SQL: Multiple Where Clause based on multiple filters

Question

How do I write an SQL query that has nested filters.

Info: 2 search filters

1st filter: Id, Name, Date
2nd filter: depending on first, for Id: exact, range; for Name: exact, like; for Date: exact, range.

In LINQ code its was done something like:

theList = somelistFromDb;
case filter1
   case "Id"
       if filter2 == "exact"
          theList.where(x => x == searchkey);
       else if filter 2 == "range"
          theList.where(x => x >= searchkey && x<=searchkey2);
   case "Name"
       if filter2 == "exact"
          theList.where(x => x == searchkey);
       else if filter2 == "like"
          theList.where(x => x.contains("searchkey));
...

How do I convert the above LINQ Pseudocode to SQL?

Answer 1

SELECT
...
WHERE
  (:filterParam='Id' AND <all the Id filter conditions> here)
OR
  (:filterParam='Name' AND <all the Name filter conditions> here)

Answer 2

select * from [Table] where 
((@filter1='Id') and 
  ((filter2='exact' and [Table].[Id]=@searchkey) OR
   (filter2='range' and [Table].[Id]>=@searchkey and [Table].[Id]<=@searchkey2) ))
OR
((@filter1='Name') and 
.....

Answer 3

Writing a single TSQL query that satisfies or excludes all the conditions in one go is usually very suboptimal - it leads to terrible query plans. Trying to do all the thinking in TSQL is... somewhat ugly - TSQL simply isn't a great language for it.

So: the way I would usually do this is by building the query in C#, for example:

static void AppendFilter(StringBuilder filter, string clause)
{
    filter.Append(filter.Length == 0 ? " where " : " and ").Append(clause);
}

StringBuilder filter = new StringBuilder();
if(/* some condition */)
    AppendFilter(filter, "row.Foo = @foo");
if(/* some condition */)
    AppendFilter(filter, "row.Bar > @bar"); 
// ...
string tsql = "select * from SomeTable row" + filter.ToString();
// pack params, exec

This:

• extends to any number of filters
• generates appropriate TSQL per filter combination, for optimal query-plans
• is simple to maintain

Personally, I'd also use dapper to do the execution, as that has (basic) parameter analysis built in, allowing simple:

var rows = conn.Query<SomeType>(tsql, new { foo, bar, ... }).ToList();

(yet it will still send only the necessary parameters)

An alternative, though, is to add the parameters in each if .