## using multiple UNION in query

 Author Message Phil Parkin SSC Guru Group: General Forum Members Points: 127862 Visits: 22561 Thanks Sean - that was the point that I was hoping to make. Help us to help you. For better, quicker and more-focused answers to your questions, consider following the advice in this link.If the answer to your question can be found with a brief Google search, please perform the search yourself, rather than expecting one of the SSC members to do it for you.Please surround any code or links you post with the appropriate IFCode formatting tags. It helps readability a lot. John Mitchell-245523 SSC Guru Group: General Forum Members Points: 79955 Visits: 17920 Sean Lange (5/16/2013)John Mitchell-245523 (5/16/2013)Phil Parkin (5/16/2013)lnardozi 61862 (5/16/2013)Just for completeness sake, you never use more than one UNION, which is the last one. All the rest should be UNION ALL.Why not?I suppose because UNION eliminates duplicates you only need to do it once - as long as you make sure that UNION is the last one evaluated. I don't know whether the query optimizer is smart enough to do that implicitly if you use more than one UNION - so it may or may not make a difference from a performance point of view.JohnBut that is only good if you want to eliminate ALL duplicates. It is very possible that you have 3 tables and in the first 2 tables you want duplicates but if the same value shows up in the third table you want only distinct values. Other times maybe you want to include all duplicates.The blanket statement that "you never use more than one UNION, which is the last one. All the rest should be UNION ALL." is just completely not true. There are time when that is correct but not always.I'm not sure I understand. Those duplicates from the first two tables will be eliminated by the UNION operator. Try this:`SELECT * FROM (VALUES (1), (1), (2)) x(A)UNION ALLSELECT * FROM (VALUES (2), (3), (3)) y(B)UNION SELECT * FROM (VALUES (4)) z(C)`John Eugene Elutin SSC-Dedicated Group: General Forum Members Points: 31836 Visits: 5478 John Mitchell-245523 (5/16/2013)Sean Lange (5/16/2013)John Mitchell-245523 (5/16/2013)Phil Parkin (5/16/2013)lnardozi 61862 (5/16/2013)Just for completeness sake, you never use more than one UNION, which is the last one. All the rest should be UNION ALL.Why not?I suppose because UNION eliminates duplicates you only need to do it once - as long as you make sure that UNION is the last one evaluated. I don't know whether the query optimizer is smart enough to do that implicitly if you use more than one UNION - so it may or may not make a difference from a performance point of view.JohnBut that is only good if you want to eliminate ALL duplicates. It is very possible that you have 3 tables and in the first 2 tables you want duplicates but if the same value shows up in the third table you want only distinct values. Other times maybe you want to include all duplicates.The blanket statement that "you never use more than one UNION, which is the last one. All the rest should be UNION ALL." is just completely not true. There are time when that is correct but not always.I'm not sure I understand. Those duplicates from the first two tables will be eliminated by the UNION operator. Try this:`SELECT * FROM (VALUES (1), (1), (2)) x(A)UNION ALLSELECT * FROM (VALUES (2), (3), (3)) y(B)UNION SELECT * FROM (VALUES (4)) z(C)`Johntry this:`SELECT * FROM (VALUES (1), (1), (2)) x(A)UNION SELECT * FROM (VALUES (2), (3), (3)) y(B)UNION allSELECT * FROM (VALUES (4),(3)) z(C)`However, until this sort of functionality is really required, I wouldn't mix both UNION and UNION ALL in one query. _____________________________________________"The only true wisdom is in knowing you know nothing""O skol'ko nam otkrytiy chudnyh prevnosit microsofta duh!":-D(So many miracle inventions provided by MS to us...)How to post your question to get the best and quick help Sean Lange SSC Guru Group: General Forum Members Points: 148158 Visits: 18570 John Mitchell-245523 (5/16/2013)Sean Lange (5/16/2013)John Mitchell-245523 (5/16/2013)Phil Parkin (5/16/2013)lnardozi 61862 (5/16/2013)Just for completeness sake, you never use more than one UNION, which is the last one. All the rest should be UNION ALL.Why not?I suppose because UNION eliminates duplicates you only need to do it once - as long as you make sure that UNION is the last one evaluated. I don't know whether the query optimizer is smart enough to do that implicitly if you use more than one UNION - so it may or may not make a difference from a performance point of view.JohnBut that is only good if you want to eliminate ALL duplicates. It is very possible that you have 3 tables and in the first 2 tables you want duplicates but if the same value shows up in the third table you want only distinct values. Other times maybe you want to include all duplicates.The blanket statement that "you never use more than one UNION, which is the last one. All the rest should be UNION ALL." is just completely not true. There are time when that is correct but not always.I'm not sure I understand. Those duplicates from the first two tables will be eliminated by the UNION operator. Try this:`SELECT * FROM (VALUES (1), (1), (2)) x(A)UNION ALLSELECT * FROM (VALUES (2), (3), (3)) y(B)UNION SELECT * FROM (VALUES (4)) z(C)`JohnThe difference is that you don't have any values in z that are also in either of the first two tables.Let's say you wanted distinct values from x and y but you wanted to include duplicates from z if they exist. In this scenario you have to change the order of the UNION and UNION ALL. `SELECT * FROM (VALUES (1), (1), (2)) x(A)UNION SELECT * FROM (VALUES (2), (3), (3)) y(B)UNION ALLSELECT * FROM (VALUES (4), (1)) z(C)`--EDIT--Didn't see Eugene's post before I hit reply. I agree that you should not mix them unless it is really necessary. It does however prove that the blanket statement posted above is just not true. As with nearly everything in SQL "it depends". :-P _______________________________________________________________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 Modens 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) John Mitchell-245523 SSC Guru Group: General Forum Members Points: 79955 Visits: 17920 But that's because you changed the order. The proviso was that the UNION operator is the last one to be evaluated. If you use parentheses to make it so, you get the same results as before:`SELECT * FROM (VALUES (1), (1), (2)) x(A)UNION (SELECT * FROM (VALUES (2), (3), (3)) y(B)UNION allSELECT * FROM (VALUES (4),(3)) z(C))`JohnEdit - I think I understand what you're saying. I need to play around and see what happens when you have more than one UNION. I'll see if I get time to do that tomorrow. Sean Lange SSC Guru Group: General Forum Members Points: 148158 Visits: 18570 John Mitchell-245523 (5/16/2013)But that's because you changed the order. The proviso was that the UNION operator is the last one to be evaluated. If you use parentheses to make it so, you get the same results as before:`SELECT * FROM (VALUES (1), (1), (2)) x(A)UNION (SELECT * FROM (VALUES (2), (3), (3)) y(B)UNION allSELECT * FROM (VALUES (4),(3)) z(C))`JohnEdit - I think I understand what you're saying. I need to play around and see what happens when you have more than one UNION. I'll see if I get time to do that tomorrow.Right but they made a blanket statement that you should NEVER us more than one union and it should always be the last one. That is just simply false. It depends on the requirements of the result set. _______________________________________________________________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 Modens 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) John Mitchell-245523 SSC Guru Group: General Forum Members Points: 79955 Visits: 17920 Yes, I agree that it was worded too strongly. Can you think of any cases where it isn't possible to rewrite a series of UNIONs so that it contains only one UNION, with the rest being converted to UNION ALLs?John