postgresql - PostgreSQL 中包含 insert into 子句的合并语句失败

Question

这是我的简单测试表，

create table test (
  id          int not null generated always as identity,
  first_name. varchar,

  primary key (id),
  unique(first_name)
);

作为 insert-into-on-conflict 句子的替代方案，我尝试使用 coalesce 懒惰来执行 select 每当仅当 select 找不到行时才可能或 insert。

coalesce 懒惰在文档中进行了描述。请参阅https://www.postgresql.org/docs/current/functions-conditional.html

Like a CASE expression, COALESCE only evaluates the arguments that are needed to determine the result; that is, arguments to the right of the first non-null argument are not evaluated. This SQL-standard function provides capabilities similar to NVL and IFNULL, which are used in some other database systems.

我还想获取该行的 id 值，无论是否插入。

我开始于:

select coalesce (
  (select id from test where first_name='carlos'),
  (insert into test(first_name) values('carlos') returning id)
);

但是在“into”处或附近发现了错误语法错误。 在另一个 DBFiddle 上查看它 https://www.db-fiddle.com/f/t7TVkoLTtWU17iaTAbEhDe/0

然后我尝试了:

select coalesce (
  (select id from test where first_name='carlos'),
  (with r as (
    insert into test(first_name) values('carlos') returning id
   ) select id from r
  )
);

这里我收到一个 WITH 子句包含数据修改语句必须位于顶层 错误，我不明白，因为 insert 是第一个并且with 中唯一的句子。

我正在使用 DBFiddle 和 PostgreSQL 13 对此进行测试。源代码可以在以下位置找到 https://www.db-fiddle.com/f/hp8T1iQ8eS4wozDCBhBXDw/5

Answer 1

不同的方法:链式 CTE:

---

CREATE TABLE test
        ( id          INTEGER NOT NULL GENERATED ALWAYS AS IDENTITY PRIMARY KEY
        , first_name VARCHAR UNIQUE
        );

WITH sel AS (
        SELECT id FROM test WHERE first_name = 'carlos'
        )
, ins AS (
        INSERT INTO test(first_name) 
        SELECT 'carlos'
        WHERE NOT EXISTS (SELECT 1 FROM test WHERE first_name = 'carlos')
        RETURNING id
        )
, omg AS (
        SELECT id FROM sel
        UNION ALL 
        SELECT id FROM ins
        )
SELECT id
FROM omg
        ;