为什么SQL在需要外连接的内部连接中进行

Question

我有两个表，我想使用SQL“外部”连接（然后获取）。确切的SQL查询（有问题）是：

SELECT
    LEFT(a.cusip, 6) AS cusip6, 
    a.date, a.prc, a.ret, a.vol, a.spread, a.shrout,
    b.epsf12, (b.seqq-b.pstkq) / b.cshoq AS bps
FROM
    crsp.msf a 
FULL JOIN 
    compa.fundq b ON (LEFT(a.cusip, 6) = LEFT(b.cusip, 6) 
                  AND a.date = b.datadate)
WHERE 
    (b.datadate BETWEEN '2010-01-01' and '2015-12-31') 
    AND (a.date BETWEEN '2010-01-01' and '2015-12-31') 
    AND (b.cshoq > 0)

这将返回670'293行。

但是当我分别获取两个数据集并且（外部）通过R-merge（）连接它们时，我得到1'182'093行。我使用的两个单独的查询是：

SELECT  
    LEFT(cusip, 6) AS cusip6, date, prc, ret, vol, spread, shrout 
FROM
    crsp.msf 
WHERE 
    date BETWEEN '2010-01-01' and '2015-12-31'

SELECT 
    LEFT(cusip, 6) AS cusip6, datadate AS date, epsf12, 
    (seqq-pstkq)/cshoq AS bps 
FROM
    compa.fundq 
WHERE 
    datadate BETWEEN '2010-01-01' and '2015-12-31' 
    AND cshoq > 0

然后我合并（外部联接）使用：

merge(x = data_1, y = data_2, by.x = c("cusip6", "date"), by.y = c("cusip6", "date"), all = T)

这将返回1'182'093行，这是正确的。因此，当我明确指定外部联接时，我的原始（第一个）SQL查询实际上正在执行“内部联接”。下面的R-merge（）返回670'293行，重新验证从SQL获取的数据确实是内连接。

merge(x = data_1, y = data_2, by.x = c("cusip6", "date"), by.y = c("cusip6", "date"))

我的SQL查询出错了什么？

Answer 1

因为WHERE子句是在JOIN之后应用的。此时存在NULL值（由于'失败'JOIN），这些行无法通过WHERE子句。

如果需要OUTER JOIN和过滤器，请将过滤器放在JOIN或子查询中。

SELECT
    LEFT(a.cusip, 6) AS cusip6, 
    a.date, a.prc, a.ret, a.vol, a.spread, a.shrout,
    b.epsf12, (b.seqq-b.pstkq) / b.cshoq AS bps
FROM
    (SELECT * FROM crsp.msf WHERE date BETWEEN '2010-01-01' and '2015-12-31') a
FULL JOIN 
    (SELECT * FROM compa.fundq WHERE datadate BETWEEN '2010-01-01' and '2015-12-31' AND cshoq > 0) b
        ON  LEFT(a.cusip, 6) = LEFT(b.cusip, 6) 
        AND a.date = b.datadate