+----------------+
| contact_id | deleted_date | deleted_by |
+------------+--------------+----------------+
| 1 | 2019-05-11 | root@localhost |
+------------+--------------+----------------+
1 row in set (0.0007 sec) 6. 删除触发器后：
顾名思义，这个触发器是在删除发生后，或者在执行删除操作后调用的。
例子：
考虑以下表格：
create table contacts (contact_id int (11) NOT NULL AUTO_INCREMENT,
last_name VARCHAR (30) NOT NULL, first_name VARCHAR (25),
birthday DATE, created_date DATE, created_by VARCHAR (30),
CONSTRAINT contacts_pk PRIMARY KEY (contact_id));
create table contacts_audit (contact_id integer, deleted_date date, deleted_by varchar(20));删除发生后触发将contact_id和联系人删除日期/用户信息插入contacts_audit记录：
create trigger contacts_after_delete
-> after delete
-> on contacts for each row
-> begin
->
-> DECLARE vUser varchar(50);
->
-> -- Find username of person performing the DELETE into table
-> SELECT USER() into vUser;
->
-> -- Insert record into audit table
-> INSERT into contacts_audit
-> ( contact_id,
-> deleted_date,
-> deleted_by)
-> VALUES
-> ( OLD.contact_id,
-> SYSDATE(),
-> vUser );
-> end; // 进行插入和删除操作以调用触发器：
delimiter;
insert into contacts values (1, "Newton", "Isaac",
str_to_date ("19-08-1985", "%d-%m-%Y"),
str_to_date ("23-07-2018", "%d-%m-%Y"), "xyz");
delete from contacts where first_name="Isaac"; 输出：
select *from contacts_audit;
+------------+--------------+----------------+
| contact_id | deleted_date | deleted_by |
+------------+--------------+----------------+
| 1 | 2019-05-11 | root@localhost |
+------------+--------------+----------------+
1 row in set (0.0009 sec)