Question

我在PostgreSQL中有一个简单的表，它有三列：

id serial primary key
key varchar
value varchar

我已经在SO上发现了这个问题：Insert, on duplicate update in PostgreSQL?但是我想知道如果它存在，如何获取id，而不是更新。如果标准做法是始终“插入”或“如果存在则更新”，为什么会这样？ SELECT（LIMIT 1）的成本是否大于UPDATE？

我有以下代码

INSERT INTO tag
    ("key", "value")
    SELECT 'key1', 'value1'
WHERE
    NOT EXISTS (
        SELECT id,"key","value" FROM tag WHERE key = 'key1' AND value = 'value1'
    );

在某种意义上起作用，如果存在则不插入，但我想得到id。是否有“RETURNING id”条款或类似的内容我可以在那里点击？

Answer 1

是的，有returning

INSERT INTO tag ("key", "value")
SELECT 'key1', 'value1'
WHERE NOT EXISTS (
    SELECT id, "key", "value"
    FROM node_tag
    WHERE key = 'key1' AND value = 'value1'
    )
returning id, "key", "value"

如果已存在，则返回该行

with s as (
    select id, "key", "value"
    from tag
    where key = 'key1' and value = 'value1'
), i as (
    insert into tag ("key", "value")
    select 'key1', 'value1'
    where not exists (select 1 from s)
    returning id, "key", "value"
)
select id, "key", "value"
from i
union all
select id, "key", "value"
from s

如果该行不存在，则会返回已插入的行，否则为现有行。

顺便说一句，如果对“key”/“value”使其唯一，则它是主键，并且不需要id列。除非“key”/“value”对中的一个或两个都可以为null。

Answer 2

with vals as (
  select 'key5' as key, 'value2' as value
)
insert into Test1 (key, value)
select v.key, v.value
from vals as v
where not exists (select * from Test1 as t where t.key = v.key and t.value = v.value)
returning id

sql fiddle demo

Answer 3

并且您可以将返回值的值存储为... RETURNING field1，field2，... INTO var1，var2，...

RETURNING通常会返回一个返回错误的查询错误'查询没有结果数据的目标'如果你在plpgsql中调用它而不使用它返回的结果集。

如果不存在则插入，否则在postgresql中返回id

3 个答案: