Question

假设我有这样的 SQL：

SELECT
    directed_events.id as event_id,
    directed_events.event_type as event_type,
    issuers.referable_id as issuer_id,
    issuers.referable_type as issuer_type,
    targets.referable_id as target_id,
    targets.referable_type as target_type
FROM
    events AS directed_events
INNER JOIN "event_refs" "issuers" 
  ON "directed_events"."id" = "issuers"."event_id"
  AND "issuers"."connection_type" = 'issuer'
INNER JOIN "event_refs" "targets" 
  ON "directed_events"."id" = "targets"."event_id"
  AND "targets"."connection_type" = 'target'

我想确保值集（event_type、issuer_id、target_id）的数据库内容是唯一的，因此必须拒绝重复插入。

当然，我可以在应用程序级别做到这一点。但是数据库级别的约束是更可靠的方法。

Answer 1

你可以使用 concat + distinct on

SELECT DISTINCT ON (conc_id) *
FROM (
    SELECT
        directed_events.id as event_id,
        directed_events.event_type as event_type,
        issuers.referable_id as issuer_id,
        issuers.referable_type as issuer_type,
        targets.referable_id as target_id,
        targets.referable_type as target_type,

        -- concat columns to have only one column for unique check
        CONCAT(directed_events.event_type::text, '-', issuers.referable_id::text, '-', targets.referable_id::text) as conc_id
    FROM
        events AS directed_events
    INNER JOIN "event_refs" "issuers" 
    ON "directed_events"."id" = "issuers"."event_id"
    AND "issuers"."connection_type" = 'issuer'
    INNER JOIN "event_refs" "targets" 
    ON "directed_events"."id" = "targets"."event_id"
    AND "targets"."connection_type" = 'target'
) as data

ORDER BY conc_id ASC

PostgreSQL：在连接表上创建唯一约束（多表约束）

1 个答案: