Linq查询从两个表中获取不同的记录

Question

I have two Tables - tblExpenses and tblCategories as follows

tblExpenses

ID (PK),
Place,
DateSpent,
CategoryID (FK)

tblCategory

ID (PK),
Name

I tried various LINQ approaches to get all distinct records from the above two tables but not with much success. I tried using UNION and DISTINCT but it didnt work.

The above two tables are defined in my Model section of my project which in turn will create tables in SQLite. I need to retrieve all the distinct records from both the tables to display values in gridview.

Kindly provide me some inputs to accomplish this task. I did some research to find answer to this question but nothing seemed close to what I wanted. Excuse me if I duplicated this question.

Here is the UNION, DISTINCT approaches I tried:

DISTINCT # ==> Gives me Repetitive values

(from exp in db.Table<tblExpenses >()
                                   from cat in db.Table<tblCategory>()                                       
                                   select new { exp.Id, exp.CategoryId, exp.DateSpent, exp.Expense, exp.Place, cat.Name }).Distinct();

UNION # ==> Got an error while using UNION

Answer 1

I think union already does the distict when you join the two tables you can try somethin like

 var query=(from c in db.tblExpenses select c).Concat(from c in 
            db.tblCategory select c).Distinct().ToList();

Answer 2

You will always get DISTINCT records, since you are selecting the tblExpenses.ID too. ( Unless there are multiple categories with the same ID. But that of course would be really, really bad design. )

Remember, when making a JOIN in LINQ, both field names and data types should be the same. Is the field tblExpenses.CategoryID a nullable field?

If so, try this JOIN :

db.Table<tblExpenses>()
.Join(db.Table<tblCategory>(), 
       exp => new { exp.CategoryId }, 
       cat => new { CategoryId = (int?)cat.ID }, 
       (exp, cat) => new { 
                           exp.Id, 
                           exp.CategoryId, 
                           exp.DateSpent, 
                           exp.Expense, 
                           exp.Place, 
                           cat.Name 
                         })
.Select(j => new { 
                   j.Id, 
                   j.CategoryId, 
                   j.DateSpent, 
                   j.Expense, 
                   j.Place, 
                   j.Name 
                 });

Answer 3

You can try this queries:

A SELECT DISTINCT query like this:

SELECT DISTINCT Name FROM tblCategory INNER JOIN tblExpenses ON tblCategory.categoryID = tblExpenses.categoryID;

limits the results to unique values in the output field. The query results are not updateable.

or

A SELECT DISTINCTROW query like this:

SELECT DISTINCTROW Name FROM tblCategory INNER JOIN tblExpenses ON tblCategory.categoryID = tblExpenses.categoryID;<br/><br/>

looks at the entire underlying tables, not just the output fields, to find unique rows.

reference: http://www.fmsinc.com/microsoftaccess/query/distinct_vs_distinctrow/unique_values_records.asp