SELECT * usage

  • Comments posted to this topic are about the item SELECT * usage

    Wayne
    Microsoft Certified Master: SQL Server 2008
    Author - SQL Server T-SQL Recipes


    If you can't explain to another person how the code that you're copying from the internet works, then DON'T USE IT on a production system! After all, you will be the one supporting it!
    Links:
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    Performance Problems
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  • Good question, learned something.

    However, the question itself was a bit weird to read (at least for me, a non-native English speaker). I had to read it a few times to finally get it. And then answer wrong of course 😀

    The CREATE TABLE reference doesn't describe the behaviour asked in the question. The following article does:

    Use Column Sets

    Adding a column set changes the behavior of SELECT * queries. The query will return the column set as an XML column and not return the individual sparse columns. Schema designers and software developers must be careful not to break existing applications.

    Need an answer? No, you need a question
    My blog at https://sqlkover.com.
    MCSE Business Intelligence - Microsoft Data Platform MVP

  • Good question, Wayne. And thanks Koen for the additional reference.

    Also, the result set in the explanation is not really correct. The ColumnSet row will display XML data. I assume that the internet interface of the website somehow has stripped a lot of the XML tags embedded in that result set. I encourage everyone to copy, paste and execute the demo code posted by Wayne.


    Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
    Visit my SQL Server blog: https://sqlserverfast.com/blog/
    SQL Server Execution Plan Reference: https://sqlserverfast.com/epr/

  • Tricky question indeed.

    Well, but I can't find any difference between the result obtained and my response: When the table contains sparse columns and a sparse column set, then any null sparse columns will not be returned.

    Isn't it right?

  • jalvarocrespo (5/10/2012)


    Tricky question indeed.

    Well, but I can't find any difference between the result obtained and my response: When the table contains sparse columns and a sparse column set, then any null sparse columns will not be returned.

    Isn't it right?

    Your answer says that "any null sparse columns" (emphasis mine) won't be returned.

    The correct answer says that "any sparse columns" (null or non-null) won't be returned.


    Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
    Visit my SQL Server blog: https://sqlserverfast.com/blog/
    SQL Server Execution Plan Reference: https://sqlserverfast.com/epr/

  • This was removed by the editor as SPAM

  • Good question, but wrong answer given as right.

    The wrong answer made no points difference to me, because I managed to get it utterly wrong even after reading the Use Column Set page. I supose I could try to hide behind language, like Koen, but I've been speaking English all day every day almost all of my life, so in my case it would be a silly sham - it was just plain sloppy carelessness on my part.

    Incidentally, it is a poor explanation too, since it references a page that tells us exactly nothing about how sparse columns are treated instead of the page with the information; and the disappearance of the XML tags surely shouldn't have been allowed to slip through.

    Hugo Kornelis (5/10/2012)


    jalvarocrespo (5/10/2012)


    Tricky question indeed.

    Well, but I can't find any difference between the result obtained and my response: When the table contains sparse columns and a sparse column set, then any null sparse columns will not be returned.

    Isn't it right?

    Your answer says that "any null sparse columns" (emphasis mine) won't be returned.

    The correct answer says that "any sparse columns" (null or non-null) won't be returned.

    I guess that depends on how you interpreted "returned". The non-null sparse columns are returned in the XML returned for the columnset column, so it isn't really true to say they are not returned. That in fact is the whole point of that columnset column, to enable sparse columns to be returned only for rows where they are not null, so it seems somewhat perverse to claim that they aren't returned when they are not null. You can only get away with that interpretation by assertimng that "returned" means ""returned as individual columns in stead of as XML data in the columnset" which isn't a definition offered in any dictionary I've ever seen.

    Tom

  • L' Eomot Inversé (5/10/2012)


    Good question, but wrong answer given as right.

    The wrong answer made no points difference to me, because I managed to get it utterly wrong even after reading the Use Column Set page. I supose I could try to hide behind language, like Koen, but I've been speaking English all day every day almost all of my life, so in my case it would be a silly sham - it was just plain sloppy carelessness on my part.

    Incidentally, it is a poor explanation too, since it references a page that tells us exactly nothing about how sparse columns are treated instead of the page with the information; and the disappearance of the XML tags surely shouldn't have been allowed to slip through.

