Question

我需要用SQL计算人口密度。我有一列人口，一列面积，但我不知道如何执行它，我得到除以零误差。

这是我尝试过的：

import React, {useState, useRef, useEffect} from 'react';
import products from './mock.json';

export default function App() {
  return (
    <div style={{display: 'flex', flexDirection: 'row'}}>
      <ShoppingCart products={products} />
    </div>
  );
}
function List({products, title, addProduct, removeProduct}) {
  return (
    <div
      style={{
        display: 'flex',
        flexDirection: 'column',
        alignItems: 'start-end',
      }}
    >
      {title}
      <ul
        style={{
          listStyle: 'none',
          paddingInlineStart: '0px',
          width: '500px',
          height: '260px',
          overflow: 'scroll',
          paddingRight: '10px',
        }}
      >
        {products.length === 0 ? (
          <div>No products</div>
        ) : (
          products.map(({category, price, stocked, name}, index) => (
            <SuspendDisplayIfOverflowing>
              {ref => (
                <li
                  style={{
                    padding: '10px',
                    border: '1px solid blue',
                    display: 'flex',
                    justifyContent: 'space-between',
                    height: '18px',
                    overflow: 'scroll',
                  }}
                  ref={ref}
                  key={price}
                >
                  <div style={{color: `${stocked ? 'green' : 'red'}`}}>
                    {`${name} - ${category} - available: ${stocked} - price: ${price}`}
                  </div>
                  {addProduct && stocked ? (
                    <button
                      onClick={() =>
                        addProduct({category, price, stocked, name})
                      }
                    >
                      Add
                    </button>
                  ) : null}
                  {removeProduct ? (
                    <button onClick={() => removeProduct(index)}>Remove</button>
                  ) : null}
                </li>
              )}
            </SuspendDisplayIfOverflowing>
          ))
        )}
      </ul>
      <Total prices={products.map(({price}) => getCleanPrice(price))} />
    </div>
  );
}

function ShoppingCart({products}) {
  const [productsInCart, manipulateProduct] = useState([]);
  const addProduct = product =>
    manipulateProduct(productsInCart.concat(product));
  const removeProduct = productIndex =>
    manipulateProduct(
      productsInCart.filter((_, index) => index !== productIndex),
    );

  return (
    <>
      <List
        title="Available Products"
        products={products}
        addProduct={addProduct}
      />
      <List
        title="Shopping Cart"
        products={productsInCart}
        removeProduct={removeProduct}
      />
    </>
  );
}

function Total({prices}) {
  const total = prices.reduce((accum, item) => (accum += parseFloat(item)), 0);
  return <div>Total amount to pay: {total.toFixed(2)}</div>;
}

function getCleanPrice(price) {
  return price.slice(1);
}

function SuspendDisplayIfOverflowing(props) {
  const overflowingRef = useRef(null);
  useEffect(() => {
    console.log(
      'oveflowing scroll height',
      overflowingRef.current.scrollHeight,
    );
    console.log(
      'oveflowing client height',
      overflowingRef.current.clientHeight,
    );
    console.log(
      'oveflowing client height',
      overflowingRef.current.offsetHeight,
    );
    console.log(overflowingRef.current);
  }, [overflowingRef]);
  return props.children(overflowingRef);
}

Answer 1

您为什么需要group by？
无需分组。
即使您确实需要group by，也无法选择非聚集列，例如：
population, gdp, area。
只需检查null列中的0或area：

select 
  name, population, gdp, area, 
  case coalesce(area, 0) 
    when 0 then null 
    else population / area 
  end as pop_density 
from country 
order by gdp

Answer 2

使用nullif()：

select name, population, gdp, area,
       population / nullif(area, 0) as pop_density
from country
order by gdp;

group by似乎没有必要。

nullif()是一个标准函数，当传入的两个值相等时会返回NULL。如果您的数据库不支持它，则可以使用case表达式来做同样的事情。

Answer 3

只需添加where clause as

       select name, population, gdp, area, 
      [population]/[area] as pop_density 
        from country where area
       NOT IN (NULL,0) group by name 
     order by gdp

或者，您可以通过Case语句来处理除数区域的值0 ...

  .......Case when Area IN (Null,0)
         then 0
          else
           [population]/[area]

Answer 4

select name, population, gdp, area, [population]/[area] as pop_density 
from country 
where area is not null and area is not 0
group by name 
order by gdp;

SQL划分2列

4 个答案: