--创建测试表
MariaDB [test]> create table account (acct_num int, amount decimal(10,2));
Query OK, 0 rows affected (0.16 sec)

--创建触发器
MariaDB [test]> CREATE TRIGGER ins_sum BEFORE INSERT ON account
    -> FOR EACH ROW SET @sum = @sum + NEW.amount;
Query OK, 0 rows affected (0.08 sec)

MariaDB [test]> select @sum;
+------+
| @sum |
+------+
| NULL |
+------+
1 row in set (0.00 sec)

MariaDB [test]> SET @sum = 0;
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> INSERT INTO account VALUES(137,14.98),(141,1937.50),(97,-100.00);
Query OK, 3 rows affected (0.08 sec)
Records: 3  Duplicates: 0  Warnings: 0

MariaDB [test]> select @sum;
+---------+
| @sum    |
+---------+
| 1852.48 |
+---------+
1 row in set (0.00 sec)

MariaDB [test]> select sum(amount) from account;
+-------------+
| sum(amount) |
+-------------+
|     1852.48 |
+-------------+
1 row in set (0.00 sec)

--查看触发器
MariaDB [test]> show triggers\G
*************************** 1. row ***************************
             Trigger: ins_sum
               Event: INSERT
               Table: account
           Statement: SET @sum = @sum + NEW.amount
              Timing: BEFORE
             Created: NULL
            sql_mode: NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
             Definer: root@localhost
character_set_client: utf8
collation_connection: utf8_general_ci
  Database Collation: utf8_general_ci
1 row in set (0.00 sec)

MariaDB [test]> select * from information_schema.triggers where trigger_name='ins_sum' \G
*************************** 1. row ***************************
           TRIGGER_CATALOG: def
            TRIGGER_SCHEMA: test
              TRIGGER_NAME: ins_sum
        EVENT_MANIPULATION: INSERT
      EVENT_OBJECT_CATALOG: def
       EVENT_OBJECT_SCHEMA: test
        EVENT_OBJECT_TABLE: account
              ACTION_ORDER: 0
          ACTION_CONDITION: NULL
          ACTION_STATEMENT: SET @sum = @sum + NEW.amount
        ACTION_ORIENTATION: ROW
             ACTION_TIMING: BEFORE
ACTION_REFERENCE_OLD_TABLE: NULL
ACTION_REFERENCE_NEW_TABLE: NULL
  ACTION_REFERENCE_OLD_ROW: OLD
  ACTION_REFERENCE_NEW_ROW: NEW
                   CREATED: NULL
                  SQL_MODE: NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
                   DEFINER: root@localhost
      CHARACTER_SET_CLIENT: utf8
      COLLATION_CONNECTION: utf8_general_ci
        DATABASE_COLLATION: utf8_general_ci
1 row in set (0.00 sec)