如何从表中选择具有非空值的列？

I have a table containing hundreds of columns many of which are null, and I would like have my select statement so that only those columns containing a value are returned. It would help me analyze data better. Something like:

我有一个包含数百列的表，其中许多列都是null，我想要我的select语句，以便只返回包含值的列。这将有助于我更好地分析数据。就像是：

Select (non null columns) from tablename;

从tablename中选择（非空列）;

I want to select all columns which have at least one non-null value.

我想选择至少有一个非空值的所有列。

Can this be done?

可以这样做吗？

6 个解决方案

#1

Have a look as statistics information, it may be useful for you:

看看统计信息，它可能对您有用：

SQL> exec dbms_stats.gather_table_stats('SCOTT','EMP');

PL/SQL procedure successfully completed.

SQL> select num_rows from all_tables where owner='SCOTT' and table_name='EMP';

  NUM_ROWS
----------
        14

SQL> select column_name,nullable,num_distinct,num_nulls from all_tab_columns
  2  where owner='SCOTT' and table_name='EMP' order by column_id;

COLUMN_NAME                    N NUM_DISTINCT  NUM_NULLS
------------------------------ - ------------ ----------
EMPNO                          N           14          0
ENAME                          Y           14          0
JOB                            Y            5          0
MGR                            Y            6          1
HIREDATE                       Y           13          0
SAL                            Y           12          0
COMM                           Y            4         10
DEPTNO                         Y            3          0

8 rows selected.

For example you can check if NUM_NULLS = NUM_ROWS to identify "empty" columns.
Reference: ALL_TAB_COLUMNS, ALL_TABLES.

例如，您可以检查NUM_NULLS = NUM_ROWS是否标识“空”列。参考：ALL_TAB_COLUMNS，ALL_TABLES。

#2

select column_name
from user_tab_columns
where table_name='Table_name' and num_nulls=0;

Here is simple code to get non null columns..

这是获取非空列的简单代码。

#3

I don't think this can be done in a single query. You may need some plsql to first test what columns contain data and put together a statement based on that information. Of course, if the data in your table changes you have to recreate the statement.

我不认为这可以在一个查询中完成。您可能需要一些plsql来首先测试哪些列包含数据，并根据该信息汇总一个语句。当然，如果表中的数据发生更改，则必须重新创建语句。

declare

   l_table          varchar2(30) := 'YOUR_TABLE';
   l_statement      varchar2(32767);
   l_test_statement varchar2(32767);

   l_contains_value pls_integer;

   -- select column_names from your table
   cursor c is
      select column_name
            ,nullable
        from user_tab_columns
       where table_name = l_table;

begin
   l_statement := 'select ';
   for r in c
   loop
      -- If column is not nullable it will always contain a value
      if r.nullable = 'N'
      then
         -- add column to select list.
         l_statement := l_statement || r.column_name || ',';
      else
         -- check if there is a row that has a value for this column
         begin
            l_test_statement := 'select 1 from dual where exists (select 1 from ' || l_table || ' where ' ||
                                r.column_name || ' is not null)';
            dbms_output.put_line(l_test_statement);
            execute immediate l_test_statement
               into l_contains_value;


            -- Yes, add column to select list
            l_statement := l_statement || r.column_name || ',';
         exception
            when no_data_found then
               null;
         end;

      end if;
   end loop;

   -- create a select statement
   l_statement := substr(l_statement, 1, length(l_statement) - 1) || ' from ' || l_table;

end;

#4

Use the below:

使用以下：

SELECT *
FROM information_schema.columns
WHERE table_name = 'Table_Name' and is_nullable = 'NO'

Table_Name has to be replaced accordingly...

必须相应地替换Table_Name ...

#5

select rtrim (xmlagg (xmlelement (e, column_name || ',')).extract ('//text()'), ',') col
from (select column_name
from user_tab_columns
where table_name='<table_name>' and low_value is not null)

#6

-1

What you're asking to do is establish a dependency on each row in the whole result. This is in fact not ever what you want. Just think of the ramifications if in one row every column had a value of '0' -- suddenly the schema of your result set grows to include all of those previously "empty" columns. You're effectively growing the badness of '*' exponentially, now your result set is not dependent on just the table's meta-data -- but your whole result set is dependent on the plain data.

你要做的是在整个结果中建立对每一行的依赖。事实上，这并不是你想要的。只要考虑一下，如果每一列的值都为'0'，那么结果集的模式会突然增加，以包含所有以前“空”的列。你实际上是以指数方式增加了'*'的不良，现在你的结果集并不仅仅依赖于表的元数据 - 但是你的整个结果集依赖于普通数据。

What you want to do is just select the fields that have what you want, and not deviate from this simple plan.

你想要做的只是选择你想要的字段，而不是偏离这个简单的计划。

#1