Question

我有一个包含3个表的数据库：

股票
stocksplits
分红

股票和股票之间以及股票和股息之间存在一对多的关系。对于每个股权，我想显示股票分割和股息的数量：

SELECT equities.Symbol, 
       (SELECT COUNT(*) 
          FROM stocksplits 
         WHERE stocksplits.EquityID = equities.InstrumentID) as `# Splits`,
       (SELECT COUNT(*) 
          FROM dividends 
         WHERE dividends.EquityID = equities.InstrumentID) as `# Dividends`
FROM equities

查询似乎运行正常，但我怀疑它效率低下。怎样才能更快地重构？没有DBMS（通过.net到MySQL服务器的SQL查询），假设索引存在于每个表的主ID上。

Answer 1

计算PK而不是*可能已经有所帮助：

SELECT equities.Symbol, 
           (SELECT COUNT(stocksplitsID) 
              FROM stocksplits 
             WHERE stocksplits.EquityID =     equity.InstrumentID) as `# Splits`,
           (SELECT COUNT(dividendsid) 
              FROM dividends 
             WHERE dividends.EquityID = equities.InstrumentID) as `# Dividends`
FROM equities

Answer 2

这是您的原始查询

SELECT equities.Symbol, 
       (SELECT COUNT(*) 
          FROM stocksplits 
         WHERE stocksplits.EquityID = equities.InstrumentID) as `# Splits`
FROM equities

我只是认为LEFT JOIN会更清洁

SELECT equities.Symbol,
    SUM(IF(IFNULL(stocksplits.EquityID,0)=0,0,1)) StockSplits,
    SUM(IF(IFNULL(dividends.EquityID  ,0)=0,0,1)) Dividends
FROM
    equities
    LEFT JOIN stocksplits ON equities.InstrumentID = stocksplits.EquityID
    LEFT JOIN dividends   ON equities.InstrumentID = dividends.EquityID
GROUP BY equities.Symbol;

IFNULL涵盖任何没有股票分裂的股票

尝试一下，看看它运行得更快

让我解释一下SUM(IF(IFNULL(stocksplits.EquityID,0)=0,0,1))

这个词

如果LEFT JOIN在右侧表格中没有相应的条目，则IFNULL会将NULL变为0。
如果LEFT JOIN有一个右侧条目，IF函数返回1
如果LEFT JOIN没有右侧输入，则IF函数返回0
SUM会将所有的1和0相加，模拟COUNT

Answer 3

在我的经验中，MySQL的correlated subquery表现不佳。

Q1 - 加入

SELECT t1.Symbol, t1.cnt_splits, t2.cnt_dividends
FROM (
    SELECT equities.Symbol AS Symbol, COUNT(*) AS cnt_splits
    FROM equities LEFT JOIN stocksplits
        ON stocksplits.EquityID = equities.InstrumentID
    GROUP BY equities.Symbol
) t1,
(
    SELECT equities.Symbol AS Symbol, COUNT(*) AS cnt_dividends
    FROM equities LEFT JOIN dividends
        dividends.EquityID = equities.InstrumentID
    GROUP BY equities.Symbol
) t2 ON t1.Symbol = t2.Symbol;

Q2 - UNION

Q1没有correlate subquery并产生与您相同的结果。但需要额外的连接，这需要时间。以下UNION模式更快，但输出应在客户端转换。

SELECT equities.Symbol AS Symbol, COUNT(*) AS cnt_splits
FROM equities LEFT JOIN stocksplits
    ON stocksplits.EquityID = equities.InstrumentID
GROUP BY equities.Symbol

UNION

SELECT equities.Symbol AS Symbol, COUNT(*) AS cnt_dividends
FROM equities LEFT JOIN dividends
    dividends.EquityID = equities.InstrumentID
GROUP BY equities.Symbol

MySQL Count子查询

3 个答案: