仅在主表不存在时才将主键添加到PostgreSQL表

Question

我在Postgres 9.1中有简单的表创建脚本.我需要它来创建具有2个属性PK的表,只有它不存在.

CREATE TABLE IF NOT EXISTS "mail_app_recipients"
(
    "id_draft" Integer NOT NULL,
    "id_person" Integer NOT NULL
) WITH (OIDS=FALSE); -- this is OK

ALTER TABLE "mail_app_recipients" ADD PRIMARY KEY IF NOT EXISTS ("id_draft","id_person");
-- this is problem since "IF NOT EXISTS" is not allowed.

任何解决方案如何解决这个问题？提前致谢.

Answer 1

为什么不在CREATE TABLE中包含PK定义:

CREATE TABLE IF NOT EXISTS mail_app_recipients
(
    id_draft Integer NOT NULL,
    id_person Integer NOT NULL,
    constraint pk_mail_app_recipients primary key (id_draft, id_person)
)

Answer 2

您可以执行以下操作,但最好将其包含在create table中,如a_horse_with_no_name建议的那样.

if NOT exists (select constraint_name from information_schema.table_constraints where table_name = 'table_name' and constraint_type = 'PRIMARY KEY') then

ALTER TABLE table_name
  ADD PRIMARY KEY (id);

end if;

Answer 3

您可以DROP在创建它之前尝试它（DROP有IF EXISTS子句）：

ALTER TABLE mail_app_recipients DROP CONSTRAINT IF EXISTS mail_app_recipients_pkey;
ALTER TABLE mail_app_recipients ADD CONSTRAINT mail_app_recipients_pkey PRIMARY KEY ("id_draft","id_person");

请注意，这要求您为主键约束命名——在本例中mail_app_recipients_pkey。