Question

任何人都可以想到此查询的另一种解决方案（或者，如果看到这种错误，则可以解决我的错误吗？我的解决方案显然没有给我正确的结果（那是我的教授说的），并且他不会告诉我为什么。感谢您的帮助。问题，ERD，输入和我的输出如下：

输出：

2   Euronext Paris          EP  2019-04-22 13:06:36
1   London Stock Exchange   LSE 2019-03-28 13:06:36
5   Moscow Stock Exchange   MSE NULL
6   NASDAQ Stock Exchange   NASDAQ  2019-05-09 13:06:36
3   New York Stock Exchange NYSE    2019-05-20 13:06:36
4   Tokyo Stock Exchange    TSE 2019-04-10 13:06:36

对于每个证券交易所列出的每个股票，显示交易所名称，   股票代码以及该股票最后交易的日期和时间。按证券交易所名称，股票代号对输出进行排序。如果尚未交易股票，请在上次交易日期显示NULL。

SELECT
stock_exchange.stock_ex_id, 
stock_exchange.name stock_exchange_name,
stock_exchange.symbol,
nvl(to_char(max(trade.transaction_time), 'yyyy-mm-dd hh24:mi:ss'), 'NULL') as last_traded
FROM stock_exchange 
LEFT JOIN trade
ON trade.stock_ex_id = stock_exchange.stock_ex_id
GROUP BY stock_exchange.stock_ex_id, stock_exchange.name, stock_exchange.symbol
ORDER BY stock_exchange.name, stock_exchange.symbol;

ERD

Answer 1

您的ERD缺少Trade和Stock_Listing之间的关系。在Stock_Listing中可以获取股票代码：

SELECT
    stock_exchange.stock_ex_id, 
    stock_exchange.name stock_exchange_name,
    stock_listing.symbol,
    nvl(to_char(max(trade.transaction_time), 'yyyy-mm-dd hh24:mi:ss'), 'NULL') as last_traded
FROM stock_exchange 
    JOIN stock_listing
    ON stock_exchange.stock_ex_id =  stock_listing.stock_ex_id
    LEFT JOIN trade
    ON trade.stock_ex_id = stock_listing.stock_ex_id
    and trade.stock_id =  stock_listing.stock_id
GROUP BY stock_exchange.stock_ex_id, stock_exchange.name, stock_listing.symbol
ORDER BY stock_exchange.name, stock_listing.symbol;

注意：

您是否需要在结果集中添加stock_ex_id
未交易股票时，是否需要包含字符串NULL。空格（null）就够了吗？

Answer 2

我想你想要

select sl.stock_ex_id, se.name, sl.symbol, max(t.trade_time)
from stock_listing sl join  -- inner join should be fine
     stock_exchange se
     on se.stock_ex_id = sl.stock_ex_id left join
     trade t
     on t.stock_id = sl.stock_id
group by sl.stock_ex_id, se.name, sl.symbol;

该问题的措辞是“针对每个股票清单”，因此在我看来，从该表开始并保留其所有行是有意义的。

max()将返回NULL的值而不是字符串'NULL'，但这似乎与问题相符。

SQL此查询的另一个解决方案？

2 个答案: