将属性表联接为列和值？

Question

CREATE TABLE person_properties (
  person_id INT,  
  key TEXT,
  value TEXT,
  PRIMARY KEY (person_id, key)
);

CREATE TABLE persons (
  id SERIAL PRIMARY KEY
);

我已经做到了：

INSERT INTO persons DEFAULT_VALUES;  -- suppose id is 1
INSERT INTO person_properties (person_id, key, value) VALUES (1, 'age', '30')
INSERT INTO person_properties (person_id, key, value) VALUES (1, 'weight', '20lb')

我想选择person s的age和weight为列，从值person_properties （或NULL ，如果不存在的话）。 我怎样才能做到这一点？

Answer 1

SELECT * FROM persons
LEFT OUTER JOIN person_properties p1 ON persons.person_id = p1.person_id AND p1.key = 'age'
LEFT OUTER JOIN person_properties p2 ON persons.person_id = p2.person_id AND p2.key = 'weight'

Answer 2

您的架构是错误的。

key是保留键，并且唯一表中不能有2个主键。

CREATE TABLE person_properties (
  person_id INT,  
  cle TEXT,
  value TEXT,
  PRIMARY KEY (person_id)
);

CREATE TABLE persons (
  id SERIAL PRIMARY KEY
);

和

您不能拥有两次相同的主键...

INSERT INTO person_properties (person_id, cle, value) VALUES (**1**, 'age', '30');
INSERT INTO person_properties (person_id, cle, value) VALUES (**1**, 'weight', '20lb');

如果必须有两行具有相同的键，那就不好了。

我建议您重做//重新考虑您的架构。

编辑：

CREATE TABLE person_properties (
  person_id INT NOT NULL AUTO_INCREMENT,  
  pkey VARCHAR(10),
  value TEXT,
  PRIMARY KEY (person_id, pkey)
);

CREATE TABLE persons (
  id SERIAL PRIMARY KEY
);

INSERT INTO person_properties (pkey, value) VALUES ('age', '30');
INSERT INTO person_properties (pkey, value) VALUES ('weight', '20lb');