如何获得MySQL服务器上每个数据库的空值计数？

Question

我想获得MySQL服务器上每个数据库的所有列和表的空值数。结果表应该是这样的：

+---------------+------------+ 
| database_name | null_count | 
+---------------+------------+ 
| database1  |   0 | 
| database2  |  5643 | 
| database3  |   72 | 
+---------------+------------+ 

但是，我没能获得超越空的数量为单一的表：

SELECT concat('select', group_concat(' count(*) - count(', column_name, ') ' SEPARATOR ' + '), 'from ', MAX(table_schema), '.', MAX(TABLE_NAME)) 
INTO @SQL 
FROM information_schema.columns 
WHERE TABLE_SCHEMA = 'Accidents' 
AND TABLE_NAME = 'nesreca'; 

PREPARE stmt FROM @SQL; 
EXECUTE stmt; 
DEALLOCATE PREPARE stmt; 

你有一些想法？

PS：我能用MATLAB代码得到结果。但是，MySQL内的解决方案将更受欢迎。

Answer 1

我认为你在动态SQL的正确轨道上。因为你希望它为整个数据库而不是一个单一的表，查询的结构是这样的：

select table_schema, sum(cnt) 
from (select "table_schema", count(*) as cnt from "table_name" where "column_name" is null union all 
     . . . 
    ) t 
group by table_schema; 

您可以构建这样的：

set group_concat_max_len=1500000; 
set @subSQL = '(select "@table_schema" as table_schema, count(*) as cnt from `@table_schema`.`@table_name` where `@column_name` is null)'; 

select @subSQL := group_concat(replace(replace(replace(@subSQL, '@table_schema', table_schema 
                ), '@table_name', table_name 
              ), '@column_name', column_name 
            ) separator ' union all ' 
          ) 
from information_schema.columns; 

set @SQL = concat('select table_schema, sum(cnt) as NumNulls from (', 
        @subSQL, 
        ') t group by table_schema'); 

PREPARE stmt FROM @SQL; 
EXECUTE stmt; 
DEALLOCATE PREPARE stmt; 

我要指出的是，默认对于此查询，group_concat()的中间值的长度可能太小。你将不得不调整它。系统变量为group_concat_max_len（请参阅here）。