Question

我目前正在刷新我的SQL知识，并在以下查询中遇到一些困难。

要求是：

对于每个制造商，按字母顺序列出＆＃34; /＆＃34;作为他所生产的所有类型产品的分界。

演绎：制造商，产品类型＆＃39;列表

以下解决方案确实有效，但我并不完全理解......

;with
t1 as 
    (select  maker, type, DENSE_RANK() over(partition by maker  order by type) rn
    from product
),
tr(maker, type,lev) as
    (select distinct t1.maker, cast(t1.type as nvarchar) , 2 from t1 where t1.rn = 1
    union all
    select t1.maker, cast(tr.type +'/'+t1.type as nvarchar), lev + 1
    from t1 join tr on (t1.maker = tr.maker and t1.rn = tr.lev
    )
)

select maker, max(type) names from tr group by maker

这些输出：

1 | A | Laptop/PC/Printer
2 | B | Laptop/PC
3 | C | Laptop
4 | D | Printer
5 | E | PC/Printer

*第二列是制造商，第三列是动态连接的类型列表。

现在，我对lev究竟是如何动态增长感到有些困惑。我在这里缺少某种循环吗？为什么从2开始？如果没有＆＃34;演员＆＃34;？

，它为什么不起作用

如果有人能够解释这个问题背后的逻辑，我将非常感激。

非常感谢！

Answer 1

你正在看的是一个递归的CTE，一个自称的CTE。它是你认为的循环＆＃34;影响。当我第一次看到它时，递归会在我的大脑中产生扭结。它有助于查看一些示例并尝试创建一些您自己的简单示例。我仍然不是最好的，但我变得更好。我在你的代码中添加了一些注释。希望这会有所帮助。

;WITH t1
AS (
    SELECT  maker,
            type,
            --Partition says each maker is a group so restart at 1
                --Order by type is alphabetic
                --DENSE_RANK() means if there are two the of the same item, they get the same number
            DENSE_RANK() OVER (PARTITION BY maker ORDER BY type) rn
    FROM product
    ),
--This is a recursive CTE meaning a CTE that calls itself(what is doing the "looping"
tr (maker,type,lev)
AS (
    --Grab only the distinct items from your ranked table
    SELECT DISTINCT t1.maker,
        cast(t1.type AS NVARCHAR),
        2 --This is the start of your recursive loop
    FROM t1
    WHERE t1.rn = 1

    UNION ALL

    --Recursively loop through you data, adding each string at the end with the '/'
    SELECT t1.maker,
        cast(tr.type + '/' + t1.type AS NVARCHAR),
        --Plus one to grab the next value
        lev + 1
    FROM t1
    INNER JOIN tr ON (
            --Only match the same makers
            t1.maker = tr.maker
            --Match to the next value
            AND t1.rn = tr.lev
            )
    )

--I think you know what this does
SELECT maker,
    max(type) names
FROM tr
GROUP BY maker

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