One Interviewer ask this question during interview of my friend.It is easy to answer but need some specific and to the point answer.So I try to relate it with C#.
If you don't know the list of conditions at compile time and it will built at run time, Then you can made a condition with “where 1=1”. and for other conditions that will affect run time, use
and <condition>.
Example
StringBuilder sb = new StringBuilder();
sb.Append("SELECT * FROM Products"); // Your query
sb.Append(" WHERE 1=1"); // always true condition
// append query's where clause
if (catID != 0)
{
sb.Append(" AND categoryID= {0}", catID);
}
if (minPrice > 0)
{
sb.Append(" AND itemPrice >= {0}", minPrice);
}
SqlCommand cmd = new SqlCommand(sb.ToString(), cnn);
SqlDataReader dr = cmd.ExecuteReader();
// your code to read data from dr.
.
