查询从表中发现没有数据的列动态

Question

我写了一个查询找到列，而不从动态表中的数据， 但其给出的输出只能用ROWNUM = 1，这也是不正确的，

WITH x AS (SELECT column_name FROM all_tab_cols 
WHERE OWNER='HR' AND table_name='EMPLOYEES' AND ROWNUM=1)  
SELECT X.column_name,count(X.column_name) 
FROM EMPLOYEES,X 
/*CONNECT BY LEVEL <= (SELECT count(1) FROM all_tab_cols 
WHERE OWNER='HR' AND table_name='EMPLOYEES')*/ 
group by X.column_name; 

COLUMN_NAME     COUNT(X.COLUMN_NAME) 
------------------------------ ---------------------- 
EMPLOYEE_ID     20 

当我尝试使用水平，使其动态失败，

WITH x AS (SELECT column_name FROM all_tab_cols 
WHERE OWNER='HR' AND table_name='EMPLOYEES' AND ROWNUM=level)  
SELECT X.column_name,count(X.column_name) 
FROM EMPLOYEES,X 
CONNECT BY LEVEL <= (SELECT count(1) FROM all_tab_cols 
WHERE OWNER='HR' AND table_name='EMPLOYEES') 
group by X.column_name; 

Error at Command Line:14 Column:5 Error report: SQL Error: ORA-01788: CONNECT BY clause required in this query block 01788. 00000 - "CONNECT BY clause required in this query block"

CUDü纠正达到同样的查询？

Answer 1

你的查询有一些问题：

• 子查询中with条款不看到其他子查询level，
• 你不能使用where rownum = 2，你必须做出别名此列，从选择它内部查询，但是这部分不需要在你的情况下，
• 如果你想检查所有的列，不需要分层suqbquery。

更正查询是：

with x as (
    select column_name from all_tab_cols 
    where owner='HR' and table_name='EMPLOYEES') 
select x.column_name, count(x.column_name) as cnt 
    from x, employees group by x.column_name; 

...但它总是显示在employees所有行的每一列的数量。因此，如果您想查找不填充所有数据的列，则需要动态解决方案。您可以在可变v_sql添加类似...where ||'r.column_name' is null：

declare 
    v_sql varchar2(1000); 
    v_num number; 
begin 
    for r in (select column_name from all_tab_cols 
      where owner='HR' and table_name='EMPLOYEES' 
      order by column_id) 
    loop 
    v_sql := 'select count('||r.column_name||') from employees'; 
    execute immediate v_sql into v_num; 
    dbms_output.put_line(rpad(r.column_name,30)||' '||v_num); 
    end loop; 
end; 

实例数据和输出：

create table employees (id number(3), name varchar2(10), hire_date date); 
insert into employees values (1, 'John', trunc(sysdate)); 
insert into employees values (2, 'Paul', trunc(sysdate)); 
insert into employees values (3, 'Mark', trunc(sysdate)-1); 
insert into employees values (4, 'Anna', null); 

ID        4 
NAME       4 
HIRE_DATE      3