MySQL系统变量sql_safe_updates总结
在MySQL中，系统变量sql_safe_updates是个非常有意思的系统变量，在Oracle和SQL Server中都没有见过这样的参数或功能。如果这个系统变量设置为1的话，意味着update与delete将会受到限制。个人臆测，之所以提供这个功能，一方面是要避免出现更新或删除数据时，忘记添加WHERE条件，导致数据被误更新或误删的情况。相信不少人都由于疏忽或大意，遇到过这种情况； 另外一方面也是为了提高SQL性能考虑，避免DELETE与UPDATE语句的WHERE条件不走索引的情况。默认情况下，系统变量sql_safe_updates是禁用的。我们下面来简单测试、了解一下这个系统变量的细节问题
系统变量sql_safe_updates分会话级别和全局级别。可以动态修改。默认是关闭（值为0）。具体参考官方文档。
系统变量的查看
注意事项1：show variables优先显示会话级变量的系统变量值，如果这个系统变量没有会话级别的值，则显示全局级系统变量的值，当然最清晰的方式就是加上GLOBAL或SESSION关键字来区别。
注意事项2：@@GLOBAL.var_name、@@SESSION.var_name 、@@LOCAL.var_name 。如果在变量名前没有级别限定符，将优先显示会话级的值。
1：查看会话级别系统变量sql_safe_updates的值
mysql> show variables like 'sql_safe_updates';
+------------------+-------+
| Variable_name | Value |
+------------------+-------+
| sql_safe_updates | ON |
+------------------+-------+
1 row in set (0.01 sec)
mysql> show session variables like 'sql_safe_updates';
+------------------+-------+
| Variable_name | Value |
+------------------+-------+
| sql_safe_updates | ON |
+------------------+-------+
1 row in set (0.00 sec)
mysql>
mysql> select @@session.sql_safe_updates;
+----------------------------+
| @@session.sql_safe_updates |
+----------------------------+
| 1 |
+----------------------------+
1 row in set (0.00 sec)
mysql> select @@sql_safe_updates;
+--------------------+
| @@sql_safe_updates |
+--------------------+
| 1 |
+--------------------+
1 row in set (0.00 sec)
2：查看全局级别系统变量sql_safe_updates的值
mysql> show global variables like 'sql_safe_updates';
+------------------+-------+
| Variable_name | Value |
+------------------+-------+
| sql_safe_updates | OFF |
+------------------+-------+
1 row in set (0.00 sec)
mysql> select @@global.sql_safe_updates;
+---------------------------+
| @@global.sql_safe_updates |
+---------------------------+
| 0 |
+---------------------------+
1 row in set (0.00 sec)
系统变量sql_safe_updates的设置
#设置会话级别的系统变量
mysql> set sql_safe_updates=1;
Query OK, 0 rows affected (0.00 sec)
#设置全局级别的系统变量
mysql> set global sql_safe_updates=1;
Query OK, 0 rows affected (0.00 sec)
如果要设置全局级别的系统变量sql_safe_updates，最好在配置文件my.cnf中设置，这样就能避免MySQL重启后，SQL命令设置的sql_safe_updates系统全局变量失效。
实验测试案例
UPDATE语句测试
1： 没有WHERE条件。
mysql> update shop set price=price*1.5;
ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column
mysql>
2： 没有WHERE条件，但是有LIMTI限制条件
如下测试所示，没有WHERE条件，但是有LIMIT限制的话，SQL也是可以执行的。
mysql> update shop set price=price*1.5 limit 5;
Query OK, 5 rows affected (0.05 sec)
Rows matched: 5 Changed: 5 Warnings: 0
3： 有WHERE条件，但是WHERE条件的列没有索引可用
mysql> show index from shop\G;
*************************** 1. row ***************************
Table: shop
Non_unique: 0
Key_name: PRIMARY
Seq_in_index: 1
Column_name: article
Collation: A
Cardinality: 931203
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
*************************** 2. row ***************************
Table: shop
Non_unique: 0
Key_name: PRIMARY
Seq_in_index: 2
Column_name: dealer
Collation: A
Cardinality: 931203
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
*************************** 3. row ***************************
Table: shop
Non_unique: 1
Key_name: shop__price
Seq_in_index: 1
Column_name: price
Collation: A
Cardinality: 1877
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
3 rows in set (0.00 sec)
ERROR:
No query specified
mysql> update shop set price = price * 1.5 where dealer='K';
ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column
4： 有WHERE条件，但是WHERE条件列没有索引 + LIMIT限制
mysql> update shop set price = price * 1.5 where dealer='K' limit 1000;
Query OK, 1000 rows affected (0.13 sec)
Rows matched: 1000 Changed: 1000 Warnings: 0
5：有WHERE条件的UPDATE，而且字段有索引
mysql> update shop set price = price * 1.5 where article=6156;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql>
6：有WHERE条件，但是WHERE条件为常量
mysql> update shop set price = price * 1.5 where 1=1;
ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column
7：有WHERE条件，但是WHERE条件为常量+LIMIT
mysql> update shop set price = price * 1.5 where 1=1 limit 10;
Query OK, 10 rows affected (0.00 sec)
Rows matched: 10 Changed: 10 Warnings: 0
DELETE语句测试
1： DELETE语句没有WHERE条件
mysql> delete from shop;
ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column
mysql>
2: DELETE语句没有WHERE条件+ LIMIT
mysql> delete from shop limit 10;
ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column
mysql>
3：DELETE语句有WHERE条件，而且查询条件的字段有索引
mysql> delete from shop where article =6156;
Query OK, 1 row affected (0.01 sec)
mysql>
4：DELETE语句有WHERE条件，而且查询条件的字段有索引 +LIMIT
mysql> delete from shop where article< 2000 limit 1000;
Query OK, 1000 rows affected (0.02 sec)
mysql>
5：DELETE语句有WHERE条件，但是查询条件的字段没有索引
mysql> delete from shop where dealer='K';
ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column
mysql>
6：DELETE语句有WHERE条件，但是查询字段没有索引 +LIMIT语句
mysql> delete from shop where dealer='K' limit 20;
Query OK, 20 rows affected (0.05 sec)
mysql>
7：DELETE语句有WHERE条件，但是WHERE条件为常量
mysql> delete from shop where 1=1;
ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column
mysql>
8：DELETE语句有WHERE条件，但是WHERE条件为常量 +LIMIT
mysql> delete from shop where 1=1 limit 10;
ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column
mysql>
如上测试的实验结果，总结出来如下所示.至于是否要开启这个系统变量，需要结合实际情况，并且和开发人员、项目管理人员一起协商才行。
| 操作 |
Delete |
Update |
| NO WHERE |
No |
No |
| NO WHERE + LIMIT |
No |
Yes |
| WHERE KEY |
Yes |
Yes |
| WHERE KEY + LIMIT |
Yes |
Yes |
| WHERE NOKEY |
No |
No |
| WHERE NOKEY+ LIMIT |
Yes |
Yes |
| WHERE CONSTANT |
No |
No |
| WHERE CONSTANT + LIMIT |
No |
Yes |