PostgreSQL插入如果不存在

Question

我有以下查询

INSERT INTO address (house_number, street, city_id)
    values(11, 'test st', (select id from city where LOWER(city) = LOWER('somecity')))

无论如何，如果在city中不存在“ somecity”，那么是否在city表中插入“ somecity”，然后在插入之后，它将返回插入行的ID？

我确实找到了这个答案，说可以使用upsert实现此目的

https://stackoverflow.com/a/31742830/492015

但是我找不到一个如果select不返回该行插入的示例。

Answer 1

除了嵌套INSERT，您还可以使用CTE一个接一个地执行INSERT，一个语句即可 ：

WITH tmp AS (
    INSERT INTO test_city (city) VALUES ('somecity')
    ON CONFLICT (lower(city)) DO UPDATE SET city = excluded.city
    RETURNING id, city
)
INSERT INTO test_address (house_number, street, city_id)
SELECT house_number, street, id
FROM (VALUES (11, 'test st', 'somecity')) val (house_number, street, city)
LEFT JOIN tmp USING (city)
RETURNING *

---

使用此设置：

DROP TABLE IF EXISTS test_address;
DROP TABLE IF EXISTS test_city;
CREATE TABLE test_address (
    house_number int
    , street text
    , city_id int
    );
CREATE TABLE test_city (
    id int GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY
    , city text 
    );
CREATE UNIQUE INDEX test_city_uniq_idx ON test_city USING btree (lower(city));
INSERT INTO test_city (city) VALUES ('Somecity');

和上面的INSERT一起，查询

SELECT * FROM test_address;

产量

| house_number | street  | city_id |
|--------------+---------+---------|
|           11 | test st |       1 |

和

SELECT * FROM test_city;

产量

| id | city     |
|----+----------|
|  1 | somecity |

---

请注意，CTE取代了

(select id from city where LOWER(city) = LOWER('somecity'))

用INSERT .. ON CONFLICT .. DO UPDATE语句：

INSERT INTO test_city (city) VALUES ('somecity')
ON CONFLICT (lower(city)) DO UPDATE SET city = excluded.city
RETURNING id, city

我使用DO UPDATE而不是DO NOTHING以便RETURNING id, city将始终返回某些内容。 如果使用DO NOTHING ，则在发生冲突时不返回任何内容。

但是请注意，使用的结果， city = excluded.city是，原来的'Somecity'被替换为'somecity' 。 我不确定您是否会接受这种行为，但是不幸的是，在发生冲突时，我还没有弄清楚如何执行任何操作，但同时返回了id和city 。

---

您可能会遇到上述解决方案的另一个问题是，我在lower(city)上使用了唯一索引：

CREATE UNIQUE INDEX test_city_uniq_idx ON test_city USING btree (lower(city));

这使您可以在INSERT语句中使用相同的条件：

INSERT ... ON CONFLICT (lower(city))

替换出现在SELECT语句中的条件LOWER(city) = LOWER('somecity') 。 它产生了预期的效果，但是要权衡的是，现在您在(lower(city))上具有唯一索引。

---

关于如何插入两个以上表的后续问题 ：

您可以将多个CTE链接在一起 ，后续的CTE甚至可以引用先前的CTE。 例如，

CREATE UNIQUE INDEX city_uniq_idx ON city USING btree (lower(city));
CREATE UNIQUE INDEX state_uniq_idx ON state USING btree (lower(state_code));

WITH tmpcity AS 
(
   INSERT INTO
      city (city) 
   VALUES
      (
         'Miami'
      )
      ON CONFLICT (lower(city)) DO 
      UPDATE
      SET
         city = excluded.city RETURNING id, city
)
, tmpstate as 
(
   INSERT INTO
      state (state_code) 
   VALUES
      (
         'FL'
      )
      ON CONFLICT (lower(state_code)) DO 
      UPDATE
      SET
         state_code = excluded.state_code RETURNING id, state_code
)
INSERT INTO
   address (house_number, street, city_id, state_id) 
   SELECT
      house_number,
      street,
      tmpcity.id,
      tmpstate.id 
   FROM
      (
      VALUES
         (
            12,
            'fake st.',
            'Miami',
            'FL'
         )
      )
      val (house_number, street, city, state_code) 
      LEFT JOIN
         tmpcity USING (city) 
      LEFT JOIN
         tmpstate USING (state_code)
         ON CONFLICT (street) DO NOTHING