在兩個表的左聯接中，選擇左表中的所有記錄，並從右表中僅選擇與左表匹配的一行記錄

Question

我有兩個表問答

我想選擇所有數學科目的問題，但只選擇其中一個問題。

$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

使用子查詢，

選擇問題，（從questions.id = answer.que_id的答案中選擇答案）作為questions.subject ='maths'的問題的答案；

請注意：這不是正在運行的SQL-只是了解一下

Answer 2

嘗試這樣的子查詢：

$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

您的查詢應如下所示：

$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():