如何从表 doc_val 中获取每个 doc_id 表 doc_val 中最小“val”的行，其中标准 = 'L'

Question

你有两个表：1. docs 2. doc_val

重点是 table : doc_val ，它具有来自 table docs 的 doc_id FK ，字段标准将是我们的条件。

Mysql 架构：

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');

预期输出：

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;

可能存在语法错误，但我希望你能理解这个想法。