选择限制范围内的空值

Question

我想要选择限制为100的NULL值。也就是说，左连接应该使输出限制为100，并且在这个范围内我想要计算所有空值。我的subquery好像坏了。你知道我做错了什么吗？提前感谢！

我用SUM()尝试了一些东西，但也不起作用

SELECT SUM(echonest.id IS NULL) FROM (SELECT *
FROM tracks, echonest
left JOIN echonest ON tracks.id = echonest.id 
LIMIT 100)

SELECT *
FROM tracks AS t
LEFT JOIN echonest AS e ON t.id = e.id
LIMIT 100 IN 
(SELECT COUNT(e.id)
FROM e
WHERE e.id IS NULL)

预期输出应计算LIMIT 100中的所有NULL值。

Answer 1

您只需选择前100行，并像这样计算它们：-

select count(*) from (
    SELECT t.id
    FROM tracks AS t
    LEFT JOIN echonest AS e ON t.id = e.id
    where e.id is null
    limit 100
) a

Answer 2

我想这就是你想要的

SELECT COUNT(*) AS tot_nuls
FROM 
(
    SELECT E.id AS eid 
    FROM tracks T
    LEFT JOIN echonest E ON T.id = E.id 
    LIMIT 100
) T
WHERE eid IS NULL

但是，正如在注释中多次指定的那样，您应该意识到不带ORDER BY子句的LIMIT子句将返回不确定(也称为随机)结果。