如何在linq中选择所有sql

Question

In my LINQ query, I am trying to select all if a value is null :

var filteredLesson = (from l in storeDB.lessons
                      where l.statusID == SUBMITTED ||
                            l.statusID == APPROVED &&
                            l.projectID == (l.projectID.HasValue 
                                ? lesson.projectID : /*select all*/  )
                      select l).ToList();

Is there a way to do this?

Answer 1

You just want to compare it to itself if the value is null . That way your row will be included in the results if the value is null .

var filteredLesson = (from l in storeDB.lessons
                      where l.statusID == SUBMITTED ||
                      l.statusID == APPROVED &&
                      l.projectID == (l.projectID.HasValue 
                                   ? lesson.projectID : l.projectID )
                      select l).ToList();

Answer 2

Something like this perhaps?

var filteredLesson = (from l in storeDB.lessons
                      where
                          (l.statusID == SUBMITTED || l.statusID == APPROVED)
                          &&
                          (!l.projectID.HasValue || l.projectID == lesson.projectID)
                      select l
                     ).ToList();

Answer 3

Without knowing your object model it's hard to be totally sure about this, but I imagine something like the following should be what you're looking for:

var filteredLesson = storeDB.lessons
                            .Where(l => (l.statusID == SUBMITTED
                                       || l.statusID == APPROVED)
                                       && !l.projectID.HasValue)
                            .ToList();

Answer 4

Here's are many of the above answers written up using xUnit.Net tests. Note that the original author's comment in the answer he selected (which is an incorrect solution) gives the results he expected. I cleaned up his solution to remove the duplicate comparisons.

Sample values are at the bottom.

    public class SelectAllInLinq2SqlTests
    {
        [Fact]
        public void Unsuccessful()
        {
            var filteredLesson =
                (from l in Lessons
                 where l.statusID == SUBMITTED ||
                       l.statusID == APPROVED &&
                       l.projectID == (l.projectID.HasValue ? projectOne : l.projectID)
                 select l).ToList();

            Assert.Equal(4, filteredLesson.Count);

            // this returned 5 items!
            // good - filtered the null project IDs and statusID == SUBMITTED
            // good - filtered the null project IDs and statusID == APPROVED
            // good - filtered the projectID == projectOne and statusID == SUBMITTED
            // good - filtered the projectID == projectOne and statusID == APPROVED
            // FAIL - filtered the projectID == projectTwo !!!

            // two failures first selecting all records containing a null projectID and any status (we wanted to filter on SUBMITTED or APPROVED)
            // the failure is due to operator precedence the 'l.statusID == SUBMITTED ||' is short-circuiting the rest of the comparison
        }

        [Fact]
        public void Unsuccessful2()
        {
            var filteredLesson =
                (from l in Lessons
                 where !l.projectID.HasValue ||
                       (l.statusID == SUBMITTED || l.statusID == APPROVED && l.projectID == projectOne)
                 select l).ToList();

            Assert.Equal(4, filteredLesson.Count);

            // this returned 6 items!
            // good - filtered the null project IDs and statusID == SUBMITTED
            // good - filtered the null project IDs and statusID == APPROVED
            // FAIL - filtered the null project IDs and statusID == OTHER!
            // good - filtered the projectID == projectOne and statusID == SUBMITTED
            // good - filtered the projectID == projectOne and statusID == APPROVED
            // FAIL - filtered the projectID == projectTwo !!!

            // Two failures:
            //   first selecting all records containing a null projectID and any status (we wanted to filter on SUBMITTED or APPROVED)
            //   second failure is due to operator precedence the 'l.statusID == SUBMITTED ||' is short-circuiting the rest of the comparison
        }

        [Fact]
        public void Unsuccessful2NotQuiteFixed()
        {
            var filteredLesson =
                (from l in Lessons
                 where !l.projectID.HasValue
                       || ((l.statusID == SUBMITTED || l.statusID == APPROVED)
                           && l.projectID == projectOne)
                 select l).ToList();

            Assert.Equal(4, filteredLesson.Count);
            // this returned 5 items!
            // good - filtered the null project IDs and statusID == SUBMITTED
            // good - filtered the null project IDs and statusID == APPROVED
            // FAIL - filtered the null project IDs and statusID == OTHER!
            // good - filtered the projectID == projectOne and statusID == SUBMITTED with a SUBMITTED status
            // good - filtered the projectID == projectOne and statusID == APPROVED with a APPROVED status

            // failures selecting all records containing a null projectID and any status (we wanted to filter on SUBMITTED or APPROVED)
        }

        [Fact]
        public void Sucessful()
        {
            // Questioner's provided solution...

            var filteredLesson =
                (from l in Lessons
                 where !l.projectID.HasValue && (l.statusID == SUBMITTED || l.statusID == APPROVED)
                       || l.projectID == projectOne && (l.statusID == SUBMITTED || l.statusID == APPROVED)
                 select l).ToList();

            Assert.Equal(4, filteredLesson.Count);
        }

        [Fact]
        public void SucessfulRefined()
        {
            // cleaned up without the duplicate status comparisons. Note the parens are necessary.

            var filteredLesson =
                (from l in Lessons
                 where (!l.projectID.HasValue || l.projectID == projectOne)
                       && (l.statusID == SUBMITTED || l.statusID == APPROVED)
                 select l).ToList();

            Assert.Equal(4, filteredLesson.Count);
        }

        public class Lesson
        {
            public int? projectID { get; set; }
            public int statusID { get; set; }
        }

        const int OTHER = 0;
        const int SUBMITTED = 1;
        const int APPROVED = 2;

        const int projectOne = 1;

        Lesson[] Lessons = new[]
                          {
                              new Lesson { projectID = null, statusID = SUBMITTED},
                              new Lesson { projectID = null, statusID = APPROVED},
                              new Lesson { projectID = null, statusID = OTHER},
                              new Lesson { projectID = 1, statusID = SUBMITTED},
                              new Lesson { projectID = 1, statusID = APPROVED},
                              new Lesson { projectID = 1, statusID = OTHER},
                              new Lesson { projectID = 2, statusID = SUBMITTED},
                              new Lesson { projectID = 2, statusID = APPROVED},
                              new Lesson { projectID = 2, statusID = OTHER},
                          };
    }
}