SELECT wx_id from `user` WHERE wx_id = 2

也就是说
SELECT wx_id from `user` WHERE wx_id = 2
等价于
SELECT wx_id from `user` WHERE CAST(wx_id AS signed int) = 2

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.

1, 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2, 两个参数都是字符串，会按照字符串来比较，不做类型转换3, 两个参数都是整数，按照整数来比较，不做类型转换4, 十六进制的值和非数字做比较时，会被当做二进制串5, 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 timestamp6, 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7, 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> 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)