Mysql学习MySQL的隐式类型转换整理总结

作者：VEPHP 时间 2017-09-24

《Mysql学习MySQL的隐式类型转换整理总结》要点：
本文介绍了Mysql学习MySQL的隐式类型转换整理总结，希望对您有用。如果有疑问，可以联系我们。

MYSQL实例前言

MYSQL实例前几天在看到一篇文章：价值百万的 MySQL 的隐式类型转换感觉写的很不错,再加上自己之前也对MySQL的隐式转化这边并不是很清楚,所以就顺势整理了一下.希望对大家有所帮助.

MYSQL实例当我们对不同类型的值进行比较的时候,为了使得这些数值「可比较」(也可以称为类型的兼容性),MySQL会做一些隐式转化(Implicit type conversion).

MYSQL实例比如下面的例子：

MYSQL实例
mysql> SELECT 1+'1';
 -> 2
mysql> SELECT CONCAT(2,' test');
 -> '2 test'

MYSQL实例很明显,上面的SQL语句的执行过程中就出现了隐式转化.并且从结果们可以判断出,第一条SQL中,将字符串的“1”转换为数字1,而在第二条的SQL中,将数字2转换为字符串“2”.

MYSQL实例MySQL也提供了CAST()函数.我们可以使用它明确的把数值转换为字符串.当使用CONCA()函数的时候,也可能会出现隐式转化,因为它希望的参数为字符串形式,但是如果我们传递的不是字符串呢：

MYSQL实例
mysql> SELECT 38.8, CAST(38.8 AS CHAR);
 -> 38.8, '38.8'
mysql> SELECT 38.8, CONCAT(38.8);
 -> 38.8, '38.8'

MYSQL实例隐式转化规则

MYSQL实例官方文档中关于隐式转化的规则是如下描述的：

MYSQL实例If one or both arguments are NULL, the result of the comparison is NULL, except for the NULL-safe <=> equality comparison operator. For NULL <=> NULL, the result is true. No conversion is needed.

If both arguments in a comparison operation are strings, they are compared as strings.
If both arguments are integers, they are compared as integers.
Hexadecimal values are treated as binary strings if not compared to a number.
If one of the arguments is a TIMESTAMP or DATETIME column and the other argument is a constant, the constant is converted to a timestamp before the comparison is performed. This is done to be more ODBC-friendly. Note that this is not done for the arguments to IN()! To be safe, always use complete datetime, date, or time strings when doing comparisons. For example, to achieve best results when using BETWEEN with date or time values, use CAST() to explicitly convert the values to the desired data type.
A single-row subquery from a table or tables is not considered a constant. For example, if a subquery returns an integer to be compared to a DATETIME value, the comparison is done as two integers. The integer is not converted to a temporal value. To compare the operands as DATETIME values, use CAST() to explicitly convert the subquery value to DATETIME.
If one of the arguments is a decimal value, comparison depends on the other argument. The arguments are compared as decimal values if the other argument is a decimal or integer value, or as floating-point values if the other argument is a floating-point value.
In all other cases, the arguments are compared as floating-point (real) numbers.

MYSQL实例翻译为中文就是：

两个参数至少有一个是 NULL 时,比较的结果也是 NULL,例外是使用 <=> 对两个 NULL 做比较时会返回 1,这两种情况都不需要做类型转换
两个参数都是字符串,会按照字符串来比较,不做类型转换
两个参数都是整数,按照整数来比较,不做类型转换
十六进制的值和非数字做比较时,会被当做二进制串
有一个参数是 TIMESTAMP 或 DATETIME,并且另外一个参数是常量,常量会被转换为 timestamp
有一个参数是 decimal 类型,如果另外一个参数是 decimal 或者整数,会将整数转换为 decimal 后进行比较,如果另外一个参数是浮点数,则会把 decimal 转换为浮点数进行比较
所有其他情况下,两个参数都会被转换为浮点数再进行比较

MYSQL实例注意点

MYSQL实例安全问题：假如 password 类型为字符串,查询条件为 int 0 则会匹配上.

