Question

我有两个表格问题与答案

我想选择所有数学主题的问题，但只选择其中一个的答案。

$this->db->select('*');
    $this->db->from('questions');
    $this->db->join('answers','questions.id = answers.que_id', 'left'); //how to limit answers to 1
    $this->db->where('questions.subject', 'maths');
    return $this->db->get();

Answer 1

使用子查询，

选择问题，（从questions.id = answers.que_id的答案中选择答案）作为questions.subject =“数学”的问题的答案；

请注意：这不是正在运行的SQL-只是想出主意

Answer 2

尝试这样的子查询：

$this->db->select('*');
$this->db->from('answers');
$this->db->join('(select * from questions limit 1)','(questions.id = answers.que_id)', 'inner');
$this->db->where('questions.subject', 'maths');
return $this->db->get();

Answer 3

您的查询应如下所示：

$query = 'select q.*, a1.* 
from questions AS q
    left join answers AS a1
        on a1.id = (select a2.id 
            from answers AS a2 
            where q.id = a2.que_id
            ORDER BY a2.id 
            limit 1
            );';
$query = $this->db->query($query);
$result = $query->result():

在两个表的左连接中，从左表中选择所有记录，并从右表中仅选择与左表匹配的一行记录

3 个答案: