等同于 Oracle 中的 IF NOT EXISTS？

Question

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

我在 Oracle 中尝试过：

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;

但它给我一个错误 PLS-00103 "Encountered the symbol 'ALTER'..."

Answer 1

使用立即执行在 PLSQL 块内触发 ddl：

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

最近不存在的是 Tom Kyte 给出的方法 here

begin
  for i in (select count(*) cnt from dual 
            where not exists (
                SELECT *
                FROM USER_CONSTRAINTS 
                WHERE CONSTRAINT_NAME = 'FK_STATIONOBJECTSID' 
                AND TABLE_NAME = 'ATTENDANCE'
            )) loop
            if (i.cnt = 1) then
              execute immediate 'ALTER TABLE Attendance ADD CONSTRAINT FK_StationObjectsID FOREIGN KEY (StationObjectsID) REFERENCES stationobjects (stationobjectsid)';
            end if;
  end loop;
end;
/

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;
/

见 - https://dba.stackexchange.com/questions/37362/why-cant-we-write-ddl-statement-directly-into-the-pl-sql-block

Answer 2

成功了，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;
/

Answer 3

您可以在 SQL 查询中使用 EXISTS，但不能像您尝试的那样在 PLSQL 条件中使用。

您可能需要以下内容：

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;

Answer 4

如何执行 DDL，并忽略任何 "foreign key already exists" 错误。

不优雅，但不依赖于键名不变。

等同于 Oracle 中的 IF NOT EXISTS？

Equivalent to IF NOT EXISTS in Oracle?

oracle

exists