IF EXISTS 条件不适用于 PLSQL

我正在尝试在条件为 TRUE 时打印文本。选择的代码工作正常。当我只运行选择代码时，它显示 403 值。但当条件存在时我必须打印一些文本。下面的代码有什么问题。

BEGIN
IF EXISTS(
SELECT CE.S_REGNO FROM
COURSEOFFERING CO
JOIN CO_ENROLMENT CE
  ON CE.CO_ID = CO.CO_ID
WHERE CE.S_REGNO=403 AND CE.COE_COMPLETIONSTATUS = 'C' AND CO.C_ID = 803
)
THEN
    DBMS_OUTPUT.put_line('YES YOU CAN');
END;

这是错误报告：

Error report:
ORA-06550: line 5, column 1:
PLS-00103: Encountered the symbol "JOIN" when expecting one of the following:

   ) , with group having intersect minus start union where
   connect
06550. 00000 -  "line %s, column %s:\n%s"
*Cause:    Usually a PL/SQL compilation error.
*Action:

IF EXISTS()在语义上是不正确的。EXISTS条件只能在 SQL 语句内使用。因此，您可以按如下方式重写您的 pl/sql 块：

declare
  l_exst number(1);
begin
  select case 
           when exists(select ce.s_regno 
                         from courseoffering co
                         join co_enrolment ce
                           on ce.co_id = co.co_id
                        where ce.s_regno=403 
                          and ce.coe_completionstatus = 'C' 
                          and ce.c_id = 803
                          and rownum = 1
                        )
           then 1
           else 0
         end  into l_exst
  from dual;

  if l_exst = 1 
  then
    DBMS_OUTPUT.put_line('YES YOU CAN');
  else
    DBMS_OUTPUT.put_line('YOU CANNOT'); 
  end if;
end;

或者你可以简单地使用count函数确定查询返回的行数，并且rownum=1谓词 - 您只需要知道记录是否存在：

declare
  l_exst number;
begin
   select count(*) 
     into l_exst
     from courseoffering co
          join co_enrolment ce
            on ce.co_id = co.co_id
    where ce.s_regno=403 
      and ce.coe_completionstatus = 'C' 
      and ce.c_id = 803
      and rownum = 1;

  if l_exst = 0
  then
    DBMS_OUTPUT.put_line('YOU CANNOT');
  else
    DBMS_OUTPUT.put_line('YES YOU CAN');
  end if;
end;