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Conditional Delete not working Expand / Collapse
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Posted Wednesday, November 14, 2012 9:00 AM
Old Hand

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Why does this delete all my records, reguardless of company?

delete Plant_Master
where EXISTS(Select Plant_CD from Plant_Master where company like 'MSC')
Post #1384698
Posted Wednesday, November 14, 2012 9:09 AM
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dwilliscp (11/14/2012)
Why does this delete all my records, reguardless of company?

delete Plant_Master
where EXISTS(Select Plant_CD from Plant_Master where company like 'MSC')



does at least one row exist in Plant_Master where company = 'MSC'?
If so then EXISTS(Select Plant_CD from Plant_Master where company like 'MSC') evaluates to true and all records from Plant_Master are deleted.

If you want to delete records from Plant_Master where company = 'MCS' then you need to do :
delete Plant_Master
where company = 'MSC'

If you want to delete records from Plant_Master where company strats from 'MCS' then you need to do :
delete Plant_Master
where company LIKE 'MSC%'

If you want to delete records from Plant_Master where company contains 'MCS' then you need to do :
delete Plant_Master
where company LIKE '%MSC%'





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Post #1384707
Posted Wednesday, November 14, 2012 12:25 PM
Old Hand

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Hmm thought it would do a line by line check.. turns out that my thought process was off anyway. Given that we have data here that is not in SAP, I can not do any conditional deletes. I will have to use the update statement to update existing records, instead of deleting and re-loading.

However if the plan had not changed.. then the delete statement would still not have worked quite right. I still wonder how that would have worked. The idea was to have it select all records that existed and refresh only those... to get the list you would have to run the following SQL statement:

Select plant.Plant
from file_Plants as plant
left outer join Plant_Master as mas
on mas.Plant_CD = plant.Plant
where NOT(mas.Plant_CD is NULL)
Post #1384809
Posted Wednesday, November 14, 2012 12:37 PM


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dwilliscp (11/14/2012)
Hmm thought it would do a line by line check.. turns out that my thought process was off anyway. Given that we have data here that is not in SAP, I can not do any conditional deletes. I will have to use the update statement to update existing records, instead of deleting and re-loading.

However if the plan had not changed.. then the delete statement would still not have worked quite right. I still wonder how that would have worked. The idea was to have it select all records that existed and refresh only those... to get the list you would have to run the following SQL statement:

Select plant.Plant
from file_Plants as plant
left outer join Plant_Master as mas
on mas.Plant_CD = plant.Plant
where NOT(mas.Plant_CD is NULL)


Just my personal preference but I find this type of check to be far easier to read with a positive test.

where mas.Plant_CD IS NOT NULL

Of course in the case of this specific query why not just change the left join to an inner join since that is the result set you want to find.


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Post #1384813
Posted Wednesday, November 14, 2012 12:59 PM
Old Hand

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True, just a personal quirk. I find that when I am trying to think through something... I tend to code the thought process. Thus the outer join filtering where not NULL.


Sean Lange (11/14/2012)
dwilliscp (11/14/2012)
Hmm thought it would do a line by line check.. turns out that my thought process was off anyway. Given that we have data here that is not in SAP, I can not do any conditional deletes. I will have to use the update statement to update existing records, instead of deleting and re-loading.

However if the plan had not changed.. then the delete statement would still not have worked quite right. I still wonder how that would have worked. The idea was to have it select all records that existed and refresh only those... to get the list you would have to run the following SQL statement:

Select plant.Plant
from file_Plants as plant
left outer join Plant_Master as mas
on mas.Plant_CD = plant.Plant
where NOT(mas.Plant_CD is NULL)


Just my personal preference but I find this type of check to be far easier to read with a positive test.

where mas.Plant_CD IS NOT NULL

Of course in the case of this specific query why not just change the left join to an inner join since that is the result set you want to find.
Post #1384825
Posted Wednesday, November 14, 2012 3:46 PM
SSCrazy

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dwilliscp (11/14/2012)
True, just a personal quirk. I find that when I am trying to think through something... I tend to code the thought process. Thus the outer join filtering where not NULL.


Sean Lange (11/14/2012)
dwilliscp (11/14/2012)
Hmm thought it would do a line by line check.. turns out that my thought process was off anyway. Given that we have data here that is not in SAP, I can not do any conditional deletes. I will have to use the update statement to update existing records, instead of deleting and re-loading.

However if the plan had not changed.. then the delete statement would still not have worked quite right. I still wonder how that would have worked. The idea was to have it select all records that existed and refresh only those... to get the list you would have to run the following SQL statement:

Select plant.Plant
from file_Plants as plant
left outer join Plant_Master as mas
on mas.Plant_CD = plant.Plant
where NOT(mas.Plant_CD is NULL)


Just my personal preference but I find this type of check to be far easier to read with a positive test.

where mas.Plant_CD IS NOT NULL

Of course in the case of this specific query why not just change the left join to an inner join since that is the result set you want to find.


I wouldn't call it "personal preference". It's wrong type of join, which can lead to worse performance.
That is exactly what INNER JOIN is for, so better to be written as

Select plant.Plant
from file_Plants as plant
inner join Plant_Master as mas
on mas.Plant_CD = plant.Plant

Now, as you select nothing from Plant_Master, you may find that the following will perform even better than INNER JOIN:

Select plant.Plant
from file_Plants as plant
where exists (select 1 from Plant_Master as mas
where mas.Plant_CD = plant.Plant)

Above two samples, can be actually called as "personal preference", but again, you may find that EXIST will give slightly better performance, as query processor will know that you need nothing from Plant_Master table.



_____________________________________________
"The only true wisdom is in knowing you know nothing"
"O skol'ko nam otkrytiy chudnyh prevnosit microsofta duh!"
(So many miracle inventions provided by MS to us...)

How to post your question to get the best and quick help
Post #1384910
Posted Thursday, November 15, 2012 8:17 AM
Old Hand

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Ok.. but how do you put that select logic into a delete statement?
Post #1385190
Posted Thursday, November 15, 2012 8:36 AM


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dwilliscp (11/15/2012)
Ok.. but how do you put that select logic into a delete statement?


Change the word "select" to "delete".


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Post #1385202
Posted Thursday, November 15, 2012 8:40 AM


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Eugene Elutin (11/14/2012)


I wouldn't call it "personal preference". It's wrong type of join, which can lead to worse performance.
That is exactly what INNER JOIN is for, so better to be written as



My preference I meant was to conditional check, I did point out that it should be an inner join.

The personal preference was to use
where mas.Plant_CD is NOT NULL

instead of

where NOT(mas.Plant_CD is NULL)



_______________________________________________________________

Need help? Help us help you.

Read the article at http://www.sqlservercentral.com/articles/Best+Practices/61537/ for best practices on asking questions.

Need to split a string? Try Jeff Moden's splitter.

Cross Tabs and Pivots, Part 1 – Converting Rows to Columns
Cross Tabs and Pivots, Part 2 - Dynamic Cross Tabs
Understanding and Using APPLY (Part 1)
Understanding and Using APPLY (Part 2)
Post #1385206
Posted Thursday, November 15, 2012 1:12 PM
Old Hand

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Ok, thanks for the help
Post #1385332
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