mysql - MySQL 比较 VARCHAR 列

Question

我有一个名为D1_VAL

此列中的行具有如下所示的数据：

C:10
R:200
E:3
N:77

我正在尝试使用逻辑运算符对这些值进行搜索。我试过这个：

SELECT * FROM d1 WHERE CAST(`D1_VAL` AS UNSIGNED) > 5

它不返回任何行。我也试过：

SELECT * FROM d1 WHERE (0 + `D1_VAL`) > 5

我将如何正确地做到这一点？

Answer 1

这个查询会做，

SELECT * FROM d1 WHERE CAST(SUBSTRING(`D1_VAL`, 3) AS UNSIGNED) > 5

但最好将值标准化C:10为两个单独的列。

create table d1(
    letter char(1),
    number int,
    // ... other columns
)

Answer 2

您要进行的比较是什么？

如果将数字（在冒号右侧）与 5 进行比较，请尝试以下操作：

CAST(SUBSTRING(`D1_VAL`, INSTR(`D1_VAL`, `:`)) AS UNSIGNED) > 5 

有关字符串函数的更多信息，请参见此处。

Also, I would suggest having a think about the database design - i.e. would the contents of this column be better off being stored in two columns - e.g. a char and an integer? That would be more efficient - requiring less disk space and allowing faster queries. If you have lots of rows, doing various string functions, casting etc. could make your queries run quite slowly.