实际上“FOR EACH ROW”意味着更新或删除的每个匹配行。换句话说，我们可以说触发器并不应用于每一行，它只是说对每个受影响的表行执行触发器主体。我们可以通过以下示例来说明这一点 -
示例
在此示例中，我们创建两个表，Sample 和 Sample_rowaffected，如下 -
mysql> Create table Sample(id int, value varchar(20));
Query OK, 0 rows affected (0.47 sec)
mysql> Insert into Sample(id, value) values(100, 'same'),(101,
'Different'),(500, 'excellent'),(501, 'temporary');
Query OK, 4 rows affected (0.04 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> Select * from Sample;
+------+-----------+
| id | value |
+------+-----------+
| 100 | same |
| 101 | Different |
| 500 | excellent |
| 501 | temporary |
+------+-----------+
4 rows in set (0.00 sec)
mysql> Create table Sample_rowaffected(id int);
Query OK, 0 rows affected (0.53 sec)
mysql> Select Count(*) as ‘Rows Affected’ from sample_rowaffected;
+---------------+
| Rows Affected |
+---------------+
| 0 |
+---------------+
1 row in set (0.10 sec)
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现在，我们将创建一个触发器，该触发器在删除表“Sample”中的任何值之前触发，如下所示 -
mysql> Delimiter //
mysql> Create trigger trigger_before_delete_sample BEFORE DELETE on
Sample
-> FOR EACH ROW
-> BEGIN
-> SET @count = if (@count IS NULL, 1, (@count+1));
-> INSERT INTO sample_rowaffected values (@count);
-> END ;
-> //
Query OK, 0 rows affected (0.15 sec)
mysql> Delimiter ;
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现在，以下查询将从表“Sample”中删除一些值，并且删除的行数将存储在 @count 用户变量中 -
mysql> Delete from Sample WHERE ID >=500;
Query OK, 2 rows affected (0.11 sec)
mysql> Select @count;
+--------+
| @count |
+--------+
| 2 |
+--------+
1 row in set (0.03 sec)
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借助以下查询，我们可以检查受删除影响的行的值，插入到sample_rowaffected表中，如下 -
mysql> Select Count(*) as 'Rows Affected' from sample_rowaffected;
+---------------+
| Rows Affected |
+---------------+
| 2 |
+---------------+
1 row in set (0.00 sec)
mysql> Select * from Sample;
+------+-----------+
| id | value |
+------+-----------+
| 100 | same |
| 101 | Different |
+------+-----------+
2 rows in set (0.00 sec)
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在上面的示例的帮助下，很明显“FOR EACH ROW”意味着更新或删除的每个匹配行。
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