using System;
using System.Data;

public class Test
{
	public DataTable Query(string caseSwitch)
	{
		string table = string.Empty;
		switch(caseSwitch)
		{
			case "ConditionA":
				table = "tableA";
				break;
			case "ConditionB":
				table = "tableB";
				break;
			default:
				table = "tableC";
				break;
		}
		string sql = @"select * from " + table;
		
		// your sql goes here

	}
}