删除表中的行，具体取决于在另一个表中删除的行 [英] Delete table rows depending upon rows deleted in another table

问题描述

我有两个表 t1 和 t2 .我这样删除 t2 的某些行:

I have two tables t1 and t2. I am deleting certain rows of t2 this way:

delete from t2 where expiry < NOW();

t1 和 t2 共有一个列 id .当执行以上语句时，我还想删除 t1 中的对应行(如果有的话)(可能没有).

There is a column id common to both t1 and t2. When the above statement is executed, I also want to delete corresponding rows in t1 if any (there might be none).

这怎么办?
是否可以为这两个操作编写一个查询?

How can this be done?
Is it possible to write a single query for both these operations?

推荐答案

我至少看到了三种好的方法-取决于问题中遗漏的更多细节.我以您的示例为基础:

I see at least three good ways - depending on further details missing in your question. I build on your example:

• t2 是母表
• t1 子

试验台:

CREATE TABLE t2 (
  id serial PRIMARY KEY  -- primary or unique key needed
, expiry timestamp
);

CREATE TABLE t1 (
  id int references t1(id) ON DELETE CASCADE  -- ON UPDATE CASCADE, too?
);

填充表格:

INSERT INTO t2(expiry)
SELECT (now() + g * interval '1h')
FROM   generate_series(1, 10) g;   -- 10 arbitrary rows

INSERT INTO t1(id)
SELECT id FROM t2 WHERE id%2 = 0;  -- pick even numbers for t1

SELECT * FROM t1;

如果您从t2中删除行，则t1中的相应行将被自动删除:

If you delete rows from t2, corresponding rows from t1 are deleted automatically:

DELETE FROM t2 WHERE id IN (1,2,3,4,5);
-- rows 2,4 in `t1` are deleted automatically

要将这样的外键约束添加到现有表中:

To add such a foreign key constraint to existing tables:

ALTER TABLE t1 ADD CONSTRAINT t1_id_fkey FOREIGN KEY (id)
REFERENCES t2 (id) ON DELETE CASCADE;

当然，外键需要 t1.id 上的唯一键或主键.但是您可能会遇到这种情况.

Of course, a foreign key requires a unique or primary key on t1.id. But you probably have that in a scenario like that.

如果 t2.id 中的值不是唯一的，或者如果 t1.id 中的值违反外键约束，则可以在AFTER DELETE之后创建触发器代替.

If the values in t2.id are not unique, or if you have values in t1.id violating a foreign key constraint, you could create a trigger AFTER DELETE instead.

触发功能:

CREATE OR REPLACE FUNCTION t2_delaft
  RETURNS trigger 
  LANGUAGE plpgsql AS
$func$
BEGIN
   DELETE FROM t1
   WHERE  t1.id = OLD.id;

   RETURN NULL;  -- AFTER trigger can return NULL
END
$func$

触发:

CREATE TRIGGER delaft
AFTER DELETE ON t2
FOR EACH ROW EXECUTE PROCEDURE t2_delaft();

您还可以实施规则.但是我发现触发器通常更易于处理.

You could also implement a RULE. But I find triggers generally easier to handle.

请原谅我最后给出最简单的答案.无论使用哪种方法，只要您使用的是PostgreSQL 9.1或更高版本，

Forgive me for giving the simplest answer last. This works, no matter what - provided you are on PostgreSQL 9.1 or later:

WITH x AS (
    DELETE FROM t1
    WHERE  id IN (1,2,3,4,5)
    RETURNING id
    )
DELETE FROM t2
USING  x
WHERE  t2.id = x.id;

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