单个SQL查询中的多个计数 [英] Multiple counts within a single SQL query

问题描述

我尝试使用以下代码获取4个特定部分中的文档数：

I'm trying to get the count of documents within 4 specific sections using the following code:

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'类别' id，标题等。
表'category_link_section'包含category_id，section_id和doc_id之间的id链接。

The table 'category' had an id, title etc. The table 'category_link_section' contains id linkages between category_id, section_id, and doc_id.

如果任何列的计数为0，它在该列中显示0。但是如果结果不为0，它显示所有段结果的乘法结果。所以如果我的4个计数列应该返回：1，2，0，3;它实际上会显示6，6，0，6;

If the count is 0 for any column, it displays 0 in that column. But if the result is not 0 it shows the multiplication result of all the section results. So if my 4 count columns were supposed to return: 1, 2, 0, 3; it would actually show 6, 6, 0, 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：

If I use this following code for each specific category I get the results I want:

SELECT
    category.id
    , category.title
    , 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

但我需要每次在每个部分循环使用数据库。

but I then need to cycle through the database each time for each section.

，我需要依次遍历和调用每个部分，构造我的表外面的SQL，或者这可以在一个单一的查询？

So my question is, do I need to step through and call each section in turn, constructing my table outside the SQL, or can this be done in a single query?

推荐答案

@ VoteyDisciple的回答是但他的查询需要一些改进：

@VoteyDisciple's answer is on the right track, but his query needs some improvements:

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：

---

Explanations:

• IF（）表达式是多余的，因为等式已返回1或0。


• 取出连接条件中的 ts1.section_id = 1 ，或者永远不会获得其他 section_id 值。


• 仅由 c.id 分组。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，则每个类别最多有四行。


• 移动在选择列表中的逗号。漂浮在线条开始处的逗号看起来很丑陋。 ; - ）

• The IF() expressions are redundant because equality already returns 1 or 0.
• Take the ts1.section_id=1 out of the join condition, or you'll never get the other section_id values.
• Group by c.id only. I assume the OP only wants one row per category, and columns for counts of each section_id value for the respective category. If the query grouped by c.id, ts1.section_id, then there'd be up to four rows per category.
• Move the commas in the select-list. Commas floating at the start of the line look ugly. ;-)

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