我试图使用以下代码获取4个特定部分中的文档数量：
SELECT
category.id,category.title,count(ts1.section_id) AS doc1,count(ts2.section_id) AS doc2,count(ts3.section_id) AS doc3,count(ts4.section_id) AS doc4
FROM
category
LEFT JOIN category_link_section AS ts1
ON (category.id = ts1.category_id AND ts1.section_id = 1)
LEFT JOIN category_link_section AS ts2
ON (category.id = ts2.category_id AND ts2.section_id = 2)
LEFT JOIN category_link_section AS ts3
ON (category.id = ts3.category_id AND ts3.section_id = 3)
LEFT JOIN category_link_section AS ts4
ON (category.id = ts4.category_id AND ts4.section_id = 4)
GROUP BY category.id,ts1.section_id,ts2.section_id,ts3.section_id,ts4.section_id
表’category’有一个id,标题等
“category_link_section”表包含category_id,section_id和doc_id之间的id连接.@H_502_6@
如果任何列的计数为0,则在该列中显示0.但如果结果不是0,则显示所有截面结果的乘积结果.所以如果我的4个计数列应该返回：1,2,3;实际上会显示6,6,6;@H_502_6@
如果我为每个特定类别使用以下代码,我会得到我想要的结果：@H_502_6@
SELECT
category.id,count(ts1.section_id) AS doc1
FROM
category
LEFT JOIN category_link_section AS ts1
ON (category.id = ts1.category_id AND ts1.section_id = 1)
GROUP BY category.id,ts1.section_id
解决方法
@ VoteyDisciple的
answer在正确的轨道上,但他的查询需要一些改进：
SELECT c.id,c.title,SUM(ts1.section_id = 1) AS doc1,SUM(ts1.section_id = 2) AS doc2,SUM(ts1.section_id = 3) AS doc3,SUM(ts1.section_id = 4) AS doc4
FROM category AS c
LEFT JOIN category_link_section AS ts1
ON (c.id = ts1.category_id)
GROUP BY c.id;
说明：@H_502_6@
> IF()表达式是多余的,因为等式已经返回1或0.>从连接条件中取出ts1.section_id = 1,否则您将永远不会得到其他section_id值.>仅由c.id组成.我假设OP只需要每个类别一行,以及各个类别的每个section_id值的计数列.如果查询按c.id,ts1.section_id分组,那么每个类别最多可以有四行.>在选择列表中移动逗号.漂浮在线条开头的逗号看起来很丑陋.
