在PostgreSQL中使用觸發器的非空約束

Question

我有一個包含兩個可選列的表。 在表中插入一行時，兩列中的一列不應爲空。

我發現MySQL的這個解決方案：

CREATE TABLE foo (
    FieldA INT, 
    FieldB INT 
); 

DELIMITER // 
CREATE TRIGGER InsertFieldABNotNull BEFORE INSERT ON foo 
FOR EACH ROW BEGIN 
    IF (NEW.FieldA IS NULL AND NEW.FieldB IS NULL) THEN 
    SIGNAL SQLSTATE '45000' 
    SET MESSAGE_TEXT = '\'FieldA\' and \'FieldB\' cannot both be null'; 
    END IF; 
END// 
CREATE TRIGGER UpdateFieldABNotNull BEFORE UPDATE ON foo 
FOR EACH ROW BEGIN 
    IF (NEW.FieldA IS NULL AND NEW.FieldB IS NULL) THEN 
    SIGNAL SQLSTATE '45000' 
    SET MESSAGE_TEXT = '\'FieldA\' and \'FieldB\' cannot both be null'; 
    END IF; 
END// 
DELIMITER ; 

INSERT INTO foo (FieldA, FieldB) VALUES (NULL, 10); -- OK 
INSERT INTO foo (FieldA, FieldB) VALUES (10, NULL); -- OK 
INSERT INTO foo (FieldA, FieldB) VALUES (NULL, NULL); -- gives error 
UPDATE foo SET FieldA = NULL; -- gives error 

如何爲PostgreSQL的翻譯呢？

Answer 1

你可以簡單地用一個表約束：

CREATE TABLE foo (
    FieldA INT, 
    FieldB INT, 
    CHECK (NOT (FieldA IS NULL AND FieldB IS NULL))); 

這樣，你不能插入或修改與NULL這兩個值的元組。

teststar=# INSERT INTO foo (FieldA, FieldB) VALUES (NULL, 10); 
INSERT 0 1 
teststar=# INSERT INTO foo (FieldA, FieldB) VALUES (10, NULL); 
INSERT 0 1 
teststar=# INSERT INTO foo (FieldA, FieldB) VALUES (NULL, NULL); 
ERROR: new row for relation "foo" violates check constraint "foo_check" 
DETAIL: Failing row contains (null, null). 
teststar=# UPDATE foo SET FieldA = NULL; 
ERROR: new row for relation "foo" violates check constraint "foo_check" 
DETAIL: Failing row contains (null, null). 
teststar=#