Question

场景：：从上一个问题（Using a cursor in a stored procedure to loop rows MySQL）开始，我试图做一个嵌套的prepare语句，在其中我将一个日期输入到外部日期，并调用内部日期从表中获取数据。

代码：

-- Create temporary table for the Output:
drop temporary table if exists `stats`;
create temporary table `stats`
(
    col_name varchar(32) null,
    num_nulls int null,
    num_values int null
);


-- Procedure for the check:
drop procedure if exists `set_column_null_stats`;
delimiter $$
create procedure `set_column_null_stats`
(`p_col_name` varchar(128), `wanted_date` date)
begin

-- Set variables:
set @col_nme = `p_col_name`;
set @date1 = `wanted_date`;

prepare stmt from 'insert into `stats` (`col_name`) values (?);';
execute stmt using @col_nme;
deallocate prepare stmt;

-- count number of NULLS based on conditions:
set @sql_txt = concat(
            'update `stats` s join(
            select 
            count(1) as `nb`
            from `btc`
            where`btc`.`date` = ', @date1, ' and `btc`.`', @col_nme, '` is null)
            t set `num_nulls` = t.`nb` where `col_name` = \'', @col_nme, '\';');
prepare stmt from @sql_txt;
execute stmt;
deallocate prepare stmt;

-- count number of not NULLS based on conditions:
set @sql_txt = concat(
           'update `stats` s join(
            select
            count(1) as `nb`
            from `btc`
            where `btc`.`date` = ', @date1, ' and `btc`.`', @col_nme, '` is not null)
            -- t set `num_values` = t.`nb` where `col_name` = \'', @col_nme, '\';');        
set @sql_txt = concat('update `stats` s join (select count(1) as `nb` from `btc` where `', @col_nme, '` is not null) t set `num_values` = t.`nb` where `col_name` = \'', @col_nme, '\';');
prepare stmt from @sql_txt;
execute stmt;
deallocate prepare stmt;

end$$
delimiter ;


-- Procedure for looping through rows of `wanted_columns` table:
delimiter $$
drop procedure if exists `data_check_loop` $$
create procedure `data_check_loop`(`wanted_date` date)
begin

declare dateval date default null;
declare colval text default null;

-- boolean variable to indicate cursor is out of data
declare done tinyint default false;

-- declare a cursor to select the desired columns from the desired source table
declare cursor1
    cursor for
        select *
        from `wanted_columns`; 

-- catch exceptions
        declare continue handler for not found set done = true;

set dateval = `wanted_date`;

-- open the cursor
        open cursor1;
            my_loop: 
            loop
                fetch next from cursor1 into colval;
                if done then 
                    leave my_loop; 
                else  
                    call `set_column_null_stats`(colval, dateval);
                end if;
            end loop;
        close cursor1;

end $$
delimiter ;


-- Start the process with the wanted date:
call `data_check_loop`('2018-08-13');

select * from `stats`;

问题：：此代码运行没有错误，但没有给我任何结果。如果我只运行第一个准备好的语句，直接将变量一个接一个地输入，则效果很好。所以我想问题出在我的第二句话。

问题：对我在这里做错的任何想法？

obs ：第二个代码应该循环一个表中的行（需要的列），并将它们逐个馈入第一条语句（以及日期，该日期应始终相同）< / p>

Obs2：我对这个查询的目标是：从一个表中以行作为名称的列表（“ id1”，“ date1” ...），我打算读取每一行并使用它将值放入另一个表（其中的名称（“ id1”，“ date1” ...）是列），并为我想要的每个列获取NULL值和非NULL值的总和（还给出了日期的另一个约束输入）。最后，对于每个原始行（表1），我将输出一个带有#NULL和#notNULL的新行。

例如表1：

Col_names
  Id1
  Name1
  Date1
  Process
  Time
  Class

例如表2：

Id1    Name1    Date1    Process    Time    Class
aa     test1    01/01       3       NULL      A
NULL   test2    01/02       4       NULL      b
bb     test3    NULL        3       NULL     NULL

例如输出：

Col_name    #Null    #notNull
  Id1         1          2
  Name1       0          3
  Date1       1          2 
  Process     0          3
  Time        3          0
  Class       1          2

Answer 1

您希望将列转换为记录结束计数，其中该列的值为null或not null。
在大多数数据库系统中，将列转换为记录的过程称为unpivot，UNPIVOT()支持此操作，但是MySQL不支持。
因此通常使用UNION并结合一些聚合函数（例如MAX()，MIX()，SUM()和COUNT()以及CASE END子句来完成此操作在MySQL上模拟UNPIVOT()。

查询

SELECT 
   'Id1' AS Col_name
 , SUM(CASE WHEN Table1.Id1 IS NULL THEN 1 ELSE 0 END) AS `#Null`
 , SUM(CASE WHEN Table1.Id1 IS NOT NULL THEN 1 ELSE 0 END) AS `#notNull` 
FROM 
 Table1

UNION ALL

SELECT 
  'Name1' AS Col_name
 , SUM(CASE WHEN Table1.Name1 IS NULL THEN 1 ELSE 0 END) AS `#Null`
 , SUM(CASE WHEN Table1.Name1 IS NOT NULL THEN 1 ELSE 0 END) AS `#notNull` 
FROM 
 Table1

UNION ALL

SELECT 
  'Date1' AS Col_name
 , SUM(CASE WHEN Table1.Date1 IS NULL THEN 1 ELSE 0 END) AS `#Null`
 , SUM(CASE WHEN Table1.Date1 IS NOT NULL THEN 1 ELSE 0 END) AS `#notNull` 
FROM 
 Table1

UNION ALL

SELECT 
  'Process' AS Col_name
 , SUM(CASE WHEN Table1.Process IS NULL THEN 1 ELSE 0 END) AS `#Null`
 , SUM(CASE WHEN Table1.Process IS NOT NULL THEN 1 ELSE 0 END) AS `#notNull` 
FROM 
 Table1

UNION ALL

SELECT 
  'Time' AS Col_name
 , SUM(CASE WHEN Table1.Time IS NULL THEN 1 ELSE 0 END) AS `#Null`
 , SUM(CASE WHEN Table1.Time IS NOT NULL THEN 1 ELSE 0 END) AS `#notNull` 
FROM 
 Table1

UNION ALL

SELECT 
  'Class' AS Col_name
 , SUM(CASE WHEN Table1.Class IS NULL THEN 1 ELSE 0 END) AS `#Null`
 , SUM(CASE WHEN Table1.Class IS NOT NULL THEN 1 ELSE 0 END) AS `#notNull` 
FROM 
 Table1

结果

| Col_name | #Null | #notNull |
| -------- | ----- | -------- |
| Id1      | 1     | 2        |
| Name1    | 0     | 3        |
| Date1    | 1     | 2        |
| Process  | 0     | 3        |
| Time     | 3     | 0        |
| Class    | 1     | 2        |

demo

准备好的语句来循环MySQL行

1 个答案: