Question

我正在尝试在postgreSQL中执行此操作：您不能存储超过5条记录（行）。

CREATE FUNCTION contar_dest()
RETURNS TRIGGER AS
$body$
BEGIN
IF (SELECT COUNT(ID) FROM "LUGARES" WHERE demanda is not null) > 5
THEN 
DELETE FROM "LUGARES"
WHERE ID = (SELECT max(ID) FROM "LUGARES");
RETURN NULL;
END IF;
END;
$body$
LANGUAGE plpgsql;

CREATE TRIGGER contar
AFTER INSERT
ON "LUGARES"
FOR EACH ROW
EXECUTE PROCEDURE contar_dest();

当我尝试插入一行时，请告诉我：

错误：执行到达触发程序结束而未找到RETURN 语境：PL / pgSQL函数contar_dest（）

现在我按照自己的意愿行事，但错误就是这样：

CREATE FUNCTION contar_dest()
RETURNS TRIGGER AS
$body$
BEGIN
IF ((SELECT COUNT(ID) FROM "LUGARES" WHERE demanda is not null) > 5) THEN 
DELETE FROM "LUGARES"
WHERE ID = (SELECT NEW.ID FROM "LUGARES");
ELSIF ((SELECT COUNT(ID) FROM "LUGARES" WHERE demanda is not null) < 5) THEN
RETURN NULL;
END IF;
RETURN NULL;
END;
$body$
LANGUAGE plpgsql;


CREATE TRIGGER contar
AFTER INSERT
ON "LUGARES"
FOR EACH ROW
EXECUTE PROCEDURE contar_dest();

INSERT INTO "LUGARES"(
        nombre, demanda)
VALUES ('Valencia',2000);

ERROR: a subquery used as an expression returned more than one record
CONTEXT : SQL statement : " DELETE FROM "LUGARES"
    WHERE ID = ( SELECT FROM NEW.ID "LUGARES") »
PL / pgSQL contar_dest ( ) function in line 4 SQL statement
********** Error **********

Answer 1

错误是由您使用最多需要1行的语法引起的，但是在执行时会返回多行。

变化：

DELETE FROM "LUGARES"
WHERE ID = (SELECT NEW.ID FROM "LUGARES");

要：

DELETE FROM "LUGARES"
WHERE ID IN (SELECT NEW.ID FROM "LUGARES");

=仅适用于1个值，但IN允许使用多个值。

如何限制表中特定列值的行数？

1 个答案: