数据库
一、 问题原因
同事项目出现Bug，让我看看，项目在运行过程中一直出现同一条数据，而将sql在数据库执行是正常的，没有查到任何数据，但项目执行始终有一条数据，那这条数据怎么产生的呢？
二、排查问题
先排查是否链接数据库不对，排查正常，在排查是否存在缓存，排查没有，然后排查sql语句，sql排查了许久发现，传入给sql语句条件的占位符参数是""132456""这样的，而条件比较字段类型是int类型，最终查询出字段为0的数据
三、为什么会这样呢？
测试如下：
mysql> select 'sss' = 0;
+-----------+
| 'sss' = 0 |
+-----------+
| 1 |
+-----------+
1 row in set
mysql> select '0sss' = 0;
+------------+
| '0sss' = 0 |
+------------+
| 1 |
+------------+
1 row in set
mysql> select '01sss' = 0;
+-------------+
| '01sss' = 0 |
+-------------+
| 0 |
+-------------+
1 row in set
mysql> select '013sss' = 13
;
+---------------+
| '013sss' = 13 |
+---------------+
| 1 |
+---------------+
1 row in set
mysql> select '113sss' = 13;
+---------------+
| '113sss' = 13 |
+---------------+
| 0 |
+---------------+
1 row in set
查询通过查看MySQL给出的警告，都可以看到类似如下的信息
mysql> show warnings;
+---------+------+--------------------------------------------+
| Level | Code | Message |
+---------+------+--------------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: '113sss' |
+---------+------+--------------------------------------------+
1 row in set
官方文档中关于比较的章节，其中说明Comparison operations result in a value of 1 (TRUE), 0 (FALSE), or NULL. These operations work for both numbers and strings. Strings are automatically converted to numbers and numbers to strings as necessary.。也就是说字符串根据需要自动转换为字符串和数字。
mysql> select cast('123ab
c' as signed);
+--------------------------+
| cast('123abc' as signed) |
+--------------------------+
| 123 |
+--------------------------+
1 row in set
mysql> select cast('1h
23abc' as signed);
+---------------------------+
| cast('1h23abc' as signed) |
+---------------------------+
| 1 |
+---------------------------+
1 row in set
mysql> select cast('
abc' as signed);
+-----------------------+
| cast('abc' as signed) |
+-----------------------+
| 0 |
+------------
由此可见转换类型时，从左往右开始截取，如果有则截取出来，遇到不能转换的就终止，如果从开头就没有截取到匹配类型结果自然就是0。
Comparison operations result in a value of 1 (TRUE), 0 (FALSE), or NULL.
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