I have a complex SQL query that can be simplified to the below:
Select ColA,ColB,ColC,ColD
From MyTable
Where (ColA In (Select ItemID From Items Where ItemName like '%xxx%')
or ColB In (Select ItemID From Items Where ItemName like '%xxx%'))
As you can see, the sub-query appears twice. Is the compiler intelligent enough to detect this and gets the result of the sub-query only once? Or does the sub-query run twice?
FYI, table Items has about 20,000 rows and MyTable has about 200,000 rows.
Is there another way to re-write this SQL statement so that the sub-query appears/runs only once?
Update: The Where clause in the main query is dynamic and added only when needed (i.e. only when a user searches for 'xxx'). Hence changes to the main select statement or re-structuring of the query are not possible.
UPDATE Your request not to change the query, just the WHERE
You can pack the CTE directly in the place where it is called (untested):
Select ColA,ColB,ColC,ColD
From MyTable
Where EXISTS (SELECT 1 FROM (Select i.ItemID
From Items AS i
Where iItemName like '%xxx%') AS itm
WHERE itm.ItemID=MyTable.ColA OR itm.ItemID=MyTable.ColB)
previous
I think this should be the same...
WITH MyCTE AS
(
Select ItemID From Items Where ItemName like '%xxx%'
)
Select ColA,ColB,ColC,ColD
From MyTable
Where EXISTS (SELECT 1 FROM MyCTE WHERE ItemID=ColA OR ItemID=ColB)
A substring LIKE search is - for sure - not performant.
If you can reduce your "Items" to just a few rows with your LIKE filter, you must test which is fastest.
You can also write the query like this:
SELECT ColA, ColB, ColC, ColD
FROM MyTable
WHERE EXISTS(
(SELECT ItemID FROM Items WHERE ItemName LIKE '%xxx%')
INTERSECT
SELECT t.v FROM (VALUES (ColA), (ColB)) AS t(v) )
There is no guarantee that it will follow the actual execution order. It depends on how you write a query. Identical subqueries are normally only performed once.
There is a WITH clause in standard SQL.
WITH mySubQuery AS
(
[the subquery code]
)
SELECT * FROM
mySubQuery AS sq
WHERE xyz IN (mySubQuery)
SQL programmers can use CTE (Common Table Expression) in such cases
You can define a CTE using the sub-query once and use it in the SQL statement by referencing more than once.
Please refer to SQL CTE Common Table Expression tutorial for samples
CTE's are very powerful tools for SQL developers especially when used as recursive-queries
Related
I don't understand why I can't use this in my code :
SELECT MAX(SMTHNG), COUNT(MAX(SMTHNG))
FROM SomeTable;
Searched for an answer but didn't find it in documentation about these aggregate functions.
Also I get an SQL-compiler error "Invalid column name "SMTHNG"".
You want to know what the maximum SMTHNG in the table is with:
SELECT MAX(SMTHNG) FROM SomeTable;
This is an aggregation without GROUP BY and hence results in one single row containing the maximum SMTHNG.
Now you also want to know how often this SMTHNG occurs and you add COUNT(MAX(SMTHNG)). This, however, does not work, because you can not aggregate an aggregate directly.
This doesn't work either:
SELECT ANY_VALUE(max_smthng), COUNT(*)
FROM (SELECT MAX(smthng) AS max_smthng FROM sometable) t;
because the sub query only contains one row, so it's too late to count.
So, either use a sub query and select from the table again:
SELECT ANY_VALUE(smthng), COUNT(*)
FROM sometable
WHERE smthng = (SELECT MAX(smthng) FROM sometable);
Or count per SMTHNG before looking for the maximum. Here is how to get the counts:
SELECT smthng, COUNT(*)
FROM sometable
GROUP BY smthng;
And the easiest way to get the maximum from this result is:
SELECT TOP(1) smthng, COUNT(*)
FROM sometable
GROUP BY smthng
ORDER BY COUNT(*) DESC;
First of all, please read my comment.
Depending on what you're trying to achieve, the statement have to be changed.
If you want to count the highest values in SMTHNG field, you may try this:
SELECT T1.SMTHNG, COUNT(T1.SMTHNG)
FROM SomeTable T1 INNER JOIN
(
SELECT MAX(SMTHNG) AS A
FROM SomeTable
) T2 ON T1.SMTHNG = T2.A
GROUP BY T1.SMTHNG;
use cte like below or subquery
with cte as
(
select count(*) as cnt ,col from table_name
group by col
) select max(cnt) from cte
you can not use double aggregate function at a time on same column
Can one filter a table dynamically with a 'where' clause acting on a value of a field from another table under some other conditions such that it is made sure only one row is returned? Can I do something like this?
SELECT COUNT(*) FROM stud t1
WHERE t1.name==SELECT name FROM (
SELECT name, row_number() over (PARTITION BY name) AS rn
FROM stud t2) t3
WHERE t3.rn==1;
Of course, the above query is just a dummy one, but is filtering on where clause like above possible theoretically? If not how could one achieve such a functionality in the cases of more complex queries?
Yes. Query can be made like this:
SELECT COUNT(*) FROM stud t1
WHERE t1.name = (SELECT name
from sometable
where somecondition);
but you need to make sure that the subquery return zero or one row. If your query may return more than one row, you can use IN instead:
SELECT COUNT(*) FROM stud t1
WHERE t1.name IN (SELECT name
from sometable
where somecondition);
I have an oracle database table with a lot of columns. I'd like to count the number of fully unique rows. The only thing I could find is:
SELECT COUNT(DISTINCT col_name) FROM table;
This however would require me listing all the columns and I haven't been able to come up with syntax that will do that for me. I'm guessing the reason for that is that this query would be very low performance? Is there a recommended way of doing this?
How about
SELECT COUNT(*)
FROM (SELECT DISTINCT * FROM Table)
It depends on what you are trying to accomplish.
To get a count of the distinct rows by specific column, so that you know what data exists, and how many of that distinct data there are:
SELECT DISTINCT
A_CODE, COUNT(*)
FROM MY_ARCHV
GROUP BY A_CODE
--This informs me there are 93 unique codes, and how many of each of those codes there are.
Another method
--How to count how many of a type value exists in an oracle table:
select A_CDE, --the value you need to count
count(*) as numInstances --how many of each value
from A_ARCH -- the table where it resides
group by A_CDE -- sorting method
Either way, you get something that looks like this:
A_CODE Count(*)
1603 32
1600 2
1605 14
I think you want a count of all distinct rows from a table like this
select count(1) as c
from (
select distinct *
from tbl
) distinct_tbl;
SELECT DISTINCT **col_name**, count(*) FROM **table_name** group by **col_name**
I would like to do something like
(SELECT ... FROM ...) AS my_select
WHERE id IN (SELECT MAX(id) FROM my_select GROUP BY name)
Is it possible to somehow do the "AS my_select" part (i.e. assign an alias to a SELECT statement)?
(Note: This is a theoretical question. I realize that I can do it without assign an alias to a SELECT statement, but I would like to know whether I can do it with that.)
Not sure exactly what you try to denote with that syntax, but in almost all RDBMS-es you can use a subquery in the FROM clause (sometimes called an "inline-view"):
SELECT..
FROM (
SELECT ...
FROM ...
) my_select
WHERE ...
In advanced "enterprise" RDBMS-es (like oracle, SQL Server, postgresql) you can use common table expressions which allows you to refer to a query by name and reuse it even multiple times:
-- Define the CTE expression name and column list.
WITH Sales_CTE (SalesPersonID, SalesOrderID, SalesYear)
AS
-- Define the CTE query.
(
SELECT SalesPersonID, SalesOrderID, YEAR(OrderDate) AS SalesYear
FROM Sales.SalesOrderHeader
WHERE SalesPersonID IS NOT NULL
)
-- Define the outer query referencing the CTE name.
SELECT SalesPersonID, COUNT(SalesOrderID) AS TotalSales, SalesYear
FROM Sales_CTE
GROUP BY SalesYear, SalesPersonID
ORDER BY SalesPersonID, SalesYear;
(example from http://msdn.microsoft.com/en-us/library/ms190766(v=sql.105).aspx)
You can do this using the WITH clause of the SELECT statement:
;
WITH my_select As (SELECT ... FROM ...)
SELECT * FROM foo
WHERE id IN (SELECT MAX(id) FROM my_select GROUP BY name)
That's the ANSI/ISO SQL Syntax. I know that SQL Server, Oracle and DB2 support it. Not sure about the others...
Yes, but you can select only one column in your subselect
SELECT (SELECT id FROM bla) AS my_select FROM bla2
You could store this into a temporary table.
So instead of doing the CTE/sub query you would use a temp table.
Good article on these here http://codingsight.com/introduction-to-temporary-tables-in-sql-server/
Is it possible to use CTE in a WHERE clause e.g.
SELECT *
FROM Table1
WHERE Table1.PK IN (
WITH Cte AS (
-- root selection (dynamic, generated in code)
SELECT Bla FROM Table2
-- recursive part
UNION ALL
SELECT …..)
SELECT Bla FROM Cte)
The reason I’m asking is that I need to use a recursive query and the only way of doing it at the moment without updating our framework is to place it in the where clause.
No, WITH clauses need to be defined before the main SELECT. Like this:
WITH recursive_cte AS (
-- root selection (dynamic, generated in code)
SELECT Bla FROM Table2
-- recursive part
UNION ALL
SELECT …..)
SELECT t.*
FROM TABLE1 t
JOIN recursive_cte rc ON rc.key = t.pk
I also tweaked the query to use a JOIN instead, but you'll have to watch for duplicates.
Yes you can apply WHERE CLAUSE with CTE. You have to make Table-Valued Function and return result in Table.
then you can use this result set in any query with WHERE Clause.
Go to the link and find example:
http://muhammadnaveed.info/use-cte-query-in-where-clause/
Hope this will help you.