    Hugo Kornelis (5/10/2012)


    jalvarocrespo (5/10/2012)


    Tricky question indeed.

    Well, but I can't find any difference between the result obtained and my response: When the table contains sparse columns and a sparse column set, then any null sparse columns will not be returned.

    Isn't it right?

    Your answer says that "any null sparse columns" (emphasis mine) won't be returned.

    The correct answer says that "any sparse columns" (null or non-null) won't be returned.

    I guess that depends on how you interpreted "returned". The non-null sparse columns are returned in the XML returned for the columnset column, so it isn't really true to say they are not returned. That in fact is the whole point of that columnset column, to enable sparse columns to be returned only for rows where they are not null, so it seems somewhat perverse to claim that they aren't returned when they are not null. You can only get away with that interpretation by assertimng that "returned" means ""returned as individual columns in stead of as XML data in the columnset" which isn't a definition offered in any dictionary I've ever seen.

    I have to disagree with you, Tom.

    Sure, the wording could have been better. I'll immediately agree to that.

    But the question clearly asks: "When does a "SELECT *" statement not return a column as an individual column in the result set (...)" (emphasis mine). The answers only mention that specific columns "... will not be returned" without adding the "as individual column" qualification. When looking at the answers by themselves that could indeed be confusing - but in the context of the question, I think it is clear that this is intended. After all, the answers also don't repeat that this only applies to SELECT * queries. In general, it should not be necessary to repeat the entire question in each answer option.


    Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
    Visit my SQL Server blog: https://sqlserverfast.com/blog/
    SQL Server Execution Plan Reference: https://sqlserverfast.com/epr/

  • Hugo Kornelis (5/10/2012)


    L' Eomot Inversé (5/10/2012)


    Good question, but wrong answer given as right.

    The wrong answer made no points difference to me, because I managed to get it utterly wrong even after reading the Use Column Set page. I supose I could try to hide behind language, like Koen, but I've been speaking English all day every day almost all of my life, so in my case it would be a silly sham - it was just plain sloppy carelessness on my part.

    Incidentally, it is a poor explanation too, since it references a page that tells us exactly nothing about how sparse columns are treated instead of the page with the information; and the disappearance of the XML tags surely shouldn't have been allowed to slip through.

    Hugo Kornelis (5/10/2012)


    jalvarocrespo (5/10/2012)


    Tricky question indeed.

    Well, but I can't find any difference between the result obtained and my response: When the table contains sparse columns and a sparse column set, then any null sparse columns will not be returned.

    Isn't it right?

    Your answer says that "any null sparse columns" (emphasis mine) won't be returned.

    The correct answer says that "any sparse columns" (null or non-null) won't be returned.

    I guess that depends on how you interpreted "returned". The non-null sparse columns are returned in the XML returned for the columnset column, so it isn't really true to say they are not returned. That in fact is the whole point of that columnset column, to enable sparse columns to be returned only for rows where they are not null, so it seems somewhat perverse to claim that they aren't returned when they are not null. You can only get away with that interpretation by assertimng that "returned" means ""returned as individual columns in stead of as XML data in the columnset" which isn't a definition offered in any dictionary I've ever seen.

    I have to disagree with you, Tom.

    Sure, the wording could have been better. I'll immediately agree to that.

    But the question clearly asks: "When does a "SELECT *" statement not return a column as an individual column in the result set (...)" (emphasis mine). The answers only mention that specific columns "... will not be returned" without adding the "as individual column" qualification. When looking at the answers by themselves that could indeed be confusing - but in the context of the question, I think it is clear that this is intended. After all, the answers also don't repeat that this only applies to SELECT * queries. In general, it should not be necessary to repeat the entire question in each answer option.

    Hugo,

    I'm absolutely sure that Tom and Me we are not alone.

    I think that, at least, you must consider the two responses good.

  • Hugo Kornelis (5/10/2012)


    I have to disagree with you, Tom.

    Sure, the wording could have been better. I'll immediately agree to that.

