我经常发现自己在group by子句中添加表达式，我确信这些表达式是唯一的。它有时候证明我错了 - 因为我的SQL中的错误或错误的假设，并且该表达式并不是真正独特的。
在很多情况下，我宁愿这会产生SQL错误，而不是默默地扩展我的结果集，有时非常巧妙。
我希望能够做到这样的事情：
select product_id, unique description from product group by product_id
但显然我自己也无法实现 - 但在某些数据库中，用户定义的聚合可以实现几乎简洁的东西。
在所有版本的SQL中，只允许一个唯一输入值的特殊聚合是否通常有用？如果是这样，现在大多数数据库都可以实现这样的事情吗？ null值应与其他任何值一样考虑 - 与内置聚合avg通常的工作方式不同。 （我已经为postgres和Oracle添加了实现此方法的答案。）
以下示例旨在说明如何使用聚合，但这是一个简单的情况，很明显哪些表达式应该是唯一的。实际使用更可能是在更大的查询中，更容易对唯一性做出错误的假设
表：
product_id | description
------------+-------------
1 | anvil
2 | brick
3 | clay
4 | door
sale_id | product_id | cost
---------+------------+---------
1 | 1 | £100.00
2 | 1 | £101.00
3 | 1 | £102.00
4 | 2 | £3.00
5 | 2 | £3.00
6 | 2 | £3.00
7 | 3 | £24.00
8 | 3 | £25.00
查询：
> select * from product join sale using (product_id);
product_id | description | sale_id | cost
------------+-------------+---------+---------
1 | anvil | 1 | £100.00
1 | anvil | 2 | £101.00
1 | anvil | 3 | £102.00
2 | brick | 4 | £3.00
2 | brick | 5 | £3.00
2 | brick | 6 | £3.00
3 | clay | 7 | £24.00
3 | clay | 8 | £25.00
> select product_id, description, sum(cost)
from product join sale using (product_id)
group by product_id, description;
product_id | description | sum
------------+-------------+---------
2 | brick | £9.00
1 | anvil | £303.00
3 | clay | £49.00
> select product_id, solo(description), sum(cost)
from product join sale using (product_id)
group by product_id;
product_id | solo | sum
------------+-------+---------
1 | anvil | £303.00
3 | clay | £49.00
2 | brick | £9.00
错误案例：
> select solo(description) from product;
ERROR: This aggregate only allows one unique input
答案 0 :(得分：7)
ORACLE解决方案
select product_id,
case when min(description) != max(description) then to_char(1/0)
else min(description) end description,
sum(cost)
from product join sale using (product_id)
group by product_id;
而不是to_char（1/0）[引发DIVIDE_BY_ZERO错误），你可以使用一个简单的函数
CREATE OR REPLACE FUNCTION solo (i_min IN VARCHAR2, i_max IN VARCHAR2)
RETURN VARCHAR2 IS
BEGIN
IF i_min != i_max THEN
RAISE_APPLICATION_ERROR(-20001, 'Non-unique value specified');
ELSE
RETURN i_min;
END;
END;
/
select product_id,
solo(min(description),max(description)) end description,
sum(cost)
from product join sale using (product_id)
group by product_id;
您可以使用用户定义的聚合，但我担心在SQL和PL / SQL之间切换会对性能产生影响。
答案 1 :(得分：3)
以下是我对postgres的实现（已编辑为将null视为唯一值）：
create function solo_sfunc(inout anyarray, anyelement)
language plpgsql immutable as $$
begin
if $1 is null then
$1[1] := $2;
else
if ($1[1] is not null and $2 is null)
or ($1[1] is null and $2 is not null)
or ($1[1]!=$2) then
raise exception 'This aggregate only allows one unique input';
end if;
end if;
return;
end;$$;
create function solo_ffunc(anyarray) returns anyelement
language plpgsql immutable as $$
begin
return $1[1];
end;$$;
create aggregate solo(anyelement)
(sfunc=solo_sfunc, stype=anyarray, ffunc=solo_ffunc);
用于测试的示例表：
create table product(product_id integer primary key, description text);
insert into product(product_id, description)
values (1, 'anvil'), (2, 'brick'), (3, 'clay'), (4, 'door');
create table sale( sale_id serial primary key,
product_id integer not null references product,
cost money not null );
insert into sale(product_id, cost)
values (1, '100'::money), (1, '101'::money), (1, '102'::money),
(2, '3'::money), (2, '3'::money), (2, '3'::money),
(3, '24'::money), (3, '25'::money);
答案 2 :(得分：1)
你应该在（product_id，description）上定义一个UNIQUE约束，然后你就不必担心一个产品有两个描述。
答案 3 :(得分：1)
这是我对Oracle的实现 - 不幸的是，我认为每种基类型都需要一个实现：
create type SoloNumberImpl as object
(
val number,
flag char(1),
static function ODCIAggregateInitialize(sctx in out SoloNumberImpl)
return number,
member function ODCIAggregateIterate( self in out SoloNumberImpl,
value in number )
return number,
member function ODCIAggregateTerminate( self in SoloNumberImpl,
returnValue out number,
flags in number )
return number,
member function ODCIAggregateMerge( self in out SoloNumberImpl,
ctx2 in SoloNumberImpl )
return number
);
/
create or replace type body SoloNumberImpl is
static function ODCIAggregateInitialize(sctx in out SoloNumberImpl)
return number is
begin
sctx := SoloNumberImpl(null, 'N');
return ODCIConst.Success;
end;
member function ODCIAggregateIterate( self in out SoloNumberImpl,
value in number )
return number is
begin
if self.flag='N' then
self.val:=value;
self.flag:='Y';
else
if (self.val is null and value is not null)
or (self.val is not null and value is null)
or (self.val!=value) then
raise_application_error( -20001,
'This aggregate only allows one unique input' );
end if;
end if;
return ODCIConst.Success;
end;
member function ODCIAggregateTerminate( self in SoloNumberImpl,
returnValue out number,
flags in number )
return number is
begin
returnValue := self.val;
return ODCIConst.Success;
end;
member function ODCIAggregateMerge( self in out SoloNumberImpl,
ctx2 in SoloNumberImpl )
return number is
begin
if self.flag='N' then
self.val:=ctx2.val;
self.flag=ctx2.flag;
elsif ctx2.flag='Y' then
if (self.val is null and ctx2.val is not null)
or (self.val is not null and ctx2.val is null)
or (self.val!=ctx2.val) then
raise_application_error( -20001,
'This aggregate only allows one unique input' );
end if;
end if;
return ODCIConst.Success;
end;
end;
/
create function SoloNumber (input number)
return number aggregate using SoloNumberImpl;
/