I have a query using LINQ-to-SQL. It queries an underlying database table Rooms. It uses Where conditions to narrow down the results, namely:
Gender.
Current Occupancy < Max Occupancy
Available Flag is Checked
I know this should return results but it keeps returning an empty set. Any ideas? Code is below
Dim selectedHalls = (From sh In dbHalls.Rooms _
Where sh.gender = Session("gender").ToString _
Where sh.max_occupancy > sh.current_occupancy _
Where sh.is_available = 1 _
Select sh.building_name).Distinct()
UPDATE: I've verified that the issue is with the statement where sh.is_available = 1, which doesn't make sense since this is a bit field.
I think the best way to find out what the problem is to generate the SQL string and test your assumption that "it should return results" by executing it directly against the data source.
To do this:
Dim sqlQuery As String = dbHalls.GetCommand(
(From sh In dbHalls.Rooms _
Where sh.gender = Session("gender").ToString _
Where sh.max_occupancy > sh.current_occupancy _
Where sh.is_available = 1 _
Select sh.building_name).Distinct()
).CommandText
(I usually use C#, but I think that's how you declare a string in VB, right?)
Anyway, this will give you an SQL statement that will be more informative than anything we can give you without being able to look at your underlying database.
The only thing that jumps out at me as being potentially problematic is the Session("gender"). You are basically relying on your Session object to be populated, have a value for the case-sensitive string key "gender" that matches the case-sensitive string field gender in your database. This sounds like quite a few assumptions, that may or may not be tested and could be the reason for receiving empty results.
EDIT
I just saw your update. Linq-to-sql, interprets a bit field as a boolean value, not an integer value. Try changing it to just where sh.is_available
I don't see anything obviously wrong with your LINQ, so try removing the Where clauses and bringing them back one at a time. That should let you know what's causing the issue, though I agree with Andrew Vogel that the gender clause is a likely suspect.
If the query comes back empty after all the Where clauses are gone, then you'll know there's something else going on here.
Do you really record their genders as "Female" and "Male" in dbHalls.Rooms.sh?
Related
I never should've expected that knowing mySQL I'd be safe using Access.
I have two tables: users and scores
users table contains: id(auto increment primary key), username, password, etc..
scoers table contains: id(number - foreign key to users.id), highScore
I've previously asked help for INSERT command, which now works as it should. Now I've got issues with a similar UPDATE command.
The non-working command looks like this:
string updateCommand = #"UPDATE scores
SET
id = (SELECT id FROM users WHERE username = #username),
highScore = #score
WHERE highScore = (SELECT MIN(highScore) FROM scores);";
which throws a: Operation must use an updateable query.
To rationalize what I'm trying to accomplish here: I'm INSERT-ing high scores until I reach 10 scores in the table, afterwards instead of adding any new scores and filling up the database needlessly I decided It'd be more sensible to just "overwrite" the currently lowest score using UPDATE.
I am supplied a username and the high score and since the scores table contains only id I need to reach the id of the current user so that's what the first subquery is doing, the second subquery in the WHERE clause is to specify which score to replace (though there is possibly a bug here if there are multiple people with the lowest score, any ideas how to fix that?)
I've also tried using OUTER RIGHT JOIN like this:
string updateCommand = #"UPDATE scores
OUTER RIGHT JOIN users ON scores.id = users.id
SET
scores.id = users.id,
scores.highScore = #score
WHERE (highScore = (SELECT MIN(highScore) FROM scores)) AND (username = #username);";
With no luck(I get a generic Syntax error in UPDATE statement.).
Browsing the net I've found that I possibly "can't" use subqueries in UPDATE statements but I seem to find conflicting opinions on the matter.
I've also tried using the DLookup function in place of subqueries like:
#"...
id = DLookup(""id"", ""users"", ""username = #username""),
...
WHERE highScore = DLookup(""MIN(highScore)"", ""scores"");";
elipses represent extraneous code which is identical to the code above.