    But the question clearly asks: "When does a "SELECT *" statement not return a column as an individual column in the result set (...)" (emphasis mine). The answers only mention that specific columns "... will not be returned" without adding the "as individual column" qualification. When looking at the answers by themselves that could indeed be confusing - but in the context of the question, I think it is clear that this is intended. After all, the answers also don't repeat that this only applies to SELECT * queries. In general, it should not be necessary to repeat the entire question in each answer option.

    I guess I have to disagree with myself, too. I was looking at the wording of the answers, not the wording of the question, and the wording of the answers has to be understood in the context of the question and its wording. So the correct answer was indeed the right correct answer (although it was certainly not well worded). My excuse (not a good one) is that one can easily forget the question by the time one has read through all those answers and then read the comments as far as the message I was replying to (which of course repeated the two relevant answers, and not the question).

    Tom

  • Koen Verbeeck (5/9/2012)


    Good question, learned something.

    However, the question itself was a bit weird to read (at least for me, a non-native English speaker). I had to read it a few times to finally get it.

    I learned something, and I got it right. But the whole....

    When the table contains sparse columns and a sparse column set, then all of the sparse columns will not be returned

    The columns come back, but in an XML column.... so depending on semantics, either the first or last answer could be construed as correct.

    I'm glad I guessed correctl.y, but... I'd have rather not had to guess as to the intention of the author at the end.



    --Mark Tassin
    MCITP - SQL Server DBA
    Proud member of the Anti-RBAR alliance.
    For help with Performance click this link[/url]
    For tips on how to post your problems[/url]

  • And now of course... after reading the explanation to Tom, I understand my mistake at reading but not trying to fully comprehend the question. 🙂



    --Mark Tassin
    MCITP - SQL Server DBA
    Proud member of the Anti-RBAR alliance.
    For help with Performance click this link[/url]
    For tips on how to post your problems[/url]

  • Thanks for a very good question. I learned 2 new things today!

  • L' Eomot Inversé (5/10/2012)


    Hugo Kornelis (5/10/2012)


    I have to disagree with you, Tom.

    Sure, the wording could have been better. I'll immediately agree to that.

    But the question clearly asks: "When does a "SELECT *" statement not return a column as an individual column in the result set (...)" (emphasis mine). The answers only mention that specific columns "... will not be returned" without adding the "as individual column" qualification. When looking at the answers by themselves that could indeed be confusing - but in the context of the question, I think it is clear that this is intended. After all, the answers also don't repeat that this only applies to SELECT * queries. In general, it should not be necessary to repeat the entire question in each answer option.

    I guess I have to disagree with myself, too. I was looking at the wording of the answers, not the wording of the question, and the wording of the answers has to be understood in the context of the question and its wording. So the correct answer was indeed the right correct answer (although it was certainly not well worded). My excuse (not a good one) is that one can easily forget the question by the time one has read through all those answers and then read the comments as far as the message I was replying to (which of course repeated the two relevant answers, and not the question).

    Not a good excuse, but a very good reason. I remember reading somewhere that the best way to read multiple-choice questions is to skim the question, read the answers carefully, and then go back and read the question very carefully, with the answers in mind. This reinforces the context of the question, and focuses you on picking out bits that disqualify the wrong answers.

    I have no idea if that's true, but it seems to work for me.

  • Koen Verbeeck (5/9/2012)


    Good question, learned something.

    However, the question itself was a bit weird to read (at least for me, a non-native English speaker). I had to read it a few times to finally get it. And then answer wrong of course 😀

    The CREATE TABLE reference doesn't describe the behaviour asked in the question. The following article does:

    Use Column Sets

    Adding a column set changes the behavior of SELECT * queries. The query will return the column set as an XML column and not return the individual sparse columns. Schema designers and software developers must be careful not to break existing applications.

    Dang, I sure wish that I had found this reference when I was looking. This is much better than what I was able to find. Thanks Koen!

    Wayne
    Microsoft Certified Master: SQL Server 2008
    Author - SQL Server T-SQL Recipes


    If you can't explain to another person how the code that you're copying from the internet works, then DON'T USE IT on a production system! After all, you will be the one supporting it!
    Links:
    For better assistance in answering your questions
    Performance Problems
    Common date/time routines
    Understanding and Using APPLY Part 1 & Part 2

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