递归 SQL 将 CSV 拆分为表行

发布于 2024-11-07 17:21:13 字数 983 浏览 0 评论 0原文

在解决了一个不同的问题后，我偶然发现了递归 CTE，从表面上看，这似乎是解决“将 csv 拆分为表行”问题的相当简单的方法。

我把这个例子放在一起

DECLARE @InputString varchar(255) = 'a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z'

SELECT @InputString = @InputString + ','
;
with MyCTE(x,y)
as 
(
    SELECT 
        x = SUBSTRING(@InputString,0,PATINDEX('%,%',@InputString)),
        y = SUBSTRING(@InputString,PATINDEX('%,%',@InputString)+1,LEN(@InputString))
    UNION ALL
        SELECT 
            x = SUBSTRING(y,0,PATINDEX('%,%',y)),
            y = SUBSTRING(y,PATINDEX('%,%',y)+1,LEN(y))
        FROM 
            MyCTE 
        WHERE
                SUBSTRING(y,PATINDEX('%,%',y)+1,LEN(y)) <> '' OR 
            SUBSTRING(y,0,PATINDEX('%,%',y)) <> ''
)
SELECT x FROM MyCTE
OPTION (MAXRECURSION 2000);
GO

这真的是一个坏主意吗？像这样的递归查询在 SQL 中的开销是多少，这种方法有哪些潜在的陷阱。

顺便说一句，我认为这个想法/技术可能可以用来解决这个其他问题。

原文

After working on a different question here on SO, I stumbled across recursive CTEs which would on the surface seem a fairly easy way to solve the "Split a csv to table rows" problem.

I put this example together

DECLARE @InputString varchar(255) = 'a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z'

SELECT @InputString = @InputString + ','
;
with MyCTE(x,y)
as 
(
    SELECT 
        x = SUBSTRING(@InputString,0,PATINDEX('%,%',@InputString)),
        y = SUBSTRING(@InputString,PATINDEX('%,%',@InputString)+1,LEN(@InputString))
    UNION ALL
        SELECT 
            x = SUBSTRING(y,0,PATINDEX('%,%',y)),
            y = SUBSTRING(y,PATINDEX('%,%',y)+1,LEN(y))
        FROM 
            MyCTE 
        WHERE
                SUBSTRING(y,PATINDEX('%,%',y)+1,LEN(y)) <> '' OR 
            SUBSTRING(y,0,PATINDEX('%,%',y)) <> ''
)
SELECT x FROM MyCTE
OPTION (MAXRECURSION 2000);
GO

Is this really a bad idea? What is the overhead in SQL for recursive queries like this, and what are the potential pitfalls for this kind of approach.

Incidentally, I'm thinking this idea/technique could probably be leveraged to solve this other question.

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少女情怀诗 2024-11-14 17:21:13

这个将使用空字符串

DECLARE @InputString VARCHAR(1000)
    SELECT @InputString = 'a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z,,1'

    SELECT SUBSTRING(',' + @InputString + ',', Number + 1,
    CHARINDEX(',', ',' + @InputString + ',', Number + 1) - Number -1)AS VALUE
    FROM master..spt_values
    WHERE type = 'p'
    AND Number <= LEN(',' + @InputString + ',') - 1
    AND SUBSTRING(',' + @InputString + ',', Number, 1) = ','
    GO

另请看一下此处的注释：SSQL Server 2005+ CLR 与 T-SQL 中的分割字符串以获得其他一些想法

This one will work with empty strings

DECLARE @InputString VARCHAR(1000)
    SELECT @InputString = 'a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z,,1'

    SELECT SUBSTRING(',' + @InputString + ',', Number + 1,
    CHARINDEX(',', ',' + @InputString + ',', Number + 1) - Number -1)AS VALUE
    FROM master..spt_values
    WHERE type = 'p'
    AND Number <= LEN(',' + @InputString + ',') - 1
    AND SUBSTRING(',' + @InputString + ',', Number, 1) = ','
    GO

Also take a look at the comments here: Split string in SQL Server 2005+ CLR vs. T-SQL for some other ideas

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