Question

我有3行的表信息：id，monter1，monter2。 Monter1和monter2是我要加入的表monterzy（id，name，surname）的id。问题是我无法在php中进行正确的查询以获取这些值。目前我有：

function compare(a, b) {
  if (a is less than b by some ordering criterion) {
    return -1;
  }
  if (a is greater than b by the ordering criterion) {
    return 1;
  }
  // a must be equal to b
  return 0;
}

我想扩展它以获得monter2和相应的名字和姓氏。我在谷歌搜索过，有AS的例子，但我不知道怎么做。谢谢！

Answer 1

您需要使用不同的别名

连接同一个表两次

SELECT info.id, 
       m1.name as m1_name, m1.surname as m1_surname,
       m2.name as m2_name, m2.surname as m2_surname
FROM info 
INNER JOIN monterzy m1 ON info.monter1 = m1.id
INNER JOIN monterzy m2 ON info.monter2 = m2.id

Answer 2

您只需要对表进行别名。也就是说，对于同一个表的每个连接，给它一个不同的别名（名称），以便查询的其余部分知道哪个是哪个。像这样：

SELECT
  info.id,
  m1.name,
  m1.surname,
  m2.name,
  m2.surname
FROM
  info
  INNER JOIN monterzy AS m1 ON info.monter1 = m1.id
  INNER JOIN monterzy AS m2 ON info.monter2 = m2.id

注意每个联接如何为AS someAlias添加一个新名称，仅用于此查询。以及查询的其余部分如何引用该别名而不是表名。

Answer 3

您需要使用表别名：

SELECT info.id, m1.name as name1, m1.surname as surname1, m2.name as name2, m2.surname as surname2
FROM info 
INNER JOIN monterzy m1 ON info.monter1 = m1.id
INNER JOIN monterzy m2 ON info.monter2 = m2.id;

Answer 4

您可以使用左连接

SELECT
  info.id,
  m1.name,
  m1.surname,
  m2.name,
  m2.surname
FROM
  info
  INNER JOIN monterzy AS m1 ON info.monter1 = m1.id
  LEFT JOIN monterzy AS m2 ON info.monter2 = m2.id

MySQL将同一个表中的数据连接两次

4 个答案: