I have a below SQL query to get the three records for notifying purpose.
SELECT orders.msg
FROM orders
INNER JOIN
(
SELECT id
FROM orders
WHERE type_id = 12
ORDER BY id DESC LIMIT 3 OFFSET 0
) AS items
ON orders.id = items.id;
When trying to make the query optimized, i made the changes as below.
SELECT orders.msg
FROM orders
WHERE type_id = 12
ORDER BY id DESC LIMIT 3 OFFSET 0;
Is the modified query seems to be OK or did i miss anything here or any other way of doing is there??
The simplified version on the bottom looks logically identical, to me, to the one on top:
SELECT msg
FROM orders
WHERE type_id = 12
ORDER BY id DESC LIMIT 3;
Note that the above query could benefit from the following index:
CREATE INDEX idx ON orders (type_id, id, msg);
This index would completely cover the WHERE, ORDER BY, and SELECT clauses.
You can try this also:
SELECT orders.msg
FROM orders
WHERE orders.id
IN (
SELECT id
FROM orders
WHERE type_id = 12
ORDER BY id
DESC LIMIT 3 OFFSET 0
)
Related
Suppose I have query for fetching the latest 10 books for a given author like this:
SELECT *
FROM books
WHERE author_id = #author_id
ORDER BY published DESC, id
LIMIT 10
Now if I have a list of n authors I want to get the latest books for, then I can run this query n times. Note that n is reasonably small. However, this seems like an optimization opportunity.
Is there are single query that can efficiently fetch the latest 10 books for n given authors?
This query doesn't work (only fetches 10, not n * 10 books):
SELECT *
FROM books
WHERE author_id = ANY(#author_ids)
ORDER BY published DESC, id
LIMIT 10
First provided author wise book where book is serialized by recent published date for generating a number using ROW_NUMBER() and then in outer subquery add a condition for fetching the desired result.
SELECT *
FROM (SELECT *
, ROW_NUMBER() OVER (PARTITION BY author_id ORDER BY published DESC) row_num
FROM books
WHERE author_id = ANY(#author_ids)) t
WHERE t.row_num <= 10
SELECT b.*
FROM authors a
JOIN LATERAL (
SELECT *
FROM books b
WHERE b.author = a.id
ORDER BY b.published DESC, b.id
LIMIT 10
) b ON TRUE
WHERE a.id = ANY(#author_ids)
I have two tables that look like the following:
Orders
------
id
tracking_number
ShippingLogs
------
tracking_number
created_at
stage
I would like to select the IDs of Orders that have ONLY ONE ShippingLog associated with it, and the stage of the ShippingLog must be error. If it has two ShippingLog entries, I don't want it. If it has one ShippingLog bug its stage is shipped, I don't want it.
This is what I have, and it doesn't work, and I know why (it finds the log with the error, but has no way of knowing if there are others). I just don't really know how to get it the way I need it.
SELECT DISTINCT
orders.id, shipping_logs.created_at, COUNT(shipping_logs.*)
FROM
orders
JOIN
shipping_logs ON orders.tracking_number = shipping_logs.tracking_number
WHERE
shipping_logs.created_at BETWEEN '2021-01-01 23:40:00'::timestamp AND '2021-01-26 23:40:00'::timestamp AND shipping_logs.stage = 'error'
GROUP BY
orders.id, shipping_logs.created_at
HAVING
COUNT(shipping_logs.*) = 1
ORDER BY
orders.id, shipping_logs.created_at DESC;
If you want to retain every column from the join of the two tables given your requirements, then I would suggest using COUNT here as an analytic function:
WITH cte AS (
SELECT o.id, sl.created_at,
COUNT(*) OVER (PARTITION BY o.id) num_logs,
COUNT(*) FILTER (WHERE sl.stage <> 'error')
OVER (PARTITION BY o.id) non_error_cnt
FROM orders o
INNER JOIN shipping_logs sl ON sl.tracking_number = o.tracking_number
WHERE sl.created_at BETWEEN '2021-01-01 23:40:00'::timestamp AND
'2021-01-26 23:40:00'::timestamp
)
SELECT id AS order_id, created_at
FROM cte
WHERE num_logs = 1 AND non_error_cnt = 0
ORDER BY id, created_at DESC;
I have a table with duplicate entries and the objective is to get the distinct entries based on the latest time stamp.
In my case 'serial_no' will have duplicate entries but I select unique entries based on the latest time stamp.
Below query is giving me the unique results with the latest time stamp.
But my concern is I need to get the total of unique entries.
For example assume my table has 40 entries overall. With the below query I am able to get 20 unique rows based on the serial number.
But the 'total' is returned as 40 instead of 20.
Any help on this pls?
SELECT
*
FROM
(
SELECT
DISTINCT ON (serial_no) id,
serial_no,
name,
timestamp,
COUNT(*) OVER() as total
FROM
product_info
INNER JOIN my.account ON id = accountid
WHERE
lower(name) = 'hello'
ORDER BY
serial_no,
timestamp DESC OFFSET 0
LIMIT
10
) AS my_info
ORDER BY
serial_no asc
product_info table intially has this data
serial_no name timestamp
11212 pulp12 2018-06-01 20:00:01
11213 mango 2018-06-01 17:00:01
11214 grapes 2018-06-02 04:00:01
11215 orange 2018-06-02 07:05:30
11212 pulp12 2018-06-03 14:00:01
11213 mango 2018-06-03 13:00:00
After the distict query I got all unique results based on the latest
timestamp:
serial_no name timestamp total
11212 pulp12 2018-06-03 14:00:01 6
11213 mango 2018-06-03 13:00:00 6
11214 grapes 2018-06-02 04:00:01 6
11215 orange 2018-06-02 07:05:30 6
But total is appearing as 6 . I wanted the total to be 4 since it has
only 4 unique entries.
I am not sure how to modify my existing query to get this desired
result.
Postgres supports COUNT(DISTINCT column_name), so if I have understood your request, using that instead of COUNT(*) will work, and you can drop the OVER.
What you could do is move the window function to a higher level select statement. This is because window function is evaluated before distinct on and limit clauses are applied. Also, you can not include DISTINCT keyword within window functions - it has not been implemented yet (as of Postgres 9.6).
SELECT
*,
COUNT(*) OVER() as total -- here
FROM
(
SELECT
DISTINCT ON (serial_no) id,
serial_no,
name,
timestamp
FROM
product_info
INNER JOIN my.account ON id = accountid
WHERE
lower(name) = 'hello'
ORDER BY
serial_no,
timestamp DESC
LIMIT
10
) AS my_info
Additionally, offset is not required there and one more sorting is also superfluous. I've removed these.
Another way would be to include a computed column in the select clause but this would not be as fast as it would require one more scan of the table. This is obviously assuming that your total is strictly connected to your resultset and not what's beyond that being stored in the table, but gets filtered out.
select count(*), serial_no from product_info group by serial_no
will give you the number of duplicates for each serial number
The most mindless way of incorporating that information would be to join in a sub query
SELECT
*
FROM
(
SELECT
DISTINCT ON (serial_no) id,
serial_no,
name,
timestamp,
COUNT(*) OVER() as total
FROM
product_info
INNER JOIN my.account ON id = accountid
WHERE
lower(name) = 'hello'
ORDER BY
serial_no,
timestamp DESC OFFSET 0
LIMIT
10
) AS my_info
join (select count(*) as counts, serial_no from product_info group by serial_no) as X
on X.serial_no = my_info.serial_no
ORDER BY
serial_no asc
The code is roughly like this:
WITH cte AS
(
SELECT TOP 4 id, due_date, check
FROM table_a a
INNER JOIN table_b b ON a.linkid = b.linkid
WHERE
b.status = 1
AND due_date > GetDate()
ORDER BY due_date, id
)
UPDATE cte
SET check = 1
OUTPUT
INSERTED.id,
INSERTED.due_date
Note: the actual data has same due_date.
When I ran the SELECT statement only inside the cte, I could get the result, for ex: 1, 2, 3, 4.
But after the UPDATE statement, the updated results are: 4, 1, 2, 3
Why is this (order-change) happening?
How to keep or re-order the results back to 1,2,3,4 in this same 1 query?
In MSDN https://msdn.microsoft.com/pl-pl/library/ms177564(v=sql.110).aspx you can read that
There is no guarantee that the order in which the changes are applied
to the table and the order in which the rows are inserted into the
output table or table variable will correspond.
Thats mean you can't solve your problem with only one query. But you still can use one batch to do what you need. Because your output don't guarantee the order then you have to save it in another table and order it after update. This code will return your output values in order that you assume:
declare #outputTable table( id int, due_date date);
with cte as (
select top 4 id, due_date, check
from table_a a
inner join table_b b on a.linkid = b.linkid
where b.status = 1
and due_date > GetDate()
order by due_date, id
)
update cte
set check = 1
output inserted.id, inserted.due_date
into #outputTable;
select *
from #outputTable
order by due_date, id;
I have the following TSQL query:
SELECT DISTINCT MyTable1.Date
FROM MyTable1
INNER JOIN MyTable2
ON MyTable1.Id = MyTable2.Id
WHERE Name = 'John' ORDER BY MyTable1.Date DESC
It retrieves a long list of Dates, but I only need the first one, the one in the first row.
How can I get it?
Thanks a ton!
In SQL Server you can use TOP:
SELECT TOP 1 MyTable1.Date
FROM MyTable1
INNER JOIN MyTable2
ON MyTable1.Id = MyTable2.Id
WHERE Name = 'John'
ORDER BY MyTable1.Date DESC
If you need to use DISTINCT, then you can use:
SELECT TOP 1 x.Date
FROM
(
SELECT DISTINCT MyTable1.Date
FROM MyTable1
INNER JOIN MyTable2
ON MyTable1.Id = MyTable2.Id
WHERE Name = 'John'
) x
ORDER BY x.Date DESC
Or even:
SELECT MAX(MyTable1.Date)
FROM MyTable1
INNER JOIN MyTable2
ON MyTable1.Id = MyTable2.Id
WHERE Name = 'John'
--ORDER BY MyTable1.Date DESC
There are several options here. You can use TOP(1) as Taryn mentioned. But according to docs for the purposes of limiting the rows returned it is better to use OFFSET and FETCH.
We recommend that you use the OFFSET and FETCH clauses instead of the TOP clause to implement a query paging solution and limit the number of rows sent to a client application.
Using OFFSET and FETCH as a paging solution requires running the query one time for each "page" of data returned to the client application. For example, to return the results of a query in 10-row increments, you must execute the query one time to return rows 1 to 10 and then run the query again to return rows 11 to 20 and so on.
Assuming, the solution for your problem using OFFSET and FETCH approach could be:
SELECT DISTINCT MyTable1.Date
FROM MyTable1
INNER JOIN MyTable2
ON MyTable1.Id = MyTable2.Id
WHERE Name = 'John' ORDER BY MyTable1.Date DESC
OFFSET 0 ROWS
FETCH NEXT 1 ROW ONLY