计算计数

Question

可能重复：
COUNT() and DISTINCT can i use together?

我有这样的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') 

，我得到两行用相同的ID =>预计

加入GROUP BY wp_posts.ID将行数减少到一个

现在我想获得的行的数量和SQL查询

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 

我会得到的结果是“2”，而不是“1” ，即使是“GROUP BY”。

从第一条语句中获取行数的正确语句是什么？

Answer 1

试试这个：

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') 

这会给你一个唯一的ID的计数结果行。

Answer 2

由于第一个查询显示2行，因此第二个查询应该返回2，并且确实如此。这是正常的。

Answer 3

拥有GROUP BY并没有真正的逻辑意义。

一个简单的解决办法是使用你原来的SQL作为一个子查询，并以此为基础进行计数： -

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 

或者如下因素可能会确定，这取决于表之间的关系（等如何唯一wp_posts.ID是）。

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')