计算MySQL查询中的行数

Question

可能重复：
我可以一起使用COUNT（）和DISTINCT吗？

我有这个SQL语句：

SELECT * FROM wp_posts
INNER JOIN wp_term_relationships ON (wp_posts.ID = wp_term_relationships.object_id)
WHERE ( wp_term_relationships.term_taxonomy_id IN (4,3) )
AND wp_posts.post_type = 'post' AND (wp_posts.post_status = 'published')

我得到两行具有相同的ID =>预期

添加GROUP BY wp_posts.ID会将行数减少到一

现在我想使用和SQL查询获取行数

SELECT COUNT(*) AS cnt FROM wp_posts
INNER JOIN wp_term_relationships ON (wp_posts.ID = wp_term_relationships.object_id)
WHERE ( wp_term_relationships.term_taxonomy_id IN (4,3) )
AND wp_posts.post_type = 'post' AND (wp_posts.post_status = 'published')
GROUP BY wp_posts.ID

即使使用“ GROUP BY”，我也会得到结果“ 2”而不是“ 1”。

从第一条语句中获取行数的正确语句是什么？

Answer 1

尝试这个：

SELECT count(DISTINCT wp_posts.ID) as cnt FROM wp_posts INNER JOIN wp_term_relationships ON (wp_posts.ID = wp_term_relationships.object_id) WHERE ( wp_term_relationships.term_taxonomy_id IN (4,3) ) AND wp_posts.post_type = 'post' AND (wp_posts.post_status = 'published')

这将为您提供结果行中唯一ID的计数。

Answer 2

由于第一个查询显示2行，因此第二个查询应返回2 ，并且确实如此。 这是应该的。

Answer 3

在那里拥有GROUP BY没有真正的逻辑意义。

一个简单的解决方法是将您原始的sql用作子选择并根据该值进行计数：

SELECT COUNT(*) AS cnt  
FROM (SELECT wp_posts.ID  
FROM wp_posts 
INNER JOIN wp_term_relationships  
ON (wp_posts.ID = wp_term_relationships.object_id) 
WHERE ( wp_term_relationships.term_taxonomy_id IN (4,3) ) 
AND wp_posts.post_type = 'post'  
AND (wp_posts.post_status = 'published') 
GROUP BY wp_posts.ID) Sub1

还是按照表之间的关系进行排序（以及wp_posts.ID的唯一性如何）。

SELECT COUNT(DISTINCT wp_posts.ID) 
FROM wp_posts
INNER JOIN wp_term_relationships 
ON (wp_posts.ID = wp_term_relationships.object_id)
WHERE ( wp_term_relationships.term_taxonomy_id IN (4,3) )
AND wp_posts.post_type = 'post' 
AND (wp_posts.post_status = 'published')