I am making a vb.net project. In one form, I want it to work like if user presses a button it first checks if a table(Built with SQL Server) is empty or not. If it is empty it will open another form otherwise Resume functioning. How to check if the table is empty or not.
Thanks.
If you are after a sql statement that are check if there any rows in a table. THen you can do something like this:
SELECT
(
CASE WHEN NOT EXISTS(SELECT NULL FROM yourTable)
THEN 1
ELSE 0
END
) AS isEmpty
You can execute a SQL query to find the row count of your required table and then based on that count you can apply your logic by using conditional commands like If[...]Else:
Dim count As Int16
con.open()
query = "select count(*) from requiredTable"
cmd = New SqlCommand(query, con)
count = Convert.ToInt16(cmd.ExecuteScalar())
con.Close()
Alternatively
SELECT TOP(1) 1 FROM MyTable
and in your vb code check the number of rows returned ( 0 rows = table is empty)
A Bit late, but for all people googled the same:
Slow Version:
select distinct 1 from MyTable ;
select count(*) from MyTable ;
Fast Version:
select count(*) from (select 1) where exists (select * from MyTable);
Maybe different products may have different results
Related
I have this sql code where I try to display an alternative value as a result whenever the table is empty or the the single column of the top row when it is not
select top 1 case when count(*)!=0 then derrivedTable.primarykey
else 0 end endCase
from
(
select top 1 m.primarykey
from mytable m
where 0=1
)derrivedTable
The problem is that when I run this, I get the error message "column 'derrivedTable.primarykey' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause."
But when I put 'derrivedTable.primarykey' in the group by clause, I just get an empty table.
Does anyone hve a solution?
thanks in advance
You can use aggregation:
select coalesce(max(m.primarykey), 0)
from mytable m;
An aggregation query with no group by always returns exactly one row. If the table is empty (or all rows are filtered out), then the aggregation functions -- except for COUNT() -- return NULL -- which can be transformed to a value using COALESCE().
Such a construct makes me worry. If you are using this to set the primary key on an insert, then you should learn about identity columns or sequences. The database will do the work for you.
Can you try this below script-
SELECT
CASE
WHEN COUNT(*) = 1 THEN derrivedTable.primarykey
ELSE 0
END endCase
FROM
(
SELECT TOP 1 m.primarykey
FROM mytable m
WHERE 0 = 1
) derrivedTable
derrivedTable.primarykey;
I have one settings table that is storing different setting keys for a app built using Microsoft Access
One of the settings key drives how many records should be seen in a dropdown list
The query behind the list is similar to the one below:
Select Top 3 id, name FROM tblRegular
Now, I want to achieve something like this:
Select Top (Select keyValue FROM tblSettings WHERE key="rowNumber") id, name FROM tblRegular
But, using it like this does not work as it fires errors
Could someone tell me if it can be done?
EDIT: The table structure looks similar to the one below:
tblRegular:
id | name
1 'A'
2 'B'
3 'C'
tblSettings:
id | key | keyValue
1 'rowNumber' 2
Thank you!
Consider the pure SQL solution using a correlated subquery to calculate a rowCount that is then used in outer query to filter by number of rows:
SELECT main.id, main.[name]
FROM
(SELECT t.id, t.[name],
(SELECT Count(*) FROM tblRegular sub
WHERE sub.[name] <= t.[name]) AS rowCount
FROM tblRegular t) AS main
WHERE main.rowCount <= (SELECT Max(s.keyValue) FROM tblSettings s
WHERE s.key = 'rowNumber')
Alternatively with the domain aggregate, DMax():
SELECT main.id, main.[name]
FROM
(SELECT t.id, t.[name],
(SELECT Count(*) FROM tblRegular sub
WHERE sub.[name] <= t.[name]) AS rowCount
FROM tblRegular t) AS main
WHERE main.rowCount <= DMax("keyValue", "tblSettings", "key = 'rowNumber'")
This syntax does fail in Access SQL, barking at "Select" with the (localised) message like:
The SELECT sentence contains a reserved word or argument, that is
misspelled or missing, or the punctuation is not correct.
Ok, so, just found out that using the Select statement as it was in the question would have triggered the errors mentioned. So, the approach that worked is to alter the .RowSource of the dropdown on form load and the query placed in the rowsource should look like:
Select Top (" & rowNr & ") id, name FROM tblRegular
where rowNr is fetched using another SQL query or even a DAO/ADO function to retrieve the value from the database
I'm quite new to SQL and I'm having issues with deleting a selected row/s from a table.
I've written a query that selects the desired rows from the table, but when I try to execute DELETE FROM table_name WHERE EXISTS it deletes all the rows in the database.
