原始数据库中的Top-n

Question

我使用名为“文件地理数据库”的 GIS 数据库，它具有最少的 SQL 支持。 不支持：

窗口函数

• TOP、LIMIT、FETCH FIRST 或 ROWNUM
• 相关子查询
是否支持：

标量子查询

• 派生表
• 分组依据、排序依据、连接等
用于文件地理数据库报告和分析的 SQL
ArcGIS 中使用的查询表达式的 SQL 参考

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数据：

我有一个 CITIES 表。

如果支持 SQL 客户端，INSERT 语句将如下所示：

  create table cities 
   (objectid int, 
    population_centre varchar2(255), 
    population_2021 number(38,1), 
    other_columns varchar2(255)
   ) 
insert into cities (objectid,population_centre,population_2021,other_columns) values (11,'Calgary',1305550,'a');
insert into cities (objectid,population_centre,population_2021,other_columns) values (23,'Edmonton',1151635,'b');
insert into cities (objectid,population_centre,population_2021,other_columns) values (31,'Halifax',348634,'c');
insert into cities (objectid,population_centre,population_2021,other_columns) values (32,'Hamilton',729560,'d');
insert into cities (objectid,population_centre,population_2021,other_columns) values (37,'Kelowna',181380,'e');
insert into cities (objectid,population_centre,population_2021,other_columns) values (40,'Kitchener',522888,'f');
insert into cities (objectid,population_centre,population_2021,other_columns) values (45,'London',423369,'g');
insert into cities (objectid,population_centre,population_2021,other_columns) values (51,'Montreal',3675219,'h');
insert into cities (objectid,population_centre,population_2021,other_columns) values (58,'Oshawa',335949,'i');
insert into cities (objectid,population_centre,population_2021,other_columns) values (59,'Ottawa–Gatineau',1068821,'j');
insert into cities (objectid,population_centre,population_2021,other_columns) values (65,'Quebec City',733156,'k');
insert into cities (objectid,population_centre,population_2021,other_columns) values (67,'Regina',224996,'l');
insert into cities (objectid,population_centre,population_2021,other_columns) values (76,'Saskatoon',264637,'m');
insert into cities (objectid,population_centre,population_2021,other_columns) values (82,'St. Catharines – Niagara Falls',242460,'n');
insert into cities (objectid,population_centre,population_2021,other_columns) values (83,'St. John''s',185565,'o');
insert into cities (objectid,population_centre,population_2021,other_columns) values (90,'Toronto',5647656,'p');
insert into cities (objectid,population_centre,population_2021,other_columns) values (92,'Vancouver',2426160,'q');
insert into cities (objectid,population_centre,population_2021,other_columns) values (94,'Victoria',363222,'r');
insert into cities (objectid,population_centre,population_2021,other_columns) values (98,'Windsor',306519,'s');
insert into cities (objectid,population_centre,population_2021,other_columns) values (99,'Winnipeg',758515,'t');

select * from cities

  OBJECTID POPULATION_CENTRE POPULATION_2021 OTHER_COLUMNS
---------- --------------------------------- -------------
        11 Calgary                 1305550   a              
        23 Edmonton                1151635   b              
        31 Halifax                  348634   c              
        32 Hamilton                 729560   d              
        37 Kelowna                  181380   e              
        40 Kitchener                522888   f              
        45 London                   423369   g              
        51 Montreal                3675219   h              
        58 Oshawa                   335949   i              
        59 Ottawa–Gatineau         1068821   j              
        65 Quebec City              733156   k              
        67 Regina                   224996   l              
        76 Saskatoon                264637   m              
        82 St. Catharines           242460   n              
        83 St. John's               185565   o              
        90 Toronto                 5647656   p              
        92 Vancouver               2426160   q              
        94 Victoria                 363222   r              
        98 Windsor                  306519   s              
        99 Winnipeg                 758515   t              

假数据库<>小提琴

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问题：

在GIS软件中，我可以在WHERE子句窗口中编写SQL表达式来选择行。

尽管 SQL 支持有限，有没有办法选择人口最多的 4 个城市？

我怀疑答案是否定的。但我不禁想知道在早期数据库的 SQL 功能有限时它是如何完成的。例如，当 SQLite 首次发布时，如果它对 SQL 支持有限，那么 top-n 查询是如何执行的？

Answer 1

db<>fiddle

工具中使用。这是一种强力消除，如果 population_2021 字段中存在具有完全相同值的行，则可能无法正常工作。它依赖于标量子查询支持，但我不确定查询引擎允许多少嵌套，所以尝试一下：

select *
from cities
where population_2021 = (select max(population_2021) from cities)
or population_2021 = (select max(population_2021) from cities where population_2021 < (select max(population_2021) from cities))
or population_2021 = (select max(population_2021) from cities where population_2021 < (select max(population_2021) from cities where population_2021 < (select max(population_2021) from cities)))
or population_2021 = (select max(population_2021) from cities where population_2021 < (select max(population_2021) from cities where population_2021 < (select max(population_2021) from cities where population_2021 < (select max(population_2021) from cities))))
order by population_2021