Question

所以我有一个查询来检索每个表中的表名和列数。

SELECT Table_Schema, Table_Name, COUNT(*)
FROM Information_Schema.Columns
GROUP BY Table_Schema, Table_Name
HAVING Table_Schema = 'schema';

如何添加此查询可以存储NULL值的列数？

Answer 1

使用is_nullable列：

SELECT Table_Schema, Table_Name, COUNT(*)
FROM Information_Schema.Columns
WHERE is_nullable = 'YES'
GROUP BY Table_Schema, Table_Name
HAVING Table_Schema = 'schema';

information_schema.columns上的documentation详细描述了该表格，包括可能感兴趣的其他列。

编辑：

如果要将信息添加到现有查询，只需使用条件聚合：

SELECT Table_Schema, Table_Name, COUNT(*), SUM( (is_nullable = 'YES')::int ) as nullable_clumns
FROM Information_Schema.Columns
GROUP BY Table_Schema, Table_Name
HAVING Table_Schema = 'schema';

Answer 2

您可以向聚合函数添加FILTER条件，以仅计算满足条件的行：

SELECT table_Schema,
       table_Name,
       count(*) AS total_columns,
       count(*) FILTER (WHERE is_nullable = 'YES') AS nullable_columns
FROM information_schema.columns
GROUP BY table_Schema, table_Name
HAVING table_schema = 'schema';

SQL：计算可以具有空值的列

2 个答案: