MySQL的隐式类型转换整理总结

2024-07-24 12:52:58

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前言

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。

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）。

比如下面的例子：

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

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

隐式转化规则

官方文档中关于隐式转化的规则是如下描述的：

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.

翻译为中文就是：

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