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sql中union和union all的区别
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union 連接兩個表查詢的結果
假設我們有一個表Student，包括以下字段與數據：
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drop?table?student;????create?table?student??(??id?int?primary?key,??name?nvarchar2(50)?not?null,??score?number?not?null??);????insert?into?student?values(1,'Aaron',78);??insert?into?student?values(2,'Bill',76);??insert?into?student?values(3,'Cindy',89);??insert?into?student?values(4,'Damon',90);??insert?into?student?values(5,'Ella',73);??insert?into?student?values(6,'Frado',61);??insert?into?student?values(7,'Gill',99);??insert?into?student?values(8,'Hellen',56);??insert?into?student?values(9,'Ivan',93);??insert?into?student?values(10,'Jay',90);????commit;?? ?
首先，我們來看一下UNION的例子：
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SQL>?select?*????2??from?student????3??where?id<4????4??union????5??select?*????6??from?student????7??where?id>2?and?id<6????8??;????????????ID?NAME????????????????????????????????SCORE??----------?------------------------------?----------???????????1?Aaron??????????????????????????????????78???????????2?Bill???????????????????????????????????76???????????3?Cindy??????????????????????????????????89???????????4?Damon??????????????????????????????????90???????????5?Ella???????????????????????????????????73????SQL>?? ?
如果換成Union All連接兩個結果集，則結果如下：
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SQL>?select?*????2??from?student????3??where?id<4????4??union?all????5??select?*????6??from?student????7??where?id>2?and?id<6????8??;????????????ID?NAME????????????????????????????????SCORE??----------?------------------------------?----------???????????1?Aaron??????????????????????????????????78???????????2?Bill???????????????????????????????????76???????????3?Cindy??????????????????????????????????89???????????3?Cindy??????????????????????????????????89???????????4?Damon??????????????????????????????????90???????????5?Ella???????????????????????????????????73????6?rows?selected.?? ?
可以看到，Union和Union All的區別之一在于對重復結果的處理。
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接下來，我們交換一個兩個SELECT語句的順序，看看結果是怎樣的。
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SQL>?select?*????2??from?student????3??where?id>2?and?id<6????4??union????5??select?*????6??from?student????7??where?id<4????8??;????????????ID?NAME????????????????????????????????SCORE??----------?------------------------------?----------???????????1?Aaron??????????????????????????????????78???????????2?Bill???????????????????????????????????76???????????3?Cindy??????????????????????????????????89???????????4?Damon??????????????????????????????????90???????????5?Ella???????????????????????????????????73????SQL>?select?*????2??from?student????3??where?id>2?and?id<6????4??union?all????5??select?*????6??from?student????7??where?id<4????8??;????????????ID?NAME????????????????????????????????SCORE??----------?------------------------------?----------???????????3?Cindy??????????????????????????????????89???????????4?Damon??????????????????????????????????90???????????5?Ella???????????????????????????????????73???????????1?Aaron??????????????????????????????????78???????????2?Bill???????????????????????????????????76???????????3?Cindy??????????????????????????????????89????6?rows?selected.?? ?
可以看到，對于UNION來說，交換兩個SELECT語句的順序后結果仍然是一樣的，這是因為UNION會自動排序。而UNION ALL在交換了SELECT語句的順序后結果則不相同，因為UNION ALL不會對結果自動進行排序。
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那么這個自動排序的規則是什么呢？我們交換一下SELECT后面選擇字段的順序（前面使用SELECT *相當于SELECT ID,NAME,SCORE），看看結果如何：
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SQL>?select?score,id,name????2??from?student????3??where?id<4????4??union????5??select?score,id,name????6??from?student????7??where?id>2?and?id<6????8??;?????????SCORE?????????ID?NAME??----------?----------?------------------------------??????????73??????????5?Ella??????????76??????????2?Bill??????????78??????????1?Aaron??????????89??????????3?Cindy??????????90??????????4?Damon?? ?
可是看到，此時是按照字段SCORE來對結果進行排序的（前面SELECT *的時候是按照ID進行排序的）。
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那么有人會問，如果我想自行控制排序，能不能使用ORDER BY呢？當然可以。不過在寫法上有需要注意的地方：
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select?score,id,name??from?student??where?id?>?2?and?id?<?7????union????select?score,id,name??from?student??where?id?<?4????union????select?score,id,name??from?student??where?id?>?8??order?by?id?desc?? ?
order by子句必須寫在最后一個結果集里，并且其排序規則將改變操作后的排序結果。對于Union、Union All、Intersect、Minus都有效。
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其他的集合操作符，如Intersect和Minus的操作和Union基本一致，這里一起總結一下：
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Union，對兩個結果集進行并集操作，不包括重復行，同時進行默認規則的排序；
Union All，對兩個結果集進行并集操作，包括重復行，不進行排序；
Intersect，對兩個結果集進行交集操作，不包括重復行，同時進行默認規則的排序；
Minus，對兩個結果集進行差操作，不包括重復行，同時進行默認規則的排序。
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可以在最后一個結果集中指定Order by子句改變排序方式。
轉載于:https://www.cnblogs.com/chyg/archive/2012/11/24/2785424.html
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