从不存在的表中选择 [英] Select from table that does not exist

问题描述

我有一个关于ORACLE的问题，我写了一个PLSQL CODE来检查一个表是否存在，如果存在，那么我从该表中选择一个东西..pseudocode类似于:

I have a question regarding ORACLE, I wrote a PLSQL CODE that checks if a table exists, if it exists then I select something from this table..pseudocode is like:

如果(表存在) 从表中选择....

if (table exists) Select from table where....

问题是，即使表条件不存在且select语句从未执行，即使表不存在，我也总是会收到错误消息.

the problem is that I always get an error if the table does not exist, even if the if condition is never met and the select statement is never executed.

我认为这是因为在编译时检查了我的代码:"select from .."，如果表不存在，则会打印错误.我该如何解决这样的问题?..这是我的代码的样子(我使用了通用名称):

I think it is because my code is checked at compile time: "select from.." and then it prints an error if the table does not exist. How can I solve such an issue?.. here is how my code looks like (I used generic names):

     DECLARE
          v_table_exists NUMBER;
       BEGIN

          SELECT NVL(MAX(1), 0)
            INTO v_table_exists
            FROM ALL_TABLES
           WHERE TABLE_NAME = 'TABLE_TEST';

          IF v_table_exists = 1 THEN
             INSERT INTO MY_TABLE(COLUMN1, COLUMN2, COLUMN3, COLUMN4)
             SELECT 1234,
                    5678,
                    T.COLUMN_TEST1,
                    T.COLUMN_TEST2


    FROM  TABLE_TEST T
          WHERE T.FLAG = 1;
      END IF;
   END;

推荐答案

问题恰好在于您的过程con未被引用，因为它引用的对象不存在.为此，您可能需要一些动态SQL；例如:

The issue is exactly in the fact that your procedure con not be compiled as it refers to a non existing object; you may need some dynamic SQL for this; for example:

create or replace procedure checkTable is
    vCheckExists  number;
    vNum          number;
begin
    -- check if the table exists
    select count(1)
    into vCheckExists
    from user_tables
    where table_name = 'NON_EXISTING_TABLE';
    -- 
    if vCheckExists = 1 then
        -- query the table with dynamic SQL
        execute immediate 'select count(1) from NON_EXISTING_TABLE'
        into vNum;
    else
        vNum := -1;
    end if;

    dbms_output.put_line(vNum);
end;

即使表不存在，过程也会编译；如果您立即拨打电话，您将获得:

The procedure compiles even if the table does not exist; if you call it now, you get:

SQL> select count(1) from NON_EXISTING_TABLE;
select count(1) from NON_EXISTING_TABLE
                     *
ERROR at line 1:
ORA-00942: table or view does not exist


SQL> exec checkTable;
-1

PL/SQL procedure successfully completed.

然后，如果您创建表并再次调用该过程:

Then, if you create the table and call the procedure again:

SQL> create table NON_EXISTING_TABLE(a) as select 1 from dual;

Table created.

SQL> exec checkTable;
1

PL/SQL procedure successfully completed.

我显示SELECT的方式相同，您可以执行UPDATE或所需的任何SQL查询；如果您执行的操作不同于SELECT，则必须删除INTO子句.

The same way I showed a SELECT, you can do an UPDATE or whatever SQL query you need; if you do something different from a SELECT, the INTO clause has to be removed.

例如，假设您需要插入另一个表中，则应以这种方式编辑上面的代码:

For example, say you need to insert into a different table, the above code should be edited this way:

if vCheckExists = 1 then
    execute immediate 'insert into target(a, b, c) select a, 1, 100 from NON_EXISTING_TABLE';
end if;

这篇关于从不存在的表中选择的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！