Also as a last resort I've tried dividing into multiple queries however userId query which looks like this:
string userIdQuery = "SELECT id FROM users WHERE username = #username"
seems to return a null judging by the NullReferenceException i recieve (Object reference not set to an instance of an object.) when trying to use the variable userId after I've done this:
int userId = 0;
userId = (Int32)command.ExecuteScalar();
I'm supposed to get an integer however I get a null I think. The almost identical query for getting the minimum highscore works flawlessly and the int variable is filled with the correct value so I'm assuming that hte problem is in the query itself somehow. I've tried adding single quotes around the #username parameter assuming that it might not be recognizing the string but it seems that's not it.
Phew.. took me a while to write this. Anyone got any ideas on how to make this all work? If you need more info let me know.
So after some messing around I've found out the causes of my troubles. The bad side is that I increased the amount of code so that I'd avoid subqueries as much as possible since, at least from my experience, Access doesn't really like the use of subqueries in UPDATE or INSERT commands.
What I did first is split the command into 3 separate ones:
"SELECT id FROM users WHERE username = ?;" - To get the id of the user whose score
I'm putting in the database.
#"SELECT scores.id, scores.highScore, scores.dateTime FROM scores WHERE (((scores.highScore)=DMin(""highScore"",""scores"")));" - which gets the id, high score
and time when the entry was... well entered, of the lowest score currently in the high scores list. Thanks to a suggestion from HansUp I used DMin function instead of a subquery with MIN to avoid the Must use an updateable query error. The extraneous parentheses are due to Access since this command was generated by the Access query designer and I'm too afraid to change anything lest I break it.
#"UPDATE scores SET scores.id = ?, scores.highScore = ?, scores.[dateTime] = Now() WHERE (((scores.id)=?) AND ((scores.highScore)=?) AND ((scores.dateTime)=?));" - The update command itself, not much to say here except that it takes the previously extracted data and uses it as values for the command.
One thing I noticed is that even if I got the command working the .ExecuteNonQuery() would always return 0 rows affected. After poking around I found out that named parameters for commands in ASP.NET / C# don't always work and that instead ? should be used instead. It's kind of inconvenient but I can't complain too much.
I am writing a code that calls a column from a dataset using a SQL query. I use two parameters to identify which rows to select. One is the ProductSerialNumber, and the other is a datetimestamp. See my SQL query below
Select TestStation FROM tblData
WHERE ProductSerialNumber = ? AND Datetimestamp = ?
In the dataset's datatable the productserialnumber is formatted as text, and the other is formatted as a date (as you would expect).
In my vb.net code, I grab the Datetimestamp from another source (don't ask why, the only thing you need to know is that it grabs a valid datetimestamp, dimensioned as a date, that matches exactly with the tblData's entry) and I use the premade query to generate a datatable. The query is a Fill query called "TestStationLookUp"
my vb.net code looks like this
Dim dt as new dataset.tbldataDataTable
Dim dta As New DataSetTableAdapters.tbldataTableAdapter
Dim ProductSerialNumber as string = "XXXXXX"
Dim DateTimeStamp as date = SomeDateVariable
dta.TestStationLookUp(dt, ProductSerialNumber, DateTimeStamp)
It is here that the code tells me:
Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints.
Line 7366: dataTable.Clear
Line 7367: End If
Error: Line 7368: Dim returnValue As Integer = Me.Adapter.Fill(dataTable)
Line 7369: Return returnValue
Line 7370: End Function
I cannot understand why this error arises, as everything is dimensioned the way it should be. This exact code setup works elsewhere in my code (except it doesn't use a date), but this specific piece won't work.
Also, if I go to the dataset in my solution, I can use the "preview data" on this query and type in the EXACT same parameters (the ProductSerialNumber and DateTimeStamp that match the record in the table AND what I use in my vb code) and it will give me produce the table I want.
Can anyone assist?
This error means that you are trying to access not valid unique id "ProductSerialNumber", maybe it does not exist
Failed to enable constraints. One or more rows contain values
violating non-null, unique, or foreign-key constraints.
Instead of passing the variable that comes from dataset ,pass a valid number that you are sure it exists in database
I have the following LINQ query:
Dim find_id = From p In dbContext.Residents _
Where p.person_name = occupant _
Select p
I then pull the first returned ID like so:
Dim building_id As String = find_building_id.FirstOrDefault.id
This works fine, except with the person has a ' in their last name, like M'arta. In this case I get a NullReferenceException. Any thoughts on how I can get around this issue?
