在一个SQL查询中有多个计数

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

我试图用下面的代码在4个特定的部分中获得文档计数:

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

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

表“类别”有一个id、标题等。表“category_link_section”包含category_id、section_id和doc_id之间的id连接。

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;

如果任何列的计数为0，则在该列中显示为0。但如果结果不是0，则显示所有部分结果的乘法结果。如果我的4个count列应该返回:1 2 0 3;它会显示6 6 0 6;

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

如果我对每个特定类别使用以下代码，我将得到我想要的结果:

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

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

但是，我需要在每个部分每次都遍历数据库。

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?

所以我的问题是，我是否需要依次遍历并调用每个部分，在SQL之外构造表，还是可以在单个查询中完成?

2 个解决方案

#1

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

@ votey门徒的回答是正确的，但是他的问题需要改进:

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;

---

Explanations:

解释:

• The IF() expressions are redundant because equality already returns 1 or 0.
• IF()表达式是冗余的，因为等式已经返回1或0。
• Take the ts1.section_id=1 out of the join condition, or you'll never get the other section_id values.
• 壹空间。section_id=1脱离join条件，否则永远不会得到其他section_id值。
• 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.
• 集团由c。id。我假设OP只希望每个类别有一行，每个类别的section_id值的计数要有列。如果查询按c分组。id,壹空间。section_id，然后每个类别最多有4行。
• Move the commas in the select-list. Commas floating at the start of the line look ugly. ;-)
• 移动选择列表中的逗号。开头的逗号看起来很难看。:-)

#2

You might want to try something like this:

你可能想试试这样的东西:

SELECT
    category.id
    , category.title
    , SUM(IF(ts1.section_id = 1, 1, 0)) AS doc1
    , SUM(IF(ts1.section_id = 2, 1, 0)) AS doc2
    , SUM(IF(ts1.section_id = 3, 1, 0)) AS doc3
    , SUM(IF(ts1.section_id = 4, 1, 0)) AS doc4
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