问题描述
假设我有一个包含发票编号列的表，数据类型是 VARCHAR，混合字符串/整数值，例如:
Lets say that I have a table which contains a column for invoice number, the data type is VARCHAR with mixed string/int values like:
invoice_number
**************
HKL1
HKL2
HKL3
.....
HKL12
HKL13
HKL14
HKL15
我尝试选择最大值，但它返回HKL9"，而不是最高值HKL15".
I tried to select max of it, but it returns with "HKL9", not the highest value "HKL15".
SELECT MAX( invoice_number )
FROM `invoice_header`
推荐答案
HKL9 (string) 大于 HKL15，因为它们是作为字符串比较的.解决您的问题的一种方法是定义一个列函数，该函数仅返回发票编号的数字部分.
HKL9 (string) is greater than HKL15, because they are compared as strings. One way to deal with your problem is to define a column function that returns only the numeric part of the invoice number.
如果您的所有发票编号都以 HKL 开头，那么您可以使用:
If all your invoice numbers start with HKL, then you can use:
SELECT MAX(CAST(SUBSTRING(invoice_number, 4, length(invoice_number)-3) AS UNSIGNED)) FROM table
它取invoice_number，不包括前3个字符，转换为int，并从中选择max.
It takes the invoice_number excluding the 3 first characters, converts to int, and selects max from it.
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