使用Linq-从数据库表中获取组，其中每个组中的一条记录与搜索匹配

Question

I am in search of assistance in working out how to perform the request that I need using Linq to Entities.

I have a table in an IBM DB2 database which, simplified, looks like this: PK {Integer} , GROUP {CHAR FOR BIT DATA [16]} , USERNAME {CHAR[15]} , ACTION {INTEGER} . (none of the values can be null)

There are more columns but these will do to get the point across. What I need to do is such:

I get user input requesting all records with a specific ACTION or USERNAME or both.

What I need to return is all records grouped by GROUP which contain the requested ACTION and USERNAME (if one of them is empty, it is ignored and I match the other).

I've tried a number of things but I keep running into walls with Linq to Entities. I get errors like "SQL0338N An ON clause associated with a JOIN [...]" and There is no stored type corresponding to the EDM Type 'Edm.Binary' of primitive 'Binary' .

I've tried to do a select where I

1. find the records which match the search,
2. get only the group column,
3. make sure I only have one of each (distinct)
4. try to use the new list in a search of the same table.

The moment I try to run .ToArray() I get smacked with an Exception, either DB2 exception or Linq exception (depending on the changes I made to the code).'

I would very much appreciate any assistance in figuring out a viable way to do this.

** Edit **

@Alex Well... the thing is that I tried several different ways of approaching the code and they each led to one of the same two errors (see above). The conclusion I came to was that the attempts I made were not just syntactically flawed but required a change to the direction with which I was approaching the problem.

@ZacharyKniebel This is the solution! Thank you! The only thing is - the example you gave would only return the groups that have a record where both fields are matched. To make it so that it is one, the other, or both, I used this:

...
var records = db.Records
    .GroupBy(r => r.Group);
if(requestedAction != null)
{
     records = records.Where(g => g.Any(r => r.Action == requestedAction));
}
if(!string.isNullOrWhiteSpace(requestedUsername))
{
     records = records.Where(g => g.Any(r => r.Username.ToLower().Contains(requestedUsername.ToLower()));
}
...

Answer 1

Try this:

...
var records = db.Records
        .GroupBy(r => r.Group)
        .Where(g => g.Any(r => r.Action == requestedAction && 
                               r.Username == requestedUsername)));
...

Here, is a breakdown of the above:

1. Grab the rows FROM the 'Records' table in the database ('implicitly' serves as a select statement)
2. GROUP the Records by the value in the 'Group' column of the 'Records' table
3. Return only those groups WHERE the requested username and action match one or more records within the group

Note that Linq will parse the Linq query into SQL before actually running the query. As such, the entirety of the above will be done in a single query.