如何從表 doc_val 中獲取每個 doc_id 表 doc_val 中最小“val”的行，其中標准 = 'L'

Question

你有兩個表：1. docs 2. doc_val

重點是 table : doc_val ，它具有來自 table docs 的 doc_id FK ，字段標准將是我們的條件。

Mysql 架構：

CREATE TABLE IF NOT EXISTS `docs` (
`id` int(6) unsigned NOT NULL,
`rev` int(3) unsigned NOT NULL,
`content` varchar(200) NOT NULL,
PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;

CREATE TABLE IF NOT EXISTS `doc_val` (
`id` int(6) unsigned NOT NULL,
`doc_id` int(6) unsigned NOT NULL,
`val` int(3) unsigned NOT NULL,
`type` varchar(2) NOT NULL,
`criteria` varchar(2) NOT NULL,
PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;

INSERT INTO `docs` (`id`, `rev`, `content`) VALUES
('1', '1', 'The earth is flat'),
('2', '1', 'One hundred angels can dance on the head of a pin'),
('3', '1', 'The earth is flat and rests on a bull\'s horn'),
('4', '4', 'The earth is like a ball.');

INSERT INTO `doc_val` (`id`, `doc_id`, `val`, `type`, `criteria`) VALUES
('1', '1', 100, 'D', 'L'),
('2', '1', 101, 'D', 'L'),
('3', '1', 80, 'H', 'L'),
('4', '2', 10, 'H', 'S'),
('5', '2', 90, 'H', 'L'),
('6', '3', 100, 'D', 'L'),
('7', '3', 100, 'D', 'L');

預期輸出：

Answer 1

DECLARE curIds CURSOR FOR SELECT DISTINCT doc_id FROM doc_val;
DECLARE id INT;
CREATE TEMPORARY TABLE temp(id int, doc_id int, val int, type char(1), criteria char(1));

OPEN curIds;

read_loop: LOOP
  FETCH curIds INTO id;
  INSERT INTO temp
   (Select * from doc_val 
    where val = (select min(val) 
                 from doc_val 
                 where doc_id = id)) AND criteria = 'L'

END LOOP;

SELECT * FROM temp;

可能存在語法錯誤，但我希望你能理解這個想法。