Difference between select single and select upto 1 rows
Can anybody tell me the exact difference between <b>select single</b> and <b>select upto 1 rows.</b> Performance wise which one is better?
<b>Knowing when to use SELECT SINGLE or SELECT ... UP TO 1 ROWS</b>
A lot of people use the SELECT SINGLE statement to check for the existence of a value in a database. Other people prefer to use the 'UP TO 1 ROWS' variant of the SELECT statement.
So what's the difference between using 'SELECT SINGLE' statement as against a 'SELECT .... UP TO 1 ROWS' statement ?
If you're considering the statements
SELECT SINGLE field INTO w_field FROM table.
SELECT field INTO w_field FROM table UP TO 1 ROWS. ENDSELECT.
then looking at the result, not much apart from the extra ENDSELECT statement. Look at the run time and memory usage and they may be worlds apart.
<b>Why is this ?? The answer is simple.</b>
The 'SELECT SINGLE' statement selects the first row in the database that it finds that fulfils the 'WHERE' clause If this results in multiple records then only the first one will be returned and therefore may not be unique.
The 'SELECT .... UP TO 1 ROWS' statement is subtly different. The database selects all of the relevant records that are defined by the WHERE clause, applies any aggregate, ordering or grouping functions to them and then returns the first record of the result set.
<b>Get the difference ??</b>
If not, here is a good example, credit for this example goes to Richard Harper, a friend of mine on sapfans.com :
Create a Ztable called ZDifference with 2 fields in it, MANDT of type MANDT and POSNR of type POSNR. Make sure both of these are keys. Also create a table maintenance dialog for it (SE11->Utilities->Table Maintenance Generator). Fill the table with ten rows 000001-000010.
<b>Then run the program shown below:</b>
Purpose: A program that demonstrates the difference
between SELECT SINGLE and SELECT UP TO n ROWS.
This program requires the data table Z_DIFFERENCE
to have been created according to the structure
outlined in the text above and populated with
at least 10 records.
Creation Date: 21/04/2004
Author: R Harper
Date Reason Transport Who
No Standard Page Heading.
Data: w_Single type Posnr,
t_Rows type standard table of Posnr
initial size 0
with header line.
Select single Posnr
into table t_Rows
up to 1 rows
order by Posnr descending.
Write :/ 'Select single:', w_Single.
Write :/ 'Up to 1 rows :'.
Loop at t_Rows.
<b>You should see the output:</b>
Select single: 000001
Up to 1 rows : 000010
The first 'SELECT' statement selected the first record in the database according to any selection criterion in the 'WHERE' clause. This is what a 'SELECT SINGLE' does. The second 'SELECT' has asked the database to reverse the order of the records before returning the first row of the result.
In order to be able to do this the database has read the entire table, sort it and then return the first record. If there was no ORDER BY clause then the results would have been identical (ie both '000001') but the second select if given a big enough table to look at would be far slower.
Note that this causes a problem in the Extended Program Check if the full key is not specified in a 'SELECT SINGLE'. Replacing the 'SELECT SINGLE' by an "UP TO 1 ROWS" will give the same exact results without any warning but the program will run slower and consume more memory. This is a good example of a warning that we should ignore... considering you are sure of what you are doing !!
<b>Reward if usefull</b>