Okay, the problem appears to be that when I am capturing the name initially from a GridView it is inserting into the value a #39 instead of ' and then LINQ is keeping things correct, so I end up with a non-match since it is attempting to match:
M#38arta = M'arta
Maybe it's an Unicode issue? I once had trouble with wrong settings of the 'Unicode' attribute of scalar string properties. Maybe your SQL connection string has defined a wrong charset? And are your sure it's a single quote and no kind of accent or a special char, looking similiar but with another code?
Just my spontanious ideas at night.
Here is my code:
I should get output of the department id (did) as an integer and the templatefilename (result) that is required.
The errors I get are: Conversion from string "ad" to type 'Integer' is not valid. I'm fairly new to asp.net and cannot see where the did variable picks up the "ad" string.
Any help would be greatly appreciated.
Thanks
When you construct the query to the table departmentsgroupings, you're changing the value of sql, but you aren't creating a new SqlCommand. This means that cmd still contains the old SQL statement (the query to the Modules table) which, when executed, returns "ad".
To fix this, change your code as follows:
sql = ("select departmentsid from departmentsgroupings where groupingid =" & pageid & "")
Set cmd = New SqlCommand(sql, conn)
did = (cmd.ExecuteScalar)
You may have expected the change you made to sql to get passed on automatically to the SqlCommand -- but it doesn't work that way.
Edit: Your code, as written, is vulnerable to SQL injection attacks. If you don't know what these are, you need to read the first answer to this:
How does the SQL injection from the "Bobby Tables" XKCD comic work?
To protect yourself against these kinds of attacks, use parameterized queries.
The mistake is in these lines:
sql = ("select departmentsid from departmentsgroupings where groupingid =" & pageid & "")
did = (cmd.ExecuteScalar) <---- Wrong command executed here.
You presumably meant to execute the code in sql, not cmd again.
I am constructing a search function in a class to be used by several of our asp pages. The idea is simple, take a search term from the user and query the database for the item. Currently I am doing this the wrong way, which is vulnerable to SQL injection attacks (and ELMAH is in there to save the day if something goes wrong):
Public Shared Function SearchByName(ByVal searchterm As String) As DataTable
SearchByName = New DataTable
Dim con As New OracleConnection(System.Configuration.ConfigurationManager.ConnectionStrings("OracleDB").ConnectionString)
Try
con.Open()
Dim SqlStr As String = "select ID_ELEMENT, ELEMENT_NAME from table_of_elements where upper(ELEMENT_NAME) like upper('%" & searchterm & "%')"
Dim cmd As New OracleCommand(SqlStr, con)
SearchByName.Load(cmd.ExecuteReader)
Catch ex As Exception
Elmah.ErrorSignal.FromCurrentContext().Raise(ex)
End Try
con.Close()
con.Dispose()
Return SearchByName
End Function
String concatenation is BAD. Next thing you know, Bobby Tables wrecks my system.
Now, the correct way to do this is to to make a proper oracle variable, by putting :searchterm in the string and adding the following line:
cmd.Parameters.Add(New OracleParameter("SEARCHTERM", searchterm))
The problem is since I am using a like statement, I need to be able to have % on either side of the search word, and I can't seem to do that with '%:searchterm%', it just gives an error of ORA-01036: illegal variable name/number.
Can I parameterize but still have my flexible like statement be a part of it?
Instead of doing the concatenation in your VB code, do the concatenation in the SQL statement. Then what you're trying to do should work. Here's some SQL illustrating what I'm talking about:
select ID_ELEMENT, ELEMENT_NAME
from table_of_elements
where upper(ELEMENT_NAME) like ('%' || upper(:searchterm) || '%')
BTW, you might end up with more efficient queries if you switch the collation on ELEMENT_NAME to case-insensitive and then remove the calls to upper().
Since you're using oracle, another option would be to use Oracle Text to perform the search.
It can take a bit to set up properly, but if you have a large amount of text to search, or have some sort of structured data, it can offer you many more options than a simple wild-card comparison.
It also has some nice features for dealing with multiple languages, if you happen to have that problem as well.