如何在存储过程中使用准备好的语句分配变量？

Question

我整理了一个简单的存储过程，在其中传递了两个参数以使其更具动态性。我已经在“前两位数和记录数”部分中准备了一条语句来完成此操作。

我不确定的是是否可以通过SET vTotalFT准备好的语句使该部分动态化。

目前，我必须对表名和字段进行硬编码。我希望vTotalFT基于准备好的动态SQL语句分配变量，但是我不确定语法。想法是，当我调用过程时，可以告诉它要用于分析的表和字段。

CREATE PROCEDURE `sp_benfords_ft_digits_analysis`(vTable varchar(255), vField varchar(255))
    SQL SECURITY INVOKER
BEGIN

    -- Variables
    DECLARE vTotalFT int(11);

    -- Removes existing table
    DROP TABLE IF EXISTS analysis_benfords_ft_digits;

    -- Builds base analysis table
    CREATE TABLE analysis_benfords_ft_digits
    (
        ID int(11) NOT NULL AUTO_INCREMENT,
        FT_Digits int(11),
        Count_of_Records int(11),
        Actual decimal(18,3),
        Benfords    decimal(18,3),
        Difference Decimal(18,3),
        AbsDiff decimal(18,3),
        Zstat decimal(18,3),
        PRIMARY KEY (ID),
        KEY id_id (ID)
    );

    -- First Two Digits and Count of Records
    SET @s = concat('INSERT INTO analysis_benfords_ft_digits
                        (FT_Digits,Count_of_Records)
                            select substring(cast(',vField,' as char(50)),1,2) as FT_Digits, count(*) as Count_of_Records
                            from ',vTable,'
                            where ',vField,' >= 10
                            group by 1');

    prepare stmt from @s;
    execute stmt;
    deallocate prepare stmt;

    SET vTotalFT = (select sum(Count_of_Records) from
                            (select substring(cast(Gross_Amount as char(50)),1,2) as FT_Digits, count(*) as Count_of_Records
                                from supplier_invoice_headers
                                where Gross_Amount >= 10
                                group by 1) a);


    -- Actual
    UPDATE analysis_benfords_ft_digits
    SET Actual = Count_of_Records / vTotalFT;

    -- Benfords
    UPDATE analysis_benfords_ft_digits
    SET Benfords = Log(1 + (1 / FT_Digits)) / Log(10);

    -- Difference
    UPDATE analysis_benfords_ft_digits
    SET Difference = Actual - Benfords;

    -- AbsDiff
    UPDATE analysis_benfords_ft_digits
    SET AbsDiff = abs(Difference);

    -- ZStat
    UPDATE analysis_benfords_ft_digits
    SET ZStat = cast((ABS(Actual-Benfords)-IF((1/(2*vTotalFT))<ABS(Actual-Benfords),(1/(2*vTotalFT)),0))/(SQRT(Benfords*(1-Benfords)/vTotalFT)) as decimal(18,3));

Answer 1

首先，要使用动态表/列名称，您需要像第一次查询一样使用字符串/ 准备语句@s。接下来，要从COUNT()查询内部获取返回值，您将需要使用SELECT .. INTO @vTotalFT。

以下是您所需要的：

SET @vTotalFTquery = CONCAT('(select sum(Count_of_Records) INTO @vTotalFT from
                        (select substring(cast(', vField, ' as char(50)),1,2) as FT_Digits, count(*) as Count_of_Records
                            from ', vTable, '
                            where ', vField, ' >= 10
                            group by 1) a);');
PREPARE stmt FROM @vTotalFTquery;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

请注意：变量名称已从更改vTotalFT为@vTotalFT。如果没有，它似乎无法工作@。而且，在@vTotalFT查询之外/之前声明该变量时，该变量将不起作用，因此，如果遇到错误或空结果可能是原因。