How to use where when variable not null - sql

Question

I want use where when input variable is not null.
For example, when #custid is not null create this query:
Select *
from customers
where custid = #custid
and when #custid is null, then use this query instead:
Select *
from customers
How can I do this?
I searched on google, but haven't found any similar answer. Thanks

Answer 1

You can use the following query:
Select *
from customers
where #custid IS NULL OR custid=#custid
In this case if your variable is null only the condition #custid IS NULL will be evaluated and you get query identical to Select * from customers
If your variable is not null then condition #custid IS NULL is false and your query is identical to Select * from customers where custid=#custid

Answer 2

I use a SqlBuilder class and provide filters that attach the WHERE clauses programmatically.
So the SqlBuilder class would look like this:
public class Program
{
public static void Main()
{
Console.WriteLine(SqlBuilder.GetSql(null));
Console.WriteLine(SqlBuilder.GetSql("CUSTOMER_ID"));
}
public class SqlBuilder{
private static string _baseSql = "select * from customers";
public static string GetSql(string customerId){
var final = _baseSql;
if(customerId != null)
final += " WHERE custId=#custId";
return final;
}
}
}
This can also be made generic through the use of parameters and enums.