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
Related
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
I have the following table:
id symbol_01 symbol_02
1 abc xyz
2 kjh okd
3 que qid
I need a query that ensures symbol_01 and symbol_02 are both contained in a list of valid symbols. In other words I would needs something like this:
select *
from mytable
where symbol_01 in (
select valid_symbols
from somewhere)
and symbol_02 in (
select valid_symbols
from somewhere)
The above example would work correctly, but the subquery used to determine the list of valid symbols is identical both times and is quite large. It would be very innefficient to run it twice like in the example.
Is there a way to do this without duplicating two identical sub queries?
Another approach:
select *
from mytable t1
where 2 = (select count(distinct symbol)
from valid_symbols vs
where vs.symbol in (t1.symbol_01, t1.symbol_02));
This assumes that the valid symbols are stored in a table valid_symbols that has a column named symbol. The query would also benefit from an index on valid_symbols.symbol
You could try use a CTE like;
WITH ValidSymbols AS (
SELECT DISTINCT valid_symbol
FROM somewhere
)
SELECT mt.*
FROM MyTable mt
INNER JOIN ValidSymbols v1
ON mt.symbol_01 = v1.valid_symbol
INNER JOIN ValidSymbols v2
ON mt.symbol_02 = v2.valid_symbol
From a performance perspective, your query is the right way to do this. I would write it as:
select *
from mytable t
where exists (select 1
from valid_symbols vs
where t.symbol_01 = vs.valid_symbol
) and
exists (select 1
from valid_symbols vs
where t.symbol_02 = vs.valid_symbol
) ;
The important component is that you need an index on valid_symbols(valid_symbol). With this index, the lookup should be pretty fast. Appropriate indexes can even work if valid_symbols is a view, although the effect depends on the complexity of the view.
You seem to have a situation where you have two foreign key relationships. If you explicitly declare these relationships, then the database will enforce that the columns in your table match the valid symbols.
I'd like to achieve something as follows, I have the following query (As simple as this),
SELECT ENT_ID,TP_ID FROM TC_LOGS WHERE ENT_ID IN (1,2,3,4,5).
Now the table TC_LOGS may not have all the items in the IN clause. So assuming that the table TC_LOGS has only 1,2. I'd like to compare the items in the IN clause i.e. 1,2,3,4,5 with 1,2(the resultset) and get a result as FOUND - 1,2 NOT FOUND - 3,4,5. I've have implemented this by applying an XSL transformation on the resultset in the application code, but I'd like to achieve this in a query, which I feel is more of an elegant solution to this problem. Also, I tried the following query with NVL, just to separate out the FOUND and NOT FOUND items as,
SELECT NVL(ENT_ID,"NOT FOUND") FROM TC_LOGS WHERE ENT_ID IN(1,2,3,4,5)
I was expecting a result as 1,2,NOT FOUND,NOT FOUND,NOT FOUND
But the above query doesn't return any result.. I'd appreciate if someone can guide me in the right path here.. Thanks much in advance.
Assuming that the items in your IN list can (or can come) from another query, you can do something like
WITH src AS (
SELECT level id
FROM dual
CONNECT BY level <= 5)
SELECT nvl(ent_id, 'Not Found' )
FROM src
LEFT OUTER JOIN tc_logs ON (src.id = tc_logs.ent_id)
In my case, the src query is just generating the numbers 1 through 5. You could just as easily fetch that data from a different table, load the numbers into a collection that you query using the TABLE operator, load the numbers into a temporary table that you query, etc. depending on how the IN list data is determined.
NVL isn't going to work because no values (including NULLS) are returned when there is no match with the IN statement.
What you can do is something like this:
SELECT NVL(ENT_ID, "NOT FOUND")
FROM TC_LOGS
RIGHT OUTER JOIN (
SELECT 1 AS 'TempID' UNION
SELECT 2 UNION
SELECT 3 UNION
SELECT 4 UNION
SELECT 5) AS Sub ON ENT_ID = TempID
The outer join will return NULLS for ENT_ID where there are no matches. Note, I'm not an Oracle person so I can't guarantee that this syntax is perfect.
if you have a table (let's use table src )contains all (1,2,3,4,5) values, you can use full join.
You can use (WITH src AS ( SELECT level id FROM dual CONNECT BY level <= 5) as the src table also)
SELECT
ent_id,tl.tp_id,src.tp_id
FROM
src
FULL JOIN
tc_logs tl
USING (ent_id)
ORDER BY
ent_id
Here is the web site for oracle full join.http://psoug.org/snippet/Oracle-PL-SQL-ANSI-Joins-FULL-JOIN_738.htm
What's the best/right/fastest/most appropriate way to detect if a row with a given rowid exists?
Or by extension, hwo to detect if at least one row matching a given condition exists?
I'm firing quite some of these requests. I am currently using
SELECT 1 FROM table WHERE condition LIMIT 1
looks a bit weird to me, but looks to me like "the least work" for the db, however, my SQL knowledge is spotty.
I would probably do it something like this:
SELECT
CASE
WHEN EXISTS(SELECT NULL FROM table1 WHERE ID=someid)
THEN 1
ELSE 0
END
To Count the rows is not that effective.
To check if something exists is in most cases more effective
Since it's sqlite, you need to use the column name "rowid" to access that id column. Using Craig Ringer's sql, the sqlite version would look like this:
SELECT EXISTS(SELECT 1 FROM table WHERE rowid = insert_number)
Use EXISTS, it sounds perfect for what you are after. e.g.
SELECT *
FROM T1
WHERE EXISTS (SELECT 1 FROM T2 WHERE T2.X = T1.X AND T2.Y = 1)
It is effectly the same as LIMIT 1 but is generally optimised better.
One could test
select true from table where id = id1
You can for example use
SELECT COUNT(*) FROM table WHERE ID = whatever
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 & ")"