union-all

问题 I have to run a select statement across several tables. I am sure the tables return different records. I am anyway using UNION ALL. Is it better to use UNION or of UNION ALL in performance terms when I am sure the tables return different records? 回答1: UNION ALL will perform better than UNION when you're not concerned about eliminating duplicate records because you're avoiding an expensive distinct sort operation. See: SQL SERVER – Difference Between Union vs. Union All – Optimal Performance

问题 i have two queries being combined with a UNION ALL 1 : --Query 1 SELECT Flavor, Color FROM Friends --Query 2 SELECT Flavor, (SELECT TOP 1 Color FROM Rainbows WHERE Rainbows.StrangerID = Strangers.StrangerID ORDER BY Wavelength DESC ) AS Color FROM Strangers Both of which, of course, work fine separately, but when combined with a UNION ALL : SELECT Flavor, Color FROM Friends UNION ALL SELECT Flavor, (SELECT TOP 1 Color FROM Rainbows WHERE Rainbows.StrangerID = Strangers.StrangerID ORDER BY

问题 The following query is perfectly valid in pretty much every database (give or take a dual dummy table), including Oracle: select 'A' as x from dual union all select 'B' from dual order by x asc Returning: | X | |---| | A | | B | Now this query is still quite standard SQL, but doesn't work on Oracle select 'A' as x from dual union all select 'B' from dual union all select 'C' from dual order by x asc I'm getting ORA-00904: "X": invalid identifier This, however, works: select 'A' as x from dual

问题 What is the difference between UNION and UNION ALL ? 回答1: UNION removes duplicate records (where all columns in the results are the same), UNION ALL does not. There is a performance hit when using UNION instead of UNION ALL , since the database server must do additional work to remove the duplicate rows, but usually you do not want the duplicates (especially when developing reports). UNION Example: SELECT 'foo' AS bar UNION SELECT 'foo' AS bar Result: +-----+ | bar | +-----+ | foo | +-----+ 1