问题
UNION joins two results and remove duplicates, while UNION ALL does not remove duplicates.UNION also sort the final output.
What I want is the UNION ALL without duplicates and without the sort. Is that possible?
The reason for this is that I want the result of the first query to be on top of the final result, and the second query at the bottom (and each sorted as if they where run individually).
回答1:
I notice this question gets quite a lot of views so I'll first address a question you didn't ask!
Regarding the title. To achieve a "Sql Union All with “distinct”" then simply replace UNION ALL with UNION. This has the effect of removing duplicates.
For your specific question, given the clarification "The first query should have "priority", so duplicates should be removed from bottom" you can use
SELECT col1,
col2,
MIN(grp) AS source_group
FROM (SELECT 1 AS grp,
col1,
col2
FROM t1
UNION ALL
SELECT 2 AS grp,
col1,
col2
FROM t2) AS t
GROUP BY col1,
col2
ORDER BY MIN(grp),
col1
回答2:
"UNION also sort the final output" - only as an implementation artifact. It is by no means guaranteed to perform the sort, and if you need a particular sort order, you should specify it with an ORDER BY clause. Otherwise, the output order is whatever is most convenient for the server to provide.
As such, your request for a function that performs a UNION ALL but that removes duplicates is easy - it's called UNION.
From your clarification, you also appear to believe that a UNION ALL will return all of the results from the first query before the results of the subsequent queries. This is also not guaranteed. Again, the only way to achieve a particular order is to specify it using an ORDER BY clause.
回答3:
SELECT *, 1 AS sort_order
FROM table1
EXCEPT
SELECT *, 1 AS sort_order
FROM table2
UNION
SELECT *, 1 AS sort_order
FROM table1
INTERSECT
SELECT *, 1 AS sort_order
FROM table2
UNION
SELECT *, 2 AS sort_order
FROM table2
EXCEPT
SELECT *, 2 AS sort_order
FROM table1
ORDER BY sort_order;
But the real answer is: other than the ORDER BY clause, the sort order will by arbitrary and not guaranteed.
回答4:
Consider these tables (Standard SQL code, runs on SQL Server 2008):
WITH A
AS
(
SELECT *
FROM (
VALUES (1),
(2),
(3),
(4),
(5),
(6)
) AS T (col)
),
B
AS
(
SELECT *
FROM (
VALUES (9),
(8),
(7),
(6),
(5),
(4)
) AS T (col)
), ...
The desired effect is this to sort table A by col ascending, sort table B by col descending then unioning the two, removing duplicates, retaining order before the union and leaving table A results on the "top" with table B on the "bottom" e.g. (pesudo code)
(
SELECT *
FROM A
ORDER
BY col
)
UNION
(
SELECT *
FROM B
ORDER
BY col DESC
);
Of course, this won't work in SQL because there can only be one ORDER BY clause and it can only be applied to the top level table expression (or whatever the output of a SELECT query is known as; I call it the "resultset").
The first thing to address is the intersection between the two tables, in this case the values 4, 5 and 6. How the intersection should be sorted needs to be specified in SQL code, therefore it is desirable that the designer specifies this too! (i.e. the person asking the question, in this case).
The implication in this case would seem to be that the intersection ("duplicates") should be sorted within the results for table A. Therefore, the sorted resultset should look like this:
VALUES (1), -- A including intersection, ascending
(2), -- A including intersection, ascending
(3), -- A including intersection, ascending
(4), -- A including intersection, ascending
(5), -- A including intersection, ascending
(6), -- A including intersection, ascending
(9), -- B only, descending
(8), -- B only, descending
(7), -- B only, descending
Note in SQL "top" and "bottom" has no inferent meaning and a table (other than a resultset) has no inherent ordering. Also (to cut a long story short) consider that UNION removes duplicate rows by implication and must be applied before ORDER BY. The conclusion has to be that each table's sort order must be explicitly defined by exposing a sort order column(s) before being unioned. For this we can use the ROW_NUMBER() windowed function e.g.
...
A_ranked
AS
(
SELECT col,
ROW_NUMBER() OVER (ORDER BY col) AS sort_order_1
FROM A -- include the intersection
),
B_ranked
AS
(
SELECT *,
ROW_NUMBER() OVER (ORDER BY col DESC) AS sort_order_1
FROM B
WHERE NOT EXISTS ( -- exclude the intersection
SELECT *
FROM A
WHERE A.col = B.col
)
)
SELECT *, 1 AS sort_order_0
FROM A_ranked
UNION
SELECT *, 2 AS sort_order_0
FROM B_ranked
ORDER BY sort_order_0, sort_order_1;
回答5:
select T.Col1, T.Col2, T.Sort
from
(
select T.Col1,
T.Col2,
T.Sort,
rank() over(partition by T.Col1, T.Col2 order by T.Sort) as rn
from
(
select Col1, Col2, 1 as Sort
from Table1
union all
select Col1, Col2, 2
from Table2
) as T
) as T
where T.rn = 1
order by T.Sort
回答6:
Try this:
SELECT DISTINCT * FROM (
SELECT column1, column2 FROM Table1
UNION ALL
SELECT column1, column2 FROM Table2
UNION ALL
SELECT column1, column2 FROM Table3
) X ORDER BY Column1
回答7:
The sort is used to eliminate the duplicates, and is implicit for DISTINCT and UNION queries (but not UNION ALL) - you could still specify the columns you'd prefer to order by if you need them sorted by specific columns.
For example, if you wanted to sort by the result sets, you could introduce an additional column, and sort by that first:
SELECT foo, bar, 1 as ResultSet
FROM Foo
WHERE bar = 1
UNION
SELECT foo, bar, 2 as ResultSet
FROM Foo
WHERE bar = 3
UNION
SELECT foo, bar, 3 as ResultSet
FROM Foo
WHERE bar = 2
ORDER BY ResultSet
回答8:
I assume your tables are table1 and table2 respectively, and your solution is;
(select * from table1 MINUS select * from table2)
UNION ALL
(select * from table2 MINUS select * from table1)
回答9:
1,1:
select 1 from dual
union all select 1 from dual
1:
select 1 from dual
union select 1 from dual
回答10:
You can do something like this.
Select distinct name from (SELECT r.name FROM outsider_role_mapping orm1
union all
SELECT r.name FROM user_role_mapping orm2
) tmp;
来源：https://stackoverflow.com/questions/7552019/how-to-execute-union-without-sorting-sql