如果Oracle中不存在，则等同于？

以下语句适用于MSSQL：

IF NOT EXISTS (SELECT * FROM sys.foreign_keys WHERE object_id = OBJECT_ID(N'[FK_StationObjectsID]') AND parent_object_id = OBJECT_ID(N'[Attendance]'))
BEGIN
    ALTER TABLE Attendance ADD CONSTRAINT FK_StationObjectsID FOREIGN KEY (StationObjectsID) REFERENCES stationobjects (stationobjectsid)
END

在Oracle中，我尝试过：

IF NOT EXISTS (SELECT * FROM USER_CONSTRAINTS WHERE CONSTRAINT_NAME = 'FK_STATIONOBJECTSID' AND TABLE_NAME = 'ATTENDANCE') THEN
BEGIN
    ALTER TABLE Attendance ADD CONSTRAINT FK_StationObjectsID FOREIGN KEY (StationObjectsID) REFERENCES stationobjects (stationobjectsid);
END;

---

但是它给了我一个错误PLS-00103“遇到符号‘ALTER’…”

使用execute immediate在PLSQL块内触发ddl：

IF <condition> THEN 
    Execute immediate 'ALTER TABLE . . .';
END if;

更简单的方法是：

declare
  n int := 0;
begin
  select count(*) into n
  from user_constraints
  where constraint_name = 'FK_STATIONOBJECTSID'
  and table_name = 'ATTENDANCE';
  if n = 0 then
    execute immediate 'ALTER TABLE Attendance ADD CONSTRAINT FK_StationObjectsID FOREIGN KEY (StationObjectsID) REFERENCES stationobjects (stationobjectsid)';
  end if;
end;
/

看

---

-

让它工作了，但可能不是最好的解决方案：

DECLARE
  foreign_key_exists number := 0;  
BEGIN
  SELECT COUNT(*) INTO foreign_key_exists FROM USER_CONSTRAINTS WHERE upper(CONSTRAINT_NAME) = upper('FK_StationObjectsID') AND upper(TABLE_NAME) = upper('Attendance');
  IF (foreign_key_exists = 0) 
    THEN
      EXECUTE IMMEDIATE 'ALTER TABLE Attendance ADD CONSTRAINT FK_StationObjectsID FOREIGN KEY (StationObjectsID) REFERENCES stationobjects (stationobjectsid)';
  END IF;
END;
/

您可以在SQL查询中使用，但不能像您尝试的那样在PLSQL条件中使用

您可能需要以下各项：

declare
    vCheck number;
begin
    select count(1)
    into vCheck
    from user_constraints
    where constraint_name = 'FK_STATIONOBJECTSID'
      and table_name = 'ATTENDANCE';
    --
    if vCheck = 0 then
        execute immediate 'ALTER TABLE Attendance ADD CONSTRAINT FK_StationObjectsID FOREIGN KEY (StationObjectsID) REFERENCES stationobjects (stationobjectsid)';
    end if;
end;

---

执行DDL并忽略任何“外键已存在”错误如何

---

不优雅，但不依赖于键名不变。

尝试

立即执行“alter…”您需要一个带有动态SQL的PL/SQL块，感谢GurV，现在得到另一个错误，即外键已经存在。好像IF有什么问题。当执行SELECT时，它返回一行，但它仍在尝试执行ALTER。。。
declare
    vCheck number;
begin
    select count(1)
    into vCheck
    from user_constraints
    where constraint_name = 'FK_STATIONOBJECTSID'
      and table_name = 'ATTENDANCE';
    --
    if vCheck = 0 then
        execute immediate 'ALTER TABLE Attendance ADD CONSTRAINT FK_StationObjectsID FOREIGN KEY (StationObjectsID) REFERENCES stationobjects (stationobjectsid)';
    end if;
end;