Here is my complete query:
DELETE FROM USR_PREF WHERE EXISTS (
SELECT *
FROM USR_PREF
WHERE USR_PREF.USR_ID = 1
AND ((USR_PREF.SRV NOT IN (SELECT SEC_ENTITY_FOR_USR_ACTION_VIEW.ENTITYT_ID
FROM SEC_ENTITY_FOR_USR_ACTION_VIEW
WHERE SEC_ENTITY_FOR_USR_ACTION_VIEW.USR_ID = 1
AND SEC_ENTITY_FOR_USR_ACTION_VIEW.ENTITYTYP_CODE = 2
AND USR_PREF.DEVICE IS NULL)
OR (USR_PREF.DEVICE NOT IN (SELECT SEC_ENTITY_FOR_USR_ACTION_VIEW.ENTITYT_ID
FROM SEC_ENTITY_FOR_USR_ACTION_VIEW
WHERE SEC_ENTITY_FOR_USR_ACTION_VIEW.USR_ID = 1
AND SEC_ENTITY_FOR_USR_ACTION_VIEW.ENTITYTYP_CODE = 3)))))
The select query returns the desired rows, but the DELETE command just deletes that entire table.
Please assist.
Your where clause WHERE EXISTS (SOME QUERY) is the problem here. You are basically saying "Delete everything if this subquery returns even one result".
You need to be more explicit. Perhaps something like:
DELETE FROM USR_PREF
WHERE USR_FIELD IN (
SELECT USR_FIELD
FROM USR_PREF
WHERE USR_PREF_T.USER_ID=1
AND ((USR_PREF.SRV NOT IN ...
and so on... With this, only records that match records returned in your subquery will be deleted.
I have a valid SQL select which returns an empty result, up and until a specific transaction has taken place in the environment.
Is there something available in SQL itself, that will allow me to return a 0 as opposed to an empty dataset? Similar to isNULL('', 0) functionality. Obviously I tried that and it didn't work.
PS. Sadly I don't have access to the database, or the environment, I have an agent installed that is executing these queries so I'm limited to solving this problem with just SQL.
FYI: Take any select and run it where the "condition" is not fulfilled (where LockCookie='777777777' for example.) If that condition is never met, the result is empty. But at some point the query will succeed based on a set of operations/tasks that happen. But I would like to return 0, up until that event has occurred.
You can store your result in a temp table and check ##rowcount.
select ID
into #T
from YourTable
where SomeColumn = #SomeValue
if ##rowcount = 0
select 0 as ID
else
select ID
from #T
drop table #T
If you want this as one query with no temp table you can wrap your query in an outer apply against a dummy table with only one row.
select isnull(T.ID, D.ID) as ID
from (values(0)) as D(ID)
outer apply
(
select ID
from YourTable
where SomeColumn = #SomeValue
) as T
alternet way is from code, you can check count of DataSet.
DsData.Tables[0].Rows.count > 0
make sure that your query matches your conditions
select Top(1)* from TableName Where columnName=value
selects only the first row just fine. However if I change the select to a delete I get an error and can't figure out how to write a query to delete only 1 record that matches my query from the db.
I'm wondering if anybody out there smarter than I knows if or how this can be done in SQL CE.
Did you try something like this?
DELETE TableName where IdColumn In ( select Top(1) IdColumn from TableName Where columnName=valuev)
I don't know specifically as I'm at home, but SQL CE is deliberately restricted in what it can do. One reason for this is that it is 'always' running locally to the process referencing it.
What means is that it is Expected that the other process is expected to handle much of the logic that may otherwise be encapsulated in the SQL Server. This often results in firing several queries at the SQL CE instance, where you may be more accustomed to firing off one.
In this case, you could do it with two queries...
1) A query to identify the record that you want to delete
2) Use that Identifier in another query to do the actual delete
You could also try using SET ROWCOUNT 1 to limit the DELETE to just 1 row. But again, I don't know if that works in CE.
You can use CTE such as
;with myTopRow(rowID)
(
select Top 1 rowID from TableName Where columnName=value
)
delete from TableName inner join myTopRow on TableName.rowID = myTopRow.rowID
Shouldn't it be rather :
DELETE FROM TableName WHERE columnName=value ORDER BY columnName LIMIT 1;
IMHO, the table logically has NO order by itself. It has it physically, but you can't rely on it. So, you HAVE to set the order in which you want to delete the first row.
The following code will delete only first row
Dim mySqlCommondDelete As String = "DELETE BOOK_ID, MemberID FROM (SELECT TOP 1 * FROM ISSUE_BOOK) where BOOK_ID = Val(" & deleteBook & ") and MemberID = Val(" & msk & ")"