sql query for reading records one at a time

Question

I need to read a Microsoft sql db table record one at a time (first in first out). we have Field1 with time stamp of insert of record into table. we have Field2 with value 'N". I need to pick the oldest record of table first with Value "N" on the basis of Field1 and Field2.

Can you advise the SQL query for this requirement ? I have a business requirement where in I need to read only one record at a time, and I am not explaining that business requirement here. I have batch job scheduled and will run every 5mins.

Thanks in Advance

kandimal

Answer 1

SELECT TOP(1) col1, col2, ...
FROM   tbl
WHERE Field2 = 'N'
ORDER BY Field1

There should be index like this:

CREATE INDEX Field1_ix ON (Field1)  INCLUDE (Field2) WHERE Field2 = 'N'

Answer 2

Hi @kandimal ,

Welcome to Microsoft Q&A!

You could also try with SET ROWCOUNT (N) which is similar with TOP (N).

Please refer below:

set rowcount 1  
  
select *   
from yourtable  
where Field2='N'   
order by Field1  

Only one note is that once you have set rowcount this will persist for the life of the connection so make sure you reset it to 0 once you are done with it.

Best regards,
Melissa

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Answer 3

Please see:

https://rusanu.com/2010/03/26/using-tables-as-queues/

Answer 4

If you do not want to delete the row which was read, you need a flag column to indicate it. See the example below:

DECLARE @tbl TABLE (
    UniqueColumn int IDENTITY,
    Field1 datetime,
    Field2 char(1),
    IsRead bit default(0)
);

INSERT INTO @tbl(Field1, Field2) VALUES
('2021-05-01', 'N'), ('2021-05-02', 'Y'), ('2021-05-03', 'N'),
('2021-05-04', 'N'), ('2021-05-05', 'Y'), ('2021-05-06', 'N');

DECLARE @UniqueColumn int;

SELECT TOP 1 @UniqueColumn = UniqueColumn 
FROM @tbl
WHERE Field2 = 'N' AND IsRead = 0
ORDER BY Field1

UPDATE @tbl
SET IsRead = 1
WHERE UniqueColumn = @UniqueColumn;