MYSQL实例
mysql> select * from test;
+----+-------+-----------+
| id | name | password |
+----+-------+-----------+
| 1 | test1 | password1 |
| 2 | test2 | password2 |
+----+-------+-----------+
2 rows in set (0.00 sec)
mysql> select * from test where name = 'test1' and password = 0;
+----+-------+-----------+
| id | name | password |
+----+-------+-----------+
| 1 | test1 | password1 |
+----+-------+-----------+
1 row in set, 1 warning (0.00 sec)
mysql> show warnings;
+---------+------+-----------------------------------------------+
| Level | Code | Message   |
+---------+------+-----------------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: 'password1' |
+---------+------+-----------------------------------------------+
1 row in set (0.00 sec)

MYSQL实例相信上面的例子,一些机灵的同学可以发现其实上面的例子也可以做sql注入.

MYSQL实例假设网站的登录那块做的比较挫,使用下面的方式：

MYSQL实例
SELECT * FROM users WHERE username = '$_POST["username"]' AND password = '$_POST["password"]'

MYSQL实例如果username输入的是a' OR 1='1,那么password随便输入,这样就生成了下面的查询：

MYSQL实例
SELECT * FROM users WHERE username = 'a' OR 1='1' AND password = 'anyvalue'

MYSQL实例就有可能登录系统.其实如果攻击者看过了这篇文章,那么就可以利用隐式转化来进行登录了.如下：

MYSQL实例
mysql> select * from test;
+----+-------+-----------+
| id | name | password |
+----+-------+-----------+
| 1 | test1 | password1 |
| 2 | test2 | password2 |
| 3 | aaa | aaaa |
| 4 | 55aaa | 55aaaa |
+----+-------+-----------+
4 rows in set (0.00 sec)
mysql> select * from test where name = 'a' + '55';
+----+-------+----------+
| id | name | password |
+----+-------+----------+
| 4 | 55aaa | 55aaaa |
+----+-------+----------+
1 row in set, 5 warnings (0.00 sec)

MYSQL实例之所以出现上述的原因是因为：

MYSQL实例
mysql> select '55aaa' = 55;
+--------------+
| '55aaa' = 55 |
+--------------+
| 1 |
+--------------+
1 row in set, 1 warning (0.00 sec)
mysql> select 'a' + '55';
+------------+
| 'a' + '55' |
+------------+
| 55 |
+------------+
1 row in set, 1 warning (0.00 sec)

MYSQL实例下面通过一些例子来复习一下上面的转换规则：

MYSQL实例
mysql> select 1+1;
+-----+
| 1+1 |
+-----+
| 2 |
+-----+
1 row in set (0.00 sec)
mysql> select 'aa' + 1;
+----------+
| 'aa' + 1 |
+----------+
| 1 |
+----------+
1 row in set, 1 warning (0.00 sec)
mysql> show warnings;
+---------+------+----------------------------------------+
| Level | Code | Message  |
+---------+------+----------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: 'aa' |
+---------+------+----------------------------------------+
1 row in set (0.00 sec)

MYSQL实例把字符串“aa”和1进行求和,得到1,因为“aa”和数字1的类型不同,MySQL官方文档告诉我们：

MYSQL实例 When an operator is used with operands of different types, type conversion occurs to make the operands compatible.

MYSQL实例查看warnings可以看到隐式转化把字符串转为了double类型.但是因为字符串是非数字型的,所以就会被转换为0,因此最终计算的是0+1=1

MYSQL实例上面的例子是类型不同,所以出现了隐式转化,那么如果我们使用相同类型的值进行运算呢?

MYSQL实例
mysql> select 'a' + 'b';
+-----------+
| 'a' + 'b' |
+-----------+
|  0 |
+-----------+
1 row in set, 2 warnings (0.00 sec)
mysql> show warnings;
+---------+------+---------------------------------------+
| Level | Code | Message    |
+---------+------+---------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: 'a' |
| Warning | 1292 | Truncated incorrect DOUBLE value: 'b' |
+---------+------+---------------------------------------+
2 rows in set (0.00 sec)

MYSQL实例是不是有点郁闷呢?

MYSQL实例之所以出现这种情况,是因为+为算术操作符arithmetic operator 这样就可以解释为什么a和b都转换为double了.因为转换之后其实就是：0+0=0了.

MYSQL实例再看一个例子：

MYSQL实例
mysql> select 'a'+'b'='c';
+-------------+
| 'a'+'b'='c' |
+-------------+
|  1 |
+-------------+
1 row in set, 3 warnings (0.00 sec)
mysql> show warnings;
+---------+------+---------------------------------------+
| Level | Code | Message    |
+---------+------+---------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: 'a' |
| Warning | 1292 | Truncated incorrect DOUBLE value: 'b' |
| Warning | 1292 | Truncated incorrect DOUBLE value: 'c' |
+---------+------+---------------------------------------+
3 rows in set (0.00 sec)

MYSQL实例现在就看也很好的理解上面的例子了吧.a+b=c结果为1,1在MySQL中可以理解为TRUE,因为'a'+'b'的结果为0,c也会隐式转化为0,因此比较其实是：0=0也就是true,也就是1.

MYSQL实例第二个需要注意点就是防止多查询或者删除数据

MYSQL实例
mysql> select * from test;
+----+-------+-----------+
| id | name | password |
+----+-------+-----------+
| 1 | test1 | password1 |
| 2 | test2 | password2 |
| 3 | aaa | aaaa |
| 4 | 55aaa | 55aaaa |
| 5 | 1212 | aaa |
| 6 | 1212a | aaa |
+----+-------+-----------+
6 rows in set (0.00 sec)
mysql> select * from test where name = 1212;
+----+-------+----------+
| id | name | password |
+----+-------+----------+
| 5 | 1212 | aaa |
| 6 | 1212a | aaa |
+----+-------+----------+
2 rows in set, 5 warnings (0.00 sec)
mysql> select * from test where name = '1212';
+----+------+----------+
| id | name | password |
+----+------+----------+
| 5 | 1212 | aaa |
+----+------+----------+
1 row in set (0.00 sec)

MYSQL实例上面的例子本意是查询id为5的那一条记录,结果把id为6的那一条也查询出来了.我想说明什么情况呢?有时候我们的数据库表中的一些列是varchar类型,但是存储的值为‘1123'这种的纯数字的字符串值,一些同学写sql的时候又不习惯加引号.这样当进行select,update或者delete的时候就可能会多操作一些数据.所以应该加引号的地方别忘记了.

MYSQL实例关于字符串转数字的一些说明

MYSQL实例
mysql> select 'a' = 0;
+---------+
| 'a' = 0 |
+---------+
| 1 |
+---------+
1 row in set, 1 warning (0.00 sec)
mysql> select '1a' = 1;
+----------+
| '1a' = 1 |
+----------+
| 1 |
+----------+
1 row in set, 1 warning (0.00 sec)
mysql> select '1a1b' = 1;
+------------+
| '1a1b' = 1 |
+------------+
|  1 |
+------------+
1 row in set, 1 warning (0.00 sec)
mysql> select '1a2b3' = 1;
+-------------+
| '1a2b3' = 1 |
+-------------+
|  1 |
+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> select 'a1b2c3' = 0;
+--------------+
| 'a1b2c3' = 0 |
+--------------+
|  1 |
+--------------+
1 row in set, 1 warning (0.00 sec)

MYSQL实例从上面的例子可以看出,当把字符串转为数字的时候,其实是从左边开始处理的.

如果字符串的第一个字符就是非数字的字符,那么转换为数字就是0
如果字符串以数字开头
如果字符串中都是数字,那么转换为数字就是整个字符串对应的数字
如果字符串中存在非数字,那么转换为的数字就是开头的那些数字对应的值

MYSQL实例总结

MYSQL实例以上就是这篇文章的全部内容了,如果你有其他更好的例子,或者被隐式转化坑过的情况,欢迎分享.希望本文的内容对大家的学习或者工作能带来一定的帮助,如果有疑问大家可以留言交流.

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