sql 数字数据库列如何与数字字符串值一起使用

Question

I have a question related to SQL.

I am recently coding an application, where sql query is generated dynamically based on user's inputs from UI. Interestingly list of values passed from UI were all of strings. I was told to add support of numeric values. But what I noticed that passing numeric string value to numeric field also works.

for example:

Select * from employee where id = 36815

and

Select * from employee where id = "36815"

My question is why second query work. Even if I change = operator to < or > , it still works. I want to know why it works and should we support it as it is. What can be disadvantages of using numeric string fields for numeric db columns.

Thanks.

Answer 1

Values in databases have types. And, you should learn not to mix types in expressions -- particularly in where and on clauses -- because such mixing can confuse the optimizer.

What is happening in your expression? The SQL compiler is seeing something like:

where <number> = <string>

By the rules of SQL, it says that the string needs to be converted to a number. MySQL does this by converting leading digits. So the result is something like:

where <number> = cast(string as int)

(although non-integers are allowed).

Note that this is not based on the string being a constant. Only on the type of the columns. So, if the expression were:

where stringcol = 5

Then stringcol would be converted to a number.