如何在Select语句中插入变量

Question

string table = "City";
string query = "Select * from '"+table+"'";

This gives me error stating incorrect symbol near ".

However,

string query = "Select * from City";

Gives the proper output.

Answer 1

You just this

string query = "Select * from '"+table+"'";

to be replaced by

string query = "Select * from " + table;

Because you Query string is not "Select * from City"; While it is forming "Select * from 'City'";

and thus you getting error

Answer 2

Best practice would be to use string.format

string table = "City";
string query = string.format("Select * from {0}", table);

Answer 3

You need to form your query like below.,

string table = "City";

//You don't need to have single quote...

string query = " Select * From " + table; 

In order to use Where condition do like below.,

//Where clause only needs single quotes, to define the SQL parameter value in between...

string query = " Select * From " + table + " Where CityId = '" + cityId + "'"; 

Hope this helps.,

Answer 4

Best-Practice should be not to do this , because it's susceptible to malicious SQL injection.

Anyway, if you have control over the table variable, you should do it as @madcow69 suggested, but I suggest to add the delimiters, so you always have a valid delimited identifier (for example if your table name is "order" or any other SQL reserved word).

string table = "City";
string query = string.format("Select * from [{0}]", table);

But what if table is the following?:

string table = "City]; DROP DATABASE [YourDB";

Answer 5

You can make it work like this:

string table ="City"
string query = "Select * from "+table;

Answer 6

Hope this helps.,

string table = "City";

//You don't need to have single quote...

string query = " Select * From " + table; 

Answer 7

If you're using .NET4.6 you could use the new "composite string formatting" feature introduced with C# 6.0 (read about it here) .
this enables you to write your statement like this:

string query = $"Select * from {table}";

However I would strongly recommend not writing queries like that and use sql parameters . this will help you avoid SQL Injection attacks .