XML 查询，按在 BaseX 中导致错误"Item expected, sequence found"排序

Question

我使用的mondial数据库既可以作为SQL也可以作为XML使用，但我对我的问题感兴趣的地方如下：

<mondial>
  <country car_code="S" area="449964" capital="cty-Sweden-Stockholm" 
           memberships="org-AfDB org-AG org-AsDB org-BIS org-CE
                        org-CBSS org-CCC org-ECE org-EBRD org-EIB 
                        org-CERN org-ESA org-EU org-FAO org-G-6 org-G-9 
                        org-G-10 org-IADB org-IAEA org-IBRD org-ICC 
                        org-ICAO org-ICFTU org-Interpol org-IDA 
                        org-IEA org-IFRCS org-IFC org-IFAD org-ILO  
                        org-IMO org-Inmarsat org-IMF org-IOC org-IOM  
                        org-ISO org-ICRM org-ITU org-Intelsat org-MTCR  
                        org-NAM org-NC org-NIB org-ANC org-EN org-NSG  
                        org-OECD org-OSCE org-PFP org-PCA org-UN  
                        org-UNAVEM-III org-UNCRO org-UNESCO org-UNIDO  
                        org-UNITAR org-UNIKOM org-UNMOGIP org-UNOMIG  
                        org-UNHCR org-UNPREDEP org-UNPROFOR org-UNTSO  
                        org-UPU org-WEU org-WFTU org-WHO org-WIPO  
                        org-WMO org-WTrO org-ZC">
    <name>Sweden</name>
    <population>8900954</population>
    <population_growth>0.56</population_growth>
    <infant_mortality>4.5</infant_mortality>
    <gdp_total>177300</gdp_total>
    <gdp_agri>2</gdp_agri>
    <gdp_ind>27</gdp_ind>
    <gdp_serv>71</gdp_serv>
    <inflation>2.6</inflation>
    <government>constitutional monarchy</government>
    <encompassed continent="europe" percentage="100"/>
    <ethnicgroup percentage="83">Swede</ethnicgroup>
    <ethnicgroup percentage="5">Finn</ethnicgroup>
    <religion percentage="87">Protestant</religion>
    <religion percentage="1.5">Roman Catholic</religion>
    <language percentage="96">Swedish</language>
    <language percentage="2.6">Finnish</language>
    <border country="SF" length="586"/>
    <border country="N" length="1619"/>
    <name>Sweden</name>
    <population>8900954</population>
    <population_growth>0.56</population_growth>
    <infant_mortality>4.5</infant_mortality>
    <gdp_total>177300</gdp_total>
    <gdp_agri>2</gdp_agri>
    <gdp_ind>27</gdp_ind>
    <gdp_serv>71</gdp_serv>
    <inflation>2.6</inflation>
    <government>constitutional monarchy</government>
    <encompassed continent="europe" percentage="100"/>
    <ethnicgroup percentage="83">Swede</ethnicgroup>
    <ethnicgroup percentage="5">Finn</ethnicgroup>
    <religion percentage="87">Protestant</religion>
    <religion percentage="1.5">Roman Catholic</religion>
    <language percentage="96">Swedish</language>
    <language percentage="2.6">Finnish</language>
    <border country="SF" length="586"/>
    <border country="N" length="1619"/>
    <province id="prov-Sweden-5" capital="cty-Sweden-3" country="S">
      <name>Vastra Gotaland</name>
      <area>24473</area>
      <population>1464971</population>
      <city id="cty-Sweden-3" is_state_cap="yes" country="S" 
            province="prov-Sweden-5">
        <name>Goteborg</name>
        <longitude>12</longitude>
        <latitude>57.4</latitude>
        <population year="95">449189</population>
        <located_at watertype="sea" sea="sea-Kattegat"/>
        <located_at watertype="river" river="river-Goetaaelv"/>
      </city>
      <city id="cty-Sweden-Vanersborg" country="S" 
            province="prov-Sweden-5">
        <name>Vanersborg</name>
        <longitude>12.2</longitude>
        <latitude>58.2</latitude>
        <population year="87">35804</population>
        <located_at watertype="river" river="river-Goetaaelv"/>
        <located_at watertype="lake" lake="lake-Vaenersee"/>
      </city>
      <city id="cty-Sweden-Mariestad" country="S" 
            province="prov-Sweden-5">
        <name>Mariestad</name>
        <longitude>13.5</longitude>
        <latitude>58.4</latitude>
        <population year="87">24255</population>
        <located_at watertype="lake" lake="lake-Vaenersee"/>
      </city>
    </province>
    <province id="prov-Sweden-3" capital="cty-Sweden-Karlskrona" country="S">
      <name>Blekinge</name>
      <area>2941</area>
      <population>151168</population>
      <city id="cty-Sweden-Karlskrona" is_state_cap="yes" country="S" 
            province="prov-Sweden-3">
        <name>Karlskrona</name>
        <longitude>15.3</longitude>
        <latitude>56.1</latitude>
        <population year="87">59007</population>
        <located_at watertype="sea" sea="sea-Ostsee"/>
      </city>
    </province>
  </country>
</mondial>

问题是，我想为以下问题编写一个查询：显示瑞典各省（län）的名称、人口和首府，按人口最多的省份排列。

到目前为止，我已经在XQuery中编写了以下内容：

let $mondial := db:open('mondial')
for $x in $mondial/mondial/country[name="Sweden"]
let $pop := $x/province/population
return $pop/string()

这给了我瑞典人口中所有运行良好的省份，然后我在查询中添加了以下内容：

let $mondial := db:open('mondial')
for $x in $mondial/mondial/country[name="Sweden"]
let $pop := $x/province/population
let $cname := $x/province/name
let $cap := $x/province/city[@is_state_cap="yes"]/name
return ($cname/string(), $pop/string(), $cap/string())

它也很好，给了我省份名称、人口和首都名称，但当我尝试以这种方式添加人口顺序时：

let $mondial := db:open('mondial')
for $x in $mondial/mondial/country[name="Sweden"]
let $pop := $x/province/population
let $cname := $x/province/name
let $cap := $x/province/city[@is_state_cap="yes"]/name
order by $pop 
return ($cname/string(), $pop/string(), $cap/string())

BaseX给我错误

应为项，找到序列：（元素填充｛……｝，…）

我似乎不明白为什么。我还尝试在排序之后添加/string()，并尝试添加降序，但到目前为止没有成功。

Answer 1

order by每次循环迭代需要一个项目。当你在所有国家而不是瑞典的省份进行循环时，你会得到瑞典每个省份的人口序列。

而是在各省上空盘旋。在下面的代码中，我更改了两件事：

• 循环使用省份而不是国家，因此也调整了子元素的检索（删除/province轴步骤）
• 更改为合理的变量名称。$x是一个省的最差的变量名之一。如果你使用了可变名称，你甚至可能会自己意识到这个问题，因为第一行表示将国家分配给省份

let $mondial := db:open('mondial')
for $province in $mondial/mondial/country[name="Sweden"]/province
let $population := $province/population
let $name := $province/name
let $capital := $province/city[@is_state_cap="yes"]/name
order by $population
return ($name/string(), $population/string(), $